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GPS Pt.1: By Geoff Graham Car Computer This project was inspired by requests from a number of readers. Essentially they said that the GPS module was great for obtaining a precise time . . . but could we produce a project that used its full capabilities for use in a vehicle? T his is the result. It is small enough to sit on the dash also use graphic symbols. The display uses a green LED backlight, with the brightor mount in the radio-sized (DIN) blank panels that ness automatically changed between day and night settings, many vehicles have. It is also relatively simple to construct and uses just a which can be easily configured. An USB interface allows you to plug the device into a few active components on a single PC board with no other wiring. There is no setup or calibration required – just plug laptop for a complete navigation experience with your position accurately pinpointed on a map. The USB interface also it in and away you go! allows you upload new firmware It is designed for use in a and re-program the device withmoving vehicle, where your atout any special hardware. tention should not be distracted GRAPHIC LCD from the road for any longer Block Diagram than is necessary. Therefore Fig.1 shows a block diagram data, such as your speed, is disof the unit. The central item is played in easily-readable 14mm GPS USB TO MICROCONTROLLER a 40-pin microcontroller which high digits, while a button press MODULE LAPTOP does most of the work, includwill show other data as needed. ing receiving data from the GPS You can also set it to autoLDR OR module, driving the LCD and matically step through the data FUEL INJECTOR HEADLIGHTS communicating via USB. screens for complete hands-off THREE PUSHBUTTONS The GPS module used is the operation. EM-408 which we have used in A key component is the Fig.1: conceptually, the GPS Car Computer is quite a number of previous projects. graphic LCD which allowed a simple. The microcontroller takes data from the Once every second this transmits great deal of design flexibility. GPS module, monitors a number of external signals, your current speed, position, Three fonts are used ranging formats the data for display and sends it to the LCD. from a jumbo font to a small While doing this it can also manage communications etc as a serial data stream to the microcontroller. detailed font, while some areas with your laptop via USB. 28  Silicon Chip siliconchip.com.au FEATURES: When a complete set of data has image is reversed in firmware and been collected the microcontrolthe data lines are reversed (D0 on • Digital speedometer ler will calculate parameters such the microcontroller connects to • Over-speed alarm as the fuel economy and format D7 on the LCD, etc). the data for display. This data is An additional output (pin 17 • Fuel economy meter then sent at high speed to the LCD or RC2) is used to control the • Distance and time to destination so that the display updates very backlight of the LCD. The mi• Digital clock with GPS accuracy quickly – in the blink of an eye! crocontroller generates a square The microcontroller also moniwave on this pin which, via Q1, • Altitude in metres tors a number of inputs, the three rapidly switches the backlight • Heading and compass pushbutton switches, a light LEDs off and on. By varying the • Current latitude and longitude dependent resistor (or the car’s duty cycle of this waveform the headlights) for night/day detecmicrocontroller can smoothly • A USB 2.0 interface so that you can tion and a signal from the fuel control the backlight brightness use it with your laptop for navigation injector system used in measuring from off to full on. or uploading firmware changes... fuel economy. It also controls the The firmware allows you to set backlight brightness of the LCD separate levels of the backlight and communicates over USB to brightness for day and night, a laptop. determined by a light sensor or The only other items of note are two voltage regulators the vehicle headlights. When the microcontroller detects a which supply 5V for the microcontroller and 3.3V for the transition between day and night it will change the brightness GPS module. The device can be powered from the vehicle gradually over a period of about a minute. This is done so that 12V battery or from a computer’s USB interface. the driver is not distracted by a sudden change in brightness. The main PC board is very small at 123 x 43mm and The crystal (X1) on pins 13 and 14 provides the main clock. piggybacks onto the LCD readout – the complete display/ Within the microcontroller the 20MHz crystal frequency is PC board stack is about 25mm high. The dimensions were divided by 5 to give 4MHz and then multiplied by 12 in a deliberately kept small so that the complete unit could be phase locked loop (PLL) to generate the processor clock of mounted in many places in addition to sitting on top of the 48MHz. This is further divided by 4 to drive the USB 2.0 dashboard. interface and also divided by 4096 to generate an interrupt to the CPU every 85µS. Circuit Details Continuing in a clockwise direction around the circuit Fig.3 shows the full circuit diagram of the GPS Car Com- diagram, the three pushbuttons go straight to inputs on the puter. As discussed before, it is not very complex and is microcontroller which are pulled high by internal resistors. dominated by the microcontroller, a Microchip PIC18F4550. As a result a button press causes the input to be pulled low. This is mostly concerned with driving the graphic LCD via The firmware switches the pullup resistors off and on as an 8-bit bus (D0 to D7) and seven control lines. needed, so if you use an oscilloscope on these inputs you The LCD is mounted upside down so that the data lines will not see a simple DC voltage. are physically on the bottom of the PC board (viewed from Pin 18 (VUSB) of the microcontroller connects to an the front) and do not short against the GPS module which internal 3.3V regulator which is used to power the USB must be mounted on top. To accommodate this, the display interface within the chip. The 220nF capacitor on that pin External Antenna Connector MMCX type for external GPS antenna (on back of box) USB 2.0 Interface For navigation software on a laptop or loading new firmware. (On end of box). Light Detector LDR (on end of box) for day/night control (vehicle light power can be used instead). Power and Signals 6-pin mini-DIN socket (on end of box) for power and external signal input. GPS CAR COMPUTER SET SILICON CHIP SET Button Switches to an option screen associated with the currently displayed screen. Using the UP/DOWN buttons allow you to change the value of the chosen option. Pressing SET again will save the option and return to the original display screen. siliconchip.com.au GRAPHIC DISPLAY 120 x 32 pixel LCD readout UP Button Will switch to the previous display. When setting an option will increase the value. DOWN Button Will switch to the next display. When setting an option will decrease the value. Fig.2: there are only three push-button controls on the GPS Car Computer – just as you would want to minimise the time your eyes are distracted from the road. Having said that, it’s delightfully easy to use. January 2010  29 helps suppress ripple and noise in the regulator. The data lines (D+ and D-) from the USB socket connect directly to pins 24 and 23 respectively of the microcontroller. This is one of the great features of the Microchip microcontrollers; you do not need external resistors, transceivers or any sort of supporting components. Everything to do with the USB connection is taken care of within the chip. Jumper JP1 is used to prevent a conflict with the USB power source when the GPS Car Computer is permanently powered from a 12V supply. The need for this is explained in the section on assembly options. GPS Module The interface to the EM-408 GPS module has some tricks D1 1N5819 REG2 LP2950CZ-3.3 OUT +5.3V IN GND 10 F 16V 11 +5.3V 4 2 +3.3V Vdd CVref EM-408 GPS MODULE 1 4 Tx 3 Rx 2 GND 3.3k RC1 RE0 5 RA3 25 Tx 6 26 JP1 CON2 1 4 23 2 24 3 18 USB TYPE B RC0 RA1 RB0 C1out RB1 RB2 Rx RD0 RD1 D– RD2 D+ RD3 Vusb RD4 220nF RD5 RD6 SET RD7 36 S1 DOWN UP 37 S2 38 S3 13 X1 20MHz 22pF 22pF 14 10 100 F 16V REG1 LM2940CT-5 OUT RB3 RC2 16 7 8 6 15 5 3 4 A 470nF D2 1N4004 K 19 Vdd CLK CS2 CS1 A0 33 +12V IN GND 2x 100nF 22k 2 6.8k +5.3V 1 +5.6V 220nF IC1 PIC18F4550 -I/P 5 EN MCLR A K 100k 32 Vdd RA0 10k V+ to it that need a little explanation. The signal line from the microcontroller’s pin 25 to the module’s receive data input (Rx) is simple enough. The microcontroller runs at over 5V while the GPS module runs at 3.3V, so the two resistors are necessary to drop the microcontroller’s output voltage to a safe level for the module. The transmit data from the module to the microcontroller is where the problems occur. The voltage level on the EM408’s transmit line is a little lower than the standard TTL threshold for a high logic level. This, coupled with noise and temperature variations will cause intermittent errors in the data stream as read by the microcontroller. If you refer to Fig.4 you can see that we overcome this LED+ Vcon LCD CONT 3 SG12232A LCD MODULE RES E R/W 18 8 9 LED– VR1 10k CON1 100nF 6-PIN MINI DIN 20 6 D7 D6 D5 D4 D3 D2 D1 D0 GND 17 16 15 14 13 12 11 10 1 5 3 4 2 34 1 35 19 20 21 82k* 22 27 28 29 30 3.3k 17 C B Q1 BC338 E RB4 RB6 RB5 RE1 RE2 OSC1 RB7 OSC2 RA5 Vss 12 82k 39 9 10 40 +5.3V 7 + Vss 31 8.2k* 47k PIEZO BUZZER RESISTORS IN RED ARE NOT INSTALLED AT PRESENT (RESERVED FOR FUTURE EXPANSION) LDR1*  * OPTIONAL 1 2 3 4 5 PC BOARD EM-408 CONNECTIONS SC 2010 GPS CAR COMPUTER 1N5819 1N4004 A A K LM2940, LP2950 BC338 K GND B E C IN GND OUT Fig.3: the full schematic for the GPS Display. It is dominated by the microcontroller which is mostly concerned with receiving data from the GPS module and driving the LCD. The only other items of note are two regulators which provide 5.3V and 3.3V. 30  Silicon Chip siliconchip.com.au SERIAL DATA (NOT TTL COMPATIBLE) 4 2 EM-408 GPS MODULE Tx 4 TTL COMPATIBLE SERIAL DATA +12V PIC 18F4550 1.35V REFERENCE COMPARATOR 5 ENGINE MANAGEMENT SYSTEM TO GPS DISPLAY (CON1 PIN 5) +12V 6 26 Rx FUEL INJECTOR SOLENOID 0V UART Fig.4: because the EM-408 GPS module does not deliver standard TTL voltage levels we must convert the signal. This is done by a comparator which compares the signal to a 1.35V reference, both of which are integrated in the microcontroller. The output of the comparator is a TTL compatible signal which is fed to the UART, also within the microcontroller. issue by feeding the transmit data to the non-inverting input of an analog comparator circuit within the microcontroller (pin 5 or RA3). The microcontroller is also set up to generate a 1.35V reference voltage which is available on pin 4 (CVREF). This in turn is connected to the inverting input of the comparator on pin 2 (RA0). The result is that the transmit data signal from the EM408 is compared to 1.35V and the clean, TTL-compatible, output is available on pin 6 (C1OUT), which is connected to the input to the serial data receiver (UART) at pin 26 (Rx). Power Supply The main power is derived from the vehicle’s 12V battery present at pin 4 of the DIN input/output connector. REG1 is a 5V linear regulator designed for automotive use so it can withstand large voltage spikes, momentay reversal of the input voltage and other abuses that are common in the auto environment. Fig.5: a simplified diagram of the fuel injector control in a standard petrol engine. The positive lead of the injector is at 12V and the engine management pulls the negative lead to zero potential to open the solenoid and inject fuel into the inlet manifold. It is this signal that you should feed to the GPS Car Computer. With the backlight turned on full, the regulator will dissipate almost 1W so it is mounted on a small heatsink. Silicon diode D2 in the common leg of the regulator serves to raise its output to 5.6V, then the 1N5819 Schottky diode in series with the output (D1) will drop about 0.3V resulting in an output of about 5.3V for the LCD and microcontroller. This is done for a number of reasons. First, diode D1 serves to isolate the main power supply if the GPS Car Computer Display is plugged into a USB port. If D1 was not present, the 5V supply from the USB port would also try to drive the output pin of the voltage regulator, resulting in an excessive current from the USB source. The second feature of this circuit is that it provides a 5.3V supply to the LCD. This is needed because the contrast of the LCD is controlled by the voltage on pin 3 (Vo) with reference to the voltage on pin 2 (Vdd). For normal contrast the voltage on pin 3 should be about -4.9V to -5.1V compared to pin 2 - although this varies with temperature and manufacturing tolerances. Without going into the subtle details, this means that either the supply voltage on pin 2 of the LCD must be noticeably higher than 5.1V or we must be prepared to The GPS Car Computer mounts on the Jiffy Box lid, which becomes the base . . . with the LCD readout showing through a cutout in the base . . . which becomes the front panel. siliconchip.com.au January 2010  31 47k CON1 82k 220nF 10 3.3k 22k 82k* S1 470nF 100k LDR* 8.2k* CON2 CON5 1 10 F 10k LP2950 X1 20MHz IC1 PIC18F4550 Q1 BC338 CON4 3.3k 100nF External connections All external connections are made through CON1, a 6 pin mini DIN connector. Ground and 12V are on pins 3 and 4 (respectively) of the connector. An external input from the vehicle’s headlight’s circuit can be wired to pin 1 to control the day/night backlight brightness (more on this in the section on assembly options). Pin 5 of the connector can be wired to a fuel injector solenoid if you want to implement the fuel economy meter function. The 82K and 47K resistors serve to drop the vehicle voltage levels to 5V for the microcontroller. There are also two spare connections (pins 6 and 2) which can be connected to pins 9 and 10 of the microcontroller. These are unused and available for future use. They can be set by the firmware to be digital inputs, digital outputs or analog inputs. TO-220 M3 x 10mm Fig.7: the 5V regulator REGULATOR SCREW HEATSINK can dissipate up to SILICONE 1W so it must be GREASE mounted on a heatsink. A smear of heat transfer M3 STAR LOCKWASHER & NUT compound must be PC BOARD applied between the regulator and the heatsink. Before tightening the bolt check that the heatsink clears the nearby solder pads. Only after you have tightened the nut and bolt should you solder the regulator’s leads to the PC board. 32  Silicon Chip REG1 LM2940 100nF VR1 6.8k 10k S3 (TO LCD MODULE) 1 supply a negative voltage to pin 3 to get a proper level of contrast. Our approach of using D1 and D2 to give 5.3V is much simpler than generating a negative voltage. The final benefit of this power supply design is that we can use the 5.6V (via a 10 current limiting resistor) for driving the backlight LEDs. Given the various voltage drops involved (about 3.6V across the LEDs and 0.7V across Q1) it is much easier to get full brightness from a supply that is a bit higher than 5V. This arrangement also means that the backlight is automatically disabled when a USB power source is used – handy because the USB interface on your computer would be overloaded if the backlight was enabled. The second voltage regulator, REG2, is a 3.3V device that supplies the GPS module. It has a low dropout voltage, so it is happy with the 2V voltage differential between its input and output and it comes in a TO92 package, which is convenient as there is not much space on our PC board. 100 F S2 REG2 D1 5819 PIEZO BUZZER 1 Future firmware updates could use these to measure voltages (eg, battery voltage or sensor outputs), detect digital inputs (eg, switch closure or tachometer output) or set them to be an output to control something. PC board The GPS Car Computer is built on a single PC board coded 05101101. The PC board is double sided and uses plated-through holes thereby allowing us to keep the size small. This type of PC board is actually easier to assemble than the single sided version commonly used. This is because the board is coated in a varnish (called solder resist) which protects tracks and areas that should not be soldered. It also has the position and values of the components printed on the top side of the board which means that assembly is mostly an exercise in following the diagrams. When soldering this type of board you should observe normal good soldering practice. Use a temperature controlled iron set to about 360°C with a fine tip (0.8mm chisel is good). Use fine resin-cored solder (0.8mm or less) and don’t hold the iron on a joint for more than three or four seconds. The use of plated-through holes means that you don’t need to install links but it does mean that it is difficult to remove a component after it has been soldered in. So, take care before you apply the soldering iron – check that the component is the right value and orientated correctly. The best way to remove a component from a board with plated through holes is to cut its leads close to the body, then pull the leads out with pliers while applying heat to the solder pad. You can then use de-soldering braid or a solder sucker to remove the excess solder from the pad and hole. 5.3V + – Fig.8: before you solder in the microcontroller and connect the GPS module you should check that these voltages are present. Both have a tolerance of ±150mV. If you cannot measure the IC1 PIC18F4550 correct voltages you should check D1, D2, IC2, IC3 and the power connection. – 1 3.3V + CON5 LP2950 D2 1N4004 1 220nF D2 1N4004 4 JP1 100nF 2 22pF 3 CON3 Fig.6: component overlay for the main PC board. Components marked with an asterisk (*) are optional – their use is explained in the text. The GPS module “plugs into” the header socket, CON5, while the LCD readout module similarly plugs into the longer socket, CON4. 22pF TO GPS MODULE siliconchip.com.au Here’s a photo of the GPS Car Computer to match the overlay at left. In this case, the LDR option is used to control the day/ night function. This an early prototype and the silk screen legend will be slightly different in the final board. If you want to make your own PC board, PDF and (zipped) EPS files can be downloaded from the SILICON CHIP website. However, given the fact that it is a double-sided board AND has plated-through holes, home fabrication is a lot more difficult than standard (single-sided) boards. Gerber format design files for the board, (used by most PCB fabricators) have been uploaded to BatchPCB and you can order it from them by following this link: http://tinyurl. com/ybulgcy. Many other PCB fabricators will also make the board for you and http://opencircuits.com/PCB_Manufacturers lists a large number of these companies. Of course, kit suppliers will include the board as part of the kit. Assembly options Before you start soldering there are a few of assembly options that you need to consider. The first is the control of the backlight brightness. The microcontroller determines if it is day or night by the voltage on pin 40 (RB7). When it is above about 1.4V it is night; less than that it is daytime. By rearranging the components on this input you can use a light dependent resistor (LDR) or the car’s headlight circuit to switch between the night and day brightness levels. The following table lists the components used: LDR Control LDR Install LDR R1 8.2k resistor R2 Leave Empty Headlight Control 47k resistor Leave Empty 82k resistor Note that for headlight control the 47k resistor is installed vertically in the position marked for the LDR. You also need to decide how you will wire up the power and other leads going to the mini DIN plug. If you are in- stalling the GPS Car Computer permanently you should connect the negative to the chassis and the positive power lead to a spare position in the fuse panel (if there is one) with a 500mA fuse. It may be easier to connect to a permanent 12V source and wire via an inline 3AG fuseholder with fuse. Refer to the Features Panel later in this article for the connection of power and other signals to the DIN connector. If you have decided on headlight brightness control you should wire pin 1 of the DIN connector to the vehicle wiring so that the voltage is at 12V when the headlights are turned on. A connection between pin 5 and a fuel injector is also necessary if you want to use the fuel economy feature. Fig.6 shows a typical circuit used in a modern car. You need to locate a negative solenoid lead and connect to that. The crude method would be to pierce the insulation of the wiring at the solenoid in the engine bay. This would expose the wire to corrosion and other dangers. A more elegant and reliable method would be to locate the engine management unit, normally behind the firewall in the passenger compartment and connect to the appropriate lead there. If you need to move the GPS Car Computer around, the best approach would be to wire the power leads to a cigarette lighter style plug with the positive power lead connected to the tip and the negative lead to the side wings. Most of LONG PINS PLASTIC SPACER SHORT PINS SOLDER PINS ON TOP (TOP) LONG PINS ON TOP (BOTTOM) Fig.9: the tactile TACTILE PUSHBUTTON switches must sit SWITCH GAP APPROX 1mm above the PC 1mm board so that the actuating rod will PINS PROTRUDE protrude far enough THROUGH PC BOARD through the front FOR SOLDERING panel. This spacing is achieved by making sure that the switch leads only protrude by a fraction of a millimetre on the underside of the PC board. siliconchip.com.au TRIM OFF PINS LCD MODULE PC BOARD (TOP) (BOTTOM) REMOVE PLASTIC SPACER Fig.10: the LCD has 20 pins which plug into the socket on the main PC board. Insert the long pins of the pin header from the underside of the display so that the plastic spacer is flush with the underside. Solder the pins on the top, lever off the plastic spacer and trim the pins on the top of the LCD board. January 2010  33 This photo shows how to remove the plastic spacer on the underside of the display board using a small screwdriver. You must remove the spacer so that the LCD can snugly mate with the main PC board. The pins on top of the board have already been trimmed. (Refer also to Fig.10). Here’s the partially assembled connector on the LCD. Note that the plastic spacer is on the underside with the long pins running up through the LCD board. The pins have been soldered on top. these connectors have a fuse in the body of the connector but if not, you should insert an in-line fuse holder fitted with a 500mA fuse. Another option for you to consider is jumper JP1, which is used to isolate the USB 5V supply. Normally this is left in place so that you can power the unit from either USB or 12V sources. You cannot use both simultaneously as this would place 5.3V from the GPS Car Computer on the USB 5V line from your computer, possibly causing damage. However, in some cases you might want to permanently wire the unit to 12V but still need to connect it to a comLEFT-HAND END puter via USB. To cover this eventuality JP1 is provided. When the jumper is removed, the USB cannot power the device, thereby removing any possible conflict. Construction Start assembly by soldering the 20-pin socket for the LCD to the PC board. Ensure that it sits fully flush with the PC board before you start soldering. The reason for starting with this socket is that it allows you to temporarily place the LCD display on top of this socket while positioning the other components. RIGHT-HAND END FRONT PANEL (BOTTOM OF BOX) 14 19 C 16 18 29 71 13 B A 32 LCD CUTOUT 71 x 29mm 13 B 16 A 13 B 32 29 15 16 B A D 114 13 17 74 8 15 E A 25 A A HOLE DRILLING DETAILS HOLES A: 3.0mm DIAM. HOLES B: 5.0mm DIAM. HOLE C: 10mm DIAM. HOLE D: 16mm DIAM. HOLE E: 6.0mm DIAM. 36 77 ALL DIMENSIONS IN MILLIMETRES (FOR EXT GPS ANTENNA) NOTE: HOLE B IN LH END IS ONLY NEEDED IF THE LDR IS INSTALLED A REAR PANEL (LID OF BOX) Fig.12: the drill holes and cutout required for the front panel, left and right ends and the box lid (which becomes the base). This diagram is reproduced at 80% – photocopy at 125% so that it can be used as a template. The centre hole in the lefthand end is only necessary if the LDR is installed. The holes in the right-hand end provide ventilation for the 5V regulator – accuracy is not so important in positioning these holes. 34  Silicon Chip siliconchip.com.au PUSHBUTTON CAP Fig.11: this shows how the main PC board and LCD are joined together and mounted in the UB3 jiffy box. It is important that the actuators of the tactile switches protrude far enough that the caps will fit without touching the front panel. It is also important that the LCD bezel is flush with the outside surface of the front panel. LABEL OVERLAPS LCD BEZEL BY 3mm 20mm M3 SCREWS WITH STAR LOCKWASHERS TOP OF LCD BEZEL LEVEL WITH TOP OF FRONT PANEL PUSHBUTTON CAP LCD MODULE BEZEL LCD MODULE PC BOARD 20-WAY SIL SOCKET MAIN PC BOARD 9mm LONG UNTAPPED SPACERS 15mm M3 TAPPED SPACERS M3 NUTS, STAR LOCKWASHERS & FLAT WASHERS BACK PANEL (LID OF UB3 BOX) 10mm M3 MACHINE SCREWS This is important as there is limited clearance (6 to 8mm) between the main PC board and the underside of the LCD display and you need to be sure that the components on the main PC board do not foul the LCD board before you solder them in. This particularly applies to the buzzer and trimpot, both of which tend to have extra plastic on their underside. You should cut this away with a sharp knife and make sure that the body of the component sits flush on the PC board and the top clears the LCD above. Other components that you need to watch out for include the transistor (Q1), the 3.3V regulator (REG2) and the 10F capacitor. For now, continue with the low profile components such as resistors and diodes and then move on to the taller components. At this stage, do not solder in the microcontroller or the push button switches. Note that resistors R3, R4, R5 and R6 are marked on the PC board but not installed as they are reserved for future expansion. Be careful with D1 and D2 as they look alike. The same applies to Q1 (the transistor) and REG1 (the regulator), both are in TO-92 packages and can easily be confused. When soldering in the crystal (X1) ensure that it sits a millimetre or two above the PC board so that there is no danger of the metal case shorting the connecting pads underneath. The LM2940 regulator (REG1) sits on top of a small heatsink as shown in Fig 7. Refer to this diagram for assembly instructions. Before screwing down the regulator check that the heatsink does not touch the solder pads on either side of the heatsink. The spacing is very close here but the heatsink can be slid around to ensure that it has adequate clearance. Once this is correct you can tighten the nut and bolt. Finally solder the regulator’s leads – this is left to last so that the solder joint is not stressed. The PC board has provision for an extra connector (CON3) which also can be seen in photographs of the prototype. Ignore this as it is intended for use with an In Circuit Debugger used only during software development. Quick Test With all components in place (except the microcontroller, switches, GPS and LCD) we can make a quick test to ensure that the voltages are correct. With the power/signal cable plugged into the mini DIN socket and the power turned on you should be able to measure about 5.3V between pins 11 and 12 of the microcontroller. Between pins 2 and 5 of the GPS connector (CON5) you should be able to measure 3.3V. Both could vary by plus or minus 150mV. Refer to Fig.8 for the measurement points. If you cannot measure the correct voltages you should check D1, D2, REG1, Loading New Firmware The GPS Display includes the facility to reprogram itself over the USB, this is sometimes called “flashing”. You don’t need any programming hardware and it uses free software. Full instructions will be included with any updates but the following will give you the flavour of how it works. To place the device into programming mode you hold down the Set button while you plug it into a USB port on your computer. This causes the GPS Display to masquerade as a Microchip PICDEM FS USB board and, as far as your computer is concerned, this is exactly what it is. This small deception allows us to use software developed by Microchip to upload new firmware to their own products – much easier than developing our own. Your computer will then ask for a device driver for the siliconchip.com.au PICDEM board. This is different from the virtual serial port driver used to receive GPS data from the GPS Display and will be included with any updates or can be downloaded from the Microchip web site. When the device driver has finished loading you can run the programming software provided by Microchip for their PICDEM FS USB board. This allows you to load the HEX file and reprogram the device, which takes only 20 seconds or so. You then unplug the USB cable and when you restart your GPS Display it will be running the new firmware. You don’t have to worry about a power failure or accidently unplugging something while it is programming. The programming code will not be affected if something does goes wrong, so you can always restart and try again or reload your original firmware. January 2010  35 REG2 and the power connection. Final PC board assembly Next you should install the microcontroller. This must be first programmed with the file 0510110A. hex (available for download from the SILICON CHIP website) using a normal PIC programmer. When handling the microcontroller and LCD you should take the standard precautions against electro static discharge (ESD) which could zap these devices. This means making sure that your work surface, your soldering iron and you are all grounded. As there is not enough clearance between the PC board and the LCD for an IC socket, the microcontroller must be directly soldered to the PC board. Normally you would need to remove it for reprogramming but as you can upload new firmware by USB, this is not a concern. However, once you have soldered the microcontroller you will not be able to remove it without destroying it. So, please read the next two paragraphs carefully. Pin 1 of the microcontroller is clearly marked on the PC board and is the top right hand pin when the PC board is viewed from the front. This must be matched to the dimple marking pin 1 on the IC’s body. This means that the microcontroller is installed upside down compared to the other components and the silk screen printing on the board. Before you apply the soldering iron, make sure that the part number printed on the chip is inverted compared to the printing on the PC board. We understand that Altronics will be producing a kit with a machine cut front panel which will make construction much easier. This will also allow each button cap to sit within a hole in the front panel and consequently there will be more space between the LCD and PC board. The main advantage of this is that an IC socket (supplied with the kit) can be used for IC1 and you do not need to panic over permanently soldering it in. When you have finished with the microcontroller you can install the three push button switches. These have been left to last because they can be easily damaged if the actuating rod is accidently forced too far to the side during assembly of the board. As illustrated in Fig.9 the body of 36  Silicon Chip GPS Car Computer - Features Summary Automatic Scan Simultaneously pressing both the Up and Down buttons will set auto scan. In this mode the unit will step to the next screen every three seconds. Pressing either Up or Down will terminate auto scan. The data displayed can be configured as described below. Show/Hide Display Screens Holding down the Up button while applying power will enter a special mode to configure what screens will be displayed or hidden. Repeatedly pressing Set will select: • Show (ie, the screen will always be displayed). • Hidden when Auto Scan is operating, otherwise shown. • Hidden Always. This is useful if, for example, the Fuel Economy Meter is not being used – it can then be hidden to reduce the visual clutter. Press Up or Down to move to the next screen to be configured. Remove power to exit this mode. USB 2.0 Interface The GPS Display creates a virtual serial port over USB and you can use this to connect to your laptop to show the GPS output, plot your location on moving maps and other functions using free and paid software. You must load the SILICON CHIP Serial Device Driver on to your PC. Firmware Updates By holding down the Set button when plugging into a USB port the GPS Display will accept firmware updates delivered via USB. Programming hardware is not required. Low Signal In a low signal situation or when first turned on the GPS Display will display a message and a count of the number of satellites found at that time. Pressing the Set button will show the Current Signal Levels screen described on the next page. Full Reset Pressing the Down button for a couple of seconds while applying power will reset the GPS module and all configurable parameters to the factory/design defaults. Automatic Brightness Control The brightness of the LCD backlight can be configured separately for day and night. Control of day/night can be from the vehicle headlights or from a light dependent resistor (LDR). When switching from day and night the brightness is slowly changed over a minute or so to avoid distracting the driver. Power and Signal Connector Pin 3 is ground Pin 4 is +12V power input PIN 5: PIN 6: Pin 5 is the fuel injector input. Connect HEADLIGHTS SPARE to the negative wire of a fuel injector (this PIN 4: is pulled to ground when the injector PIN 3: +12V POWER GROUND solenoid is activated). Pin 1 is the vehicle headlights input. PIN 1: PIN 2: It will control the day/night brightness of FUEL INJECTOR SPARE the backlight. This input is optional and is disabled if a light dependent resistor is used. Pins 2 and 6 are spare input/output lines for future enhancement. In this version they are left unconnected. siliconchip.com.au GPS Car Computer – Readout Summary Digital Clock All data is updated once a second. The currently displayed screen and all settings are saved in non-volatile memory and recalled on power up. Shows the current time in 12 hour (AM/PM) format. The seconds are shown in the bottom right. The time is derived from the GPS signal and is accurate to within 100mS (ie, any error is undetectable to a human observer). Pressing the Set button will allow adjustment of the time in steps of half an hour (ie, this sets the time zone – the exact time is always derived from the GPS satellites). Digital Speedometer with Over-Speed Alarm The digital speedometer will display your speed up to 250km/h. The current setting of the over speed alarm is shown in the bottom right. An audible alarm (two beeps) will sound when this speed is exceeded and the speedo display will switch to reverse video. Built in hysteresis prevents it from continuously beeping if you stay near this speed. The Set button will change the threshold or completely turn it off when set to zero. Fuel Economy Meter The length of the black bar shows the relative fuel consumption for the distance travelled (equivalent to litres per 100km). The longer the bar, the higher the fuel consumption so you would normally adjust your driving to keep the bar as short as possible. The graph is not calibrated but the sensitivity (ie, full scale) can be adjusted by pressing the Set button. This allows you to adjust it to suit different vehicles. Distance and Time to a Destination This display will count down the distance and time (in hours and minutes) to a destination. The time is based on your average speed over the last ten minutes, so if you get onto a slow road the time to your destination will increase accordingly. This display is reasonably accurate; the error is about one kilometre in 100. Press the Set button to setup the initial distance to the destination. Heading, Compass and Altitude The number on the left is the current heading (direction of the vehicle) in degrees. The needle can show either the heading or point to the north – this is configured by pressing the Set button. The numeric reading will always show the heading. Your current altitude in metres is shown on the right of the screen. Latitude and Longitude Your current latitude and longitude. This is shown as degrees, minutes and fraction of a minute and updates continuously as you drive. This is accurate to within a few metres depending on the number of satellites that can be found in the sky. This information (and much more) can be sent to your laptop via USB for use by navigation and mapping software. Current Signal Levels Shows how many satellites should be in the sky and the number that is currently being used by the GPS module. The bar graph shows the signal level of every satellite that can be detected. The module will not use a satellite if its signal level is below a threshold. By pressing the Set button you can adjust the backlight brightness for day and night conditions. Day/Night is determined by a light sensor or your car’s headlights. siliconchip.com.au January 2010  37 the switches must sit 1mm above the PC board. This is necessary so that the actuating rod will protrude far enough through the front panel. This spacing is easily achieved by making sure that the switch leads only protrude by a fraction of a millimetre on the underside of the PC board before soldering. The next assembly step is the connecting pins on the Parts List – GPS Car Computer 1 PC board, code 05101101, 140mm x 57mm 1 EM-408 GPS module manufactured by GlobalSat Technology (available from www.altronics.com.au) 1 SG12232A 122 x 32 dot matrix LCD (Altronics Z7052) 1   20MHz low profile crystal (X1) 1 buzzer – sealed mini PC board type, high output (Altronics S6105) 1   light dependent resistor, 10k to 1M (Altronics Z1621) * 3   tactile switches with 22mm actuating shaft (Altronics S1119) 3 button caps 7.5mm diameter (Altronics S1482) 1 mini DIN socket, 6 pin, PC board mounting 1 mini DIN plug, 6-pin 1   B-type USB socket, PC board mounting 1   2-pin header 1   20-pin header 1   20-way header socket, single row (8mm high socket base) 1   U-style micro heatsink for TO-220 (19 x 19 x 9.5mm) 1   UB3 jiffy box (130 x 67 x 43 mm) 4   15mm metal M3 tapped spacer 4   9mm metal M3 untapped spacer 4   20mm M3 bolt 5   10mm M3 bolt 9   M3 Star washer 4   M3 flat washer 5   M3 nut Semiconductors 1  PIC18F4550-I/P microcontroller (IC1) programmed with 0510110A.hex 1  LM2940CT-5 or LM2938ET-5.0 TO-220 5V voltage regulator (REG1) 1   LP2950CZ-3.3 TO-92 3.3V voltage regulator (REG2) 1   BC338 transistor (Q1) 1   1N4004 diode (D1) 1 1N5819 Schottky diode (D2) Capacitors 1   100F 16V electrolytic (105°C rating) 1   10F 16V tantalum 1   470nF MKT (code 470n or 0.47F) 2  220nF MKT (code 220n or 0.22F) 3   100nF monolithic (code 100n or 0.1F) 2   22pF ceramic (code 22p) Resistors (0.25W 5%) 1 100k 2 82k* 2 47k* 1 22k 1 10k 1 8.2k* 1 6.8k 2 3.3k 1 10 1 10k trimpot, vertical mount front adjust * = Optional – see text 38  Silicon Chip LCD. Start by inserting the long pins of the 20 way pin header strip through the matching holes on the LCD from the bottom. The plastic spacer should be flush on the underside of the display’s PC board and the shorter pins underneath. See Fig 10. Solder the pins on the top of the board while ensuring that the spacer underneath remains flush with the board. Then remove the plastic spacer leaving just the pins. This is best done with a fine screwdriver – lever down one end of the spacer by a few millimetres, then the other end followed by the middle. After repeating this a few times the plastic spacer will slide off the pins. The GPS module is supplied with a cable with identical connectors on each end. Cut off one of the connectors, bare the wires and solder to the pads marked CON5. The grey wire goes to pad 1 which is marked accordingly (the left hand pad when viewing the board from the top). The other wires should be soldered in the same sequence as they emerge from the connector. As a final check, closely inspect the board with a high power magnifying glass. Carefully check every solder joint for blobs, shorts or poor joints. You can now plug the LCD and GPS into the PC board. When you apply 12V power your GPS Car Computer should immediately start by showing the firmware version followed by a message indicating that it is searching for satellites. You will probably have to adjust the 10k trimpot to get an image on the display and then continue adjusting it for a good contrast. Consult the troubleshooting section if you run into problems. The housing The combined PC board and LCD is 123mm wide, 44mm high and about 25mm deep when viewed from the front. This means that it can be mounted in a standard car radio sized cut out in the dashboard. Many vehicles have this and are intended for holding small items. The PC board and LCD will even fit in the smaller radio slot found in newer vehicles. When mounted in this way and with a suitable front panel the GPS Car Computer appears to be part of the vehicle’s instrumentation. In this type of installation you will probably have to either extend the leads to the GPS module so that it can be moved to a more sensitive position in the vehicle or purchase and attach an external antenna. The antenna connector on the GPS module is an MMCX type and you should easily find a suitable antenna on the Internet (just google for “GPS antenna MMCX”). Getting a good signal level is very much dependent on the design of the vehicle and the position of the GPS Car Computer, so experimentation will be the order of the day. We decided to mount our prototype in a standard UB3 “jiffy box” so that it could be portable. The stacked PC board and LCD are mounted on the lid of the box, which becomes the back panel as shown in Fig.11. When the complete assembly is slid into the box the LCD bezel and the shafts of the push buttons will protrude through the base of the box (which becomes the front) as also shown in this diagram, the result is a neat appearance without visible screws. The LCD bezel should protrude through the front panel so that its front face is flush with the box’s outer surface. This means that there will be no need for a window or front glass for the LCD and the front panel label will cover any siliconchip.com.au Fig.13: front panel artwork for the GPS Car Computer, reproduced same size, ready for photocopying as described in the text. This can also be downloaded from siliconchip.com.au GPS CAR COMPUTER SET SILICON CHIP roughness in the cut out. To get this flush appearance you should use the spacers, washers and nuts as illustrated in Fig.11. If your result is not flush (due to tolerances in the hardware) you may need to add or remove some washers. To prepare the box you should drill and cut holes as illustrated in Fig.12. These diagrams are reproduced at 80% so if you photocopy them at 125% they can be used as direct templates for positioning the holes and cutout. If you have purchased the Altronics kit you should follow their instructions as their box will be already prepared and will use a slightly differentRDG_SiliconChip_0110_egx350.pdf sequence of spacers and mounting hardware for 1 1/12/2009 2:58:02 PM the PC board/LCD. To finish the assembly you should copy the front panel design in Fig.13 onto heavy-duty adhesive-backed paper (Avery 936067 is a good choice) and then either hot laminate it or cover the printed surface with adhesive backed clear plastic of the type used to cover books. Then razor cut this paper/plastic sandwich around the edges. When you cut out the centre of the label for the LCD you will notice that it is a little smaller than the LCD bezel. This is deliberate, as shown in Fig.11 the label is designed to overlap the LCD display bezel by a few millimetres. This hides any roughness in the cutout in the front panel and gives a clean, professional finish. Finally, peel off the backing paper from the label and attach to the front panel. C M Y CM MY CY CMY K siliconchip.com.au January 2010  39 The GPS module sits horizontally on top of the PC board and LCD boards with its antenna (the brown/silver assembly on the top) pointing to the sky. This is its most sensitive orientation. To make space for the module you need to use a sharp knife to remove the centre two plastic ribs on the top inside of the box. The module should then sit with its external antenna connector (the gold coloured connector) poking through the hole in the back panel and held in place by the surrounding box and PC board/LCD. If the module rattles around inside the enclosure you will have to attach padding of some type to keep it still. Troubleshooting With only a few active components in this project it should be easy to track down any faults. Firstly, check the two supply voltages as described earlier as nothing much will happen if they are not correct. Next, check the microcontroller. This is best done by measuring the voltage on pin 4 which should be between 1.2V and 1.5V. This voltage is created by the internal voltage reference and implies that the firmware is running, has enabled the reference and configured it for the correct voltage. In other words, it means that the microcontroller is perfectly OK and running its program. In the absence of this voltage, use an oscilloscope to check for a 20MHz sinewave at pins 13 and 14, indicating that the main clock is present. Also check the soldering around the microcontroller for bridges, non soldered joints, etc. If the microcontroller is working the next thing to check is that the pushbuttons are OK. Use a multimeter to check that the switches are open when not pressed and near zero ohms when pressed. If a button is stuck down it will cause the unit to go into one of several special modes when power is applied and that may trick you into thinking that the whole unit is not working. If the actuating arm of a switch is pushed too far to the side this can also cause the switch to close. This can happen if your drilling of the front panel is not accurate. Next is the LCD. First check the voltage on pin 3 of the LCD (Vo), it should be under 0.5V. This voltage is controlled by the 10k trimpot (contrast) and if not correctly adjusted the display will appear blank. The microcontroller only sends data to the LCD and does not expect any response. So, even with the LCD removed or faulty, you should still see signals on the data lines to Choosing a Microcontroller Readers might be interested in the background of why we chose a PIC18F4550 microcontroller for this project. Originally the design started with the Microchip PIC16F877A. This has been around in variou s incarnations for at least 10 yea rs (a long time in semiconductor circles) and has long been the chip of choice when large pin cou nts have been required. How ever, during development we ran into a problem with its limited RAM . Searching through the Mic rochip catalog we found the PIC18F4550. This is a much mo re modern chip which has 204 8 bytes of RAM – plenty for our requirements. As a bonus it also had four times the program me mory, ran four times faster tha n the venerable PIC16F877A and , if that was not enough, it include d a USB 2.0 interface to boot. It had also been used in previou s SILICON CHIP projects so it was not a total stranger. The clincher was when we che cked the Microchip price list. The PIC18F4550 with all its fan cy features was 20% cheaper tha n the plain old PIC16F877A! As an American would say… the decision was a “no brainer ”. the LCD. If the LCD is blank or showing rubbish the only things that you can do is check that it is plugged in correctly and that there are no solder bridges on the connector. With the microcontroller running and the LCD showing the startup message the only other fault would be with the GPS module. The microcontroller will display an error message if the module is not connected or running, so that type of fault should be obvious. The GPS Car Computer could also sit for a long time with the LCD showing a message indicating that it is searching for satellites. This indicates that the GPS module cannot get a signal and moving the unit outside or near a window should correct that. While it is searching you can press the Set button to get the Signal Levels display – this will show you how many satellites can be seen and their signal levels. Assuming all is well, we’re ready to move into some of the more esoteric aspects of operating the GPS Car Computer but alas, space has beaten us so that will have to wait until next month. However, as we said earlier, it really is very simple and quite intuitive to operate, so between now and February you’ll have a good opportunity to play with it yourself and find out many of the good things it can do. We’ve even given you a head start with the various LCD SC readout screens shown earlier! Resistor Colour Codes o o o o o o o o o      No. Value 1 100k 2* 82k 2* 47k 1 22k 1 10k 1* 8.2k 1 6.8k 2 3.3k 1 10 * - Optional, see text 40  Silicon Chip 4-Band Code (1%) brown black yellow brown grey red orange brown yellow violet orange brown red red orange brown brown black orange brown grey red brown blue grey red brown orange orange red brown brown blackblack brown 5-Band Code (1%) brown black black orange brown grey red black red brown yellow violet black red brown red red black red brown brown black black red brown grey red black brown brown blue grey black brown brown orange orange black brown brown brown black black gold brown siliconchip.com.au