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Try your hand at a surface-mount-device project... PICPROBE A versatile PIC-based logic probe that fits inside a Biro case! T HIS PROJECT CAME ABOUT through the recent trend in electronics towards lower operating voltages. If you look around at the latest chips being offered from semiconductor manufacturers you will see that most are designed to operate on 3.3V or lower. Having done a few recent designs with 3.3V circuits, I discovered that my old favourite test tool, the Logic Probe, wouldn’t operate below 5V. I looked around my usual electronic suppliers but couldn’t find anything that would work on anything less than 5V. So I thought I’d build design and one. The first requirement I had was to make it work over as wide an operating voltage as I could so that it could be used on the old legacy 5V systems and down to some of the latest processors at 2.8V. The second requirement was low cost. I took a look inside the existing probes I had, only to discover them to be full of analog components, some of which were now obsolete. 32  Silicon Chip The quickest and easiest approach seemed to be to build something out of a small microcontroller, so I went on the hunt for anything that was small, cheap and worked on a wide supply voltage. Where I ended up was at the Microchip website looking at our old friend, the PIC. One of their latest additions to the ever-expanding family is the 10F20x series which are available in DIP-8, SO-8 or SOT-23-6 packages. The SOT23-6 was my choice as these are tiny and easy to put inside some type of pen as a housing. My next mission was to find a housing for the design. Having built a Logic Pulser from a magazine article many years ago into a white board marker pen, I decided to check out the local stationery shop for ideas. If I could find, from an original by Ross Purdy say, a pen moulded in clear plastic, I wouldn’t need to drill holes to view the LEDs. This would not only make it easier to build but it would look pretty cool as well! I found a 10-pack of ballpoint pens that looked about right and cost only $2.00, making for a very cheap case – including an end cap to protect the pointy bit. The pens were a bit on the small size, allowing for a PC board only about 5mm wide and 100mm long, but it was the height that I was more concerned with. I cut out a dummy piece of circuit board and glued a few bits on and found that the micro and LEDs would fit easily down the barrel of the pen. With the micro and housing sorted out, I next concentrated on the functionality required. First and foremost was a good sharp tip that you can use to probe tiny pitch devices that were/are becoming increasingly common. A sewing needle seemed to fit the bill quite nicely here. siliconchip.com.au This photo shows the first prototype without the extra components added for higher voltage operation or input protection. Don’t forget to keep the pen cap – it can save some nasty stabs! Also note the S1 access hole in the pen body. I also wanted to have a pulse stretching or latching function to view and change very quick pulse transitions so a switch would be required to change modes and clear the pulse latch when required. Modifying the design! You will see from the schematic (Fig.1) that there isn’t much to the design. However, this has some differences to the author’s original circuit and project, with SILICON CHIP making a number of changes. First, SILICON CHIP added provision for a 5V regulator on the PC board, since there would be a lot of hobbyists who might want to use the probe for testing devices with higher voltages. This involved including the pads and tracks for a 5V SMD regulator (78L05, REG1). Due to the miniscule power drawn by the circuit, the regulator should be quite happy working up to its maximum input voltage of 30V. If you only want low-voltage operation, the regulator can be left out and a link added to connect the DC in and DC out pads (where the regulator would be). The regulator input and output filter capacitor can remain – they won’t do any harm and may even do a bit of good in decoupling a supply. We’ve specified 100nF capacitors because we have found these are the easiest to get in SMD and in small quantities. But there would be some benefit if one of the two “downstream” capacitors (ie, between the siliconchip.com.au regulator output and ground) could be larger – in fact, as large as you can get in SMD. The second change was in the input circuitry. The PIC only has six pins, two of which are the power supply. GP3, the probe input, can withstand a maximum of 13.5V. In the vast majority of circumstances this would be more than adequate but once again, we’ve “gilded the lily” somewhat by adding a pair of diodes across the input (one each to the positive supply and ground) along with a series resistor. This protects the input from accidental higher voltages and for the price is a worthwhile addition. This is very handy in case you touch something at a higher potential than the power supply. If you don’t need this protection, the diodes can simply be omitted. The 4.7kW resistor could be retained, or replaced by a wire link if you wish. It won’t matter either way. Circuit details Surface-mount LEDs, so tiny that they are almost impossible to find if you drop them on a carpeted floor REG1* 78L05 OUT +2.8 -5V OR +6 -15V* GND 100nF 100nF IN 180Ω 180Ω 100nF 180Ω A LED1 A LED2 A LED3 λ TS4148* K 5 Vdd D1 4.7k* 10k A 6 PROBE GP2 IC1 PIC10F200 GP3/VPP GP1 K * ONLY REQUIRED FOR HIGHER VOLTAGE OPERATION -- SEE TEXT K K C 4 B E 3 IC1 = PIC10F200 OR PIC10F202 Q1 MMUN2211 1 NOTE: ALL DEVICES IN THIS PROJECT ARE SURFACE-MOUNT Vss D2 TS4148* GP0 λ λ K A 2 S1 0V SC  2007 PICPROBE LEDS (UNDER SIDE) A K 78L05 TS4148 6 IN GND NC A K PIC10F20X OUT GND NC 1 5 MMUN2211 4 C B 2 3 E Fig.1: the circuit can be built in two versions – that shown here, suitable for general purpose work or without REG1, suitable only for low-voltage work. October 2007  33 The SMD LEDs are really bright, especially in normal lighting. This photo clearly shows them glowing even though they have been “swamped” by the very bright photo flash we used for the photo. Parts List – PICPROBE 1 PC board – see text 1 clear plastic ballpoint pen case with top and cap 1 35-40mm long darning needle 1 500mm length thin figure-8 cable, red & black 1 small red alligator clip 1 small black alligator clip 1 ultra-miniature (SMD) momentary action pushbutton switch Semiconductors 1 PIC10F200 or PIC10F202 SMD microcontroller programmed with PicProbe.hex 1 MMUN2211 SMD NPN resistor-equipped transistor (Q1) 1 red SMD LED (LED1) 1 green SMD LED (LED2) 1 orange SMD LED (LED3) 2 TS4148 SMD diodes (D1-D2) 1 5V SMD positive regulator (see text) (REG1) Capacitors 3 100nF SMD Resistors (all 603SMD) 1 10kW 1 4.7kW 3 180W 34  Silicon Chip (trust us!), are directly driven from the PIC’s GP2 (red LED) and GP1 (orange LED) outputs. The green LED is driven by the inverse of GP2, using transistor Q1. Even though Q1 is shown on the circuit as a standard NPN type, it’s a bit more complicated than that. It is actually a “Resistor Equipped Transistor” which has two internal resistors: a series resistor to the base and a pull-down resistor to the emitter. These “RET” devices are great for use as digital inverters. GP0 is normally held high by a 10kW resistor connected to the positive supply. It’s also connected to a pushbutton switch which grounds the input when pressed. Which PIC? The circuit shows a PIC10F200 as the microcontroller but you can also use a PIC10 F202. The program was originally written for the 200, which has 256 bytes of program, 16 bytes of RAM, and one 8-bit timer. Note that neither the PIC10F204 nor PIC10F206 will work in this circuit – you must use the 200 or the 202. Operation The probe has three LEDs and a push button. The Red LED is turned on for a logic 1 at the probe tip while the Green LED turns on for a logic 0. The Orange LED works in one of two modes – pulse stretch or latched. In pulse stretch mode, the orange LED will pulse for 50ms every time there is a change on the probe input. This makes very small pulses at the probe tip viewable. If the orange LED stays on permanently in this mode then the probe tip is changing at a rate greater than 50ms. In latched mode, the orange LED will turn on and stay on with any change on the probe tip. This is handy for detecting very infrequent changes. The latch is cleared and the LED turned off when the button is pressed. The pushbutton has three functions: (1) changing orange LED mode, (2) clearing the latch and (3) enabling a pull-up resistor on the probe tip. To change modes you press and hold the button for two seconds. After two seconds the orange LED with blink to indicate the mode is about to change. When the button is released, the mode is toggled. In latch mode, a single press of the button will immediately reset the latch. If the button is pressed when power is first applied, a pull-up resistor on siliconchip.com.au siliconchip.com.au 0V +V PICPROBE + JAYCAR 100nF JE 100nF TWICE FULL SIZE (FOR CLARITY) REG1 MC785L05 ACTUAL SIZE OF PC BOARD JAYCAR PICPROBE + ALL COMPONENTS ARE SMD AND ON COPPER SIDE OF PC BOARD A 180 K 180 20071129 A S1 10k GND LED3 LED2 JE 20071129 A, B, C AND GND ARE USED FOR IN-CIRCUIT PROGRAMMING D2 KC5457 EC8257 EC8257 SOLDER 4.7k D1 C TS4148 x2 IC1 B A 100nF 180 K LED1 MMUN2211 C K A Q1 B E DARNING NEEDLE (~35-40mm) Fig.2: install the parts on the Jaycar PC board as shown in this twice-size overlay. Note that this assembly differs slightly from the accompanying photo which shows the author’s prototype (ie, no regulator or input protection diodes for working at higher voltages). KC5457 Internally, you don’t get much room to play with in this tiny PIC. Because the device is so small and the task relatively simple, the software was written in PIC assembler using the PIC IDE 7.5 tool kit, which is available free from www.microchip.com. The IDE gives you an editor and assembler and is quite easy to learn. As this micro has no interrupts and very little resources it doesn’t take long to master but as I found out, there are a few traps for young players. The first thing to master is the internal oscillator and its calibration, if required. When the device is manufactured, it has a MOVLW instruction loaded into the last byte of the memory. On power reset, the micro starts at the last address and executes the MOVLW instruction. This loads a calibration value into the “W” register and is factory set. The program counter then rolls around to 0 and starts executing the user’s code. The problem comes when you erase the device and lose the MOVLW instruction. If you want a 4MHz calibrated oscillator you need to read the last byte and write it down then manually put it back in. All this seemed unnecessary for my application as I wanted it to run as fast as possible. As the first instruction I loaded “W” with 0x7E which makes the oscillator run at its fastest speed. The next item to master is the internal timer. This is a bit tricky as the micro has no interrupts to trigger asynchronous events. The timer is freerunning and you can only read the timer register and compare it with a constant. Any write to the timer will clear it and start timing again so you can’t use any read-modify-write instruction. This was a trap I fell into. I have run the timer at 50ms per overflow (counts from 0x3d to 0 in 50ms then is reloaded OUCH! The software with 0x3d). If you check and branch when the timer is zero you can have a routine which is executed at a regular period for timing tasks. The program begins by setting the oscillator configuration, port pin configuration (inputs or outputs), starting the timer, and resetting the LEDs. As the processor has no interrupts the only way to monitor the probe tip is to poll it. This is done in the main loop and the smaller the main loop (or the quicker it executes) the smaller the pulse transition that can be detected. This is one limitation of the design but in practice it doesn’t appear to be a problem. The main loop moves the state of the probe input to the red/green output, checks the status of the mode change flag and looks for the timer to reach zero. Every 50ms, the time function is called. The job of the time function is to check that the button has been held down for two seconds and to update the orange LED in either pulse or latch modes. First we will look at the button down timer. To do this we have a variable called CNT0 which is preloaded with 40. Every time the time function is called we decrement CNT0 if the button is pressed. If it is not pressed, we reset CNT0 back to 40. The only way CNT0 can make it to 0 is if we have 40 consecutive calls to time with the button pressed (40 x .05 = 2 seconds). When CNT0 reaches 0 we set a flag (BDOWN) to signal to the main loop that the mode change function needs executing. The orange LED is handled with different pieces of code depending on the mode set. The flag LATCH determines the mode. Every time the red/green LED changes state we set a flag (CHANGE). This flag is read by the time routine. In pulse-stretch mode, the orange LED is turned on when CHANGE is set and then CHANGE is cleared. If CHANGE is not set, the orange LED is turned off. This means that the minimum time that the orange LED is on will be 50ms which is more than enough for your eye to see. PIC the probe tip is enabled. Normally the pull-up is disabled, which makes the input impedance very high. In this configuration the LEDs will flash randomly until the probe is connected to the target test point. This is very useful for tracking down floating circuits on the target under test. If this is not an issue, then enable the pull-up and the tip will go to a “soft” logic 1. The only way to reset the pullup is to re-power the probe. October 2007  35 Where Do You Get It? PROGRAMMING THE PIC CHIP If you’re not building the PICPROBE from a kit, you must first program the 10F20x micro with the file PicProbe.hex, available from the SILICON CHIP website – www.siliconchip.com.au Since the micro is a surfacemount device, programming it presents added complications. 6 1 It must be done in circuit 2 but before the board is fully 7 3 populated. This section explains 8 how to do this. 4 9 You need both a VPP voltage 5 source of around +13V and a normal +5V supply. If you RS-232 SERIAL have decided to use the 78L05 PORT regulator, you can derive the 5V supply from that. If you have chosen to bypass the regulator, you will need to apply +5V to pin 5 of the micro and 0V to pin 2. The micro must first be soldered in place, making sure that the orientation is correct. If you are using the regulator, solder that in too, then solder both the positive and negative supply leads to the board. Special pads to access pins 1, 3 & 6 of the PIC have been provided on the board specifically for programming. These are labelled, respectively, “A”, “B” and “C” on the component overlay. The pad labelled “GND” can be connected to the external programming circuit shown above. You may solder wires to these pads for the programming phase and later, when the micro has been successfully programmed, remove these wires. Back-up pads for the links required in normal operation have also been provided on the PC board. The type of programmer we recommend is the “COM84” style programmer whose schematic appears above. A computer’s serial port will be required and the software to use is WinPic, available free to download from www.hamradioindia.org/circuits/ winpic.php We used the WinPic version compiled 9th December 2005 but other versions should be similar. After soldering the wires to the A, B and C pads, you should breadboard this circuit. The two BC546 NPN transistors are used to switch on and off the higher programming voltage, which for normal programming 36  Silicon Chip +VPP +5V 2.2k +5V 10k BC546 BC546 2.2k 2.2k 22k 2.2k PIC 10F20x “COM84” COMPATIBLE PROGRAMMER VPP APPROX. +13V 6 (”C”) 3 (”B”) 1 (”A”) 5 PIC 10F20x 2 Both Jaycar Electronics (www. jaycar.com.au) and Altronics (www. altronics.com.au) sell a kit of parts for the PICPROBE. Jaycar’s kit includes a doublesided PC board with plated-through holes and all parts, including a preprogrammed micro but not the pen or the needle. It retails for $14.95 (Cat. KC-5457). The Altronics kit is similar with a preprogrammmed micro and no pen nor needle. It also sells for $14.95 (Cat. K-2587). PIC TO BE PROGRAMMED (ON PICPROBE PC BOARD should be between 12.5V and 13.5V at pin 6. Adjust your input VPP voltage level to within this range. There will be a small voltage drop across the 10kW resistor in series between VPP and the collector of the BC546/pin 6. When the Tx line (pin 3) of the serial port is low, the voltage at pin 6 of the PIC10F20x should be around 0V. When it is high, it should be between 12.5V and 13.5V. The WinPIC software will automatically switch this voltage on or off as required. To access the serial port, we used a serial cable with an IDC 10-pin header attached, as in the photograph below. Once you are satisfied that the circuit is working correctly, you may connect the serial cable to your computer’s COM1 port. Now you should run the WinPic pro­ grammer. You must first select the “COM84” programmer for the serial port in the “Interface” tab. While you are there, check that the interface is working correctly by clicking on the “Initialize!” button. If everything is working OK, you should get the message “Interface tested OK”. If not, double check your wiring. Now go to Device -> Select . . . and select the PIC10F20x as your device. You now should be able to erase, program and read the micro. To load the firmware, go to File -> Load and select the PicProbe. hex file. Then choose Device -> Program to program the micro. If this worked, go to Verify to check that the firmware has been programmed correctly. Latch mode is similar, in that when CHANGE is set the orange LED is turned on but is not cleared until the button is pressed. This is detected using the BPRESS flag. Mode changing uses a separate function labelled “cngmode”. When this function is called it will blink the orange LED using simple delay loops until the button is released. When the button is released, the LATCH flag is inverted and the routine exits back to the main loop. Construction Basically, all the parts are installed on a double-sided PC board – see Fig.2. This board measures just 106 x 5mm and should be a relatively snug fit inside the pen case. Don’t push it all the way in to check, though – you may not be able to get it back out again. Note that the PC board shown in Fig.2 is available only as part of a kit from Jaycar Electronics. Altronics also sell a kit for the PICPROBE, using their own version of the PC board (the assembly instructions are with the kit). Note that because you’ll be building the PICPROBE from a kit, the PIC micro will be supplied ready programmed. You need to decide if you want to use your logic probe for low-voltage work only (as in the original design) or for general purpose, higher voltage work. If it is for low-voltage work only (ie, 5V or less), you can leave out the voltage regulator and place a link between its input and output positions. The first step in the assembly is to carefully solder the SMD devices to the PC board but don’t install the PIC just yet. To install these parts, you will need a soldering iron with a fine pointed tip siliconchip.com.au and a magnifying lamp. A pair of self-closing tweezers can be used to hold each device in position as it is soldered. Once these SMD parts are in, solder on the probe tip, the switch and the external connection wires. As mentioned earlier, the tip is a sewing needle. These are often nickel-plated, which makes soldering a bit difficult. Test it first – if it is difficult (or impossible) to get solder to take, you may need to file off a small section of the nickel plating. The size of the “probe” is up to you – and the type of work you’ll be doing. We’d be Rigol DS5062MA 60MHz Rigol DS5102MA 100MHz inclined to use a small darning 60MHz Bandwidth 100MHz Bandwidth needle, as these tend to have 1GS/s Real Time Sampling 1GS/s Real Time Sampling less of a point (so you won’t 2 Channels 2 Channels get stabbed!) but are still fine Mono LCD Display Mono LCD Display enough for the vast majority 4K Memory Per Channel 4K Memory Per Channel of work. 20 Automatic Measurements 20 Automatic Measurements The needle we used was Advanced Triggering on Edge, Advanced Triggering on Edge, about 35mm long and so far, Video & Pulse Video & Pulse still hasn’t been missed from Built-in FFT Built-in FFT the sewing box (;-). Built-in USB Built-in USB Don’t forget that the power 3 Year Warranty 3 Year Warranty wires (polarised figure-8 cable) need to pass through the pen top cap so it is wise to ONLY $ ONLY $ ex GST ex GST do this now, rather than later. You’ll need to drill a hole in SAVE $300 SAVE $200 the end of the cap to accom* Offer valid until 30th June 2007 or while stocks last. modate the wires. The last component to be fitted should be the PIC chip, Melbourne Brisbane Adelaide Perth Sydney Tel 03 9889 0427 Tel 07 3275 2183 Tel 08 8260 8166 Tel 08 9361 4200 as this allows you to check the Tel 02 9519 3933 Fax 03 9889 0715 Fax 07 3275 2196 Fax 08 8260 8170 Fax 08 9361 4300 Fax 02 9550 1378 LED operation before putting email testinst<at>emona.com.au web www.emona.com.au the PIC chip in. To do this, connect power and in turn short the cathode of each LED to ground (0V). Each should light in turn (you won’t do any harm to Q1 doing this). Next, remove power, wait a few of course equates to a logic high and As you do this, also check that the minutes and then fit the PIC chip to the logic low). colours are correct: red towards the board, taking care with its orientation. Assembly is now complete – all you probe, orange in the middle and green That done, apply power again – the have to do is drill a 2mm hole in the towards the switch. If your LEDs light, LEDs should be flashing in an apparent pen case as shown in the photo to acit’s a pretty good bet that you haven’t random fashion but only one should be cess S1, then slide the completed PC made any mistakes or shorted out any lit when you touch the probe tip to the board into the case until the switch is SC SMD pins. positive supply and then to 0V (which right under the hole. RIGOL SCOPES SALE ... SAVE UP TO $300 1,099 799 EMONA Issues Getting Dog-Eared? Keep your copies safe with these handy binders Available Aust, only. Price: $A13.95 plus $7 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au REAL VALUE AT $13.95 PLUS P & P October 2007  37