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Pt.1: By GEOFF GRAHAM The COLOUR MAXIMITE A PIC32-Based Microcomputer With Colour Output & Stereo Audio The original Maximite microcomputer (March-May 2011) has been a huge hit with readers and thousands are now in use all over the world, from Australia to Russia. So how do you improve on such a big hit? How about by adding eight vibrant colours and a stereo music synthesiser for a start? The result is stunning and it takes the Maximite to a whole new level. T HE MAXIMITE is a small, self-contained computer that is ideal for experimenting, learning programming and for use as an embedded controller. It uses a standard VGA monitor for display, a standard PS2 keyboard for input and an SD card for storage. It includes its own operating system and BASIC programming language (called MMBasic), so you do not need to tether it to a larger computer. It also starts up instantly, so you can just plug it in and immediately get going by entering commands and a program. The new Colour Maximite has 40 input/output (I/O) lines which can be 22  Silicon Chip independently configured as analog inputs, digital inputs or digital outputs. You can measure voltages and frequencies, detect switch closures, and so on, and get it to respond by turning on lights, closing relays, etc – all under control of your BASIC program. Perhaps the most amazing thing about the Colour Maximite is that, like its predecessor, everything (VGA, USB, colour, music, etc) is generated by a single chip that costs less than $10. For people who are familiar with the original Maximite, the Colour Maximite has a similar component count so the cost of building the colour version should be about the same. It’s also reasonably easy to build; just an hour or two is all that’s needed. Suggested uses What would you want to use the Colour Maximite for? Well, for a start, you could simply use it as a small computer that will remind you of the early days of personal computing, when computers were less complicated and much easier to use. The Colour Maximite allows you test your programming skills and have fun creating small programs that can do anything from siliconchip.com.au Now with 100-pin micro & Arduino conne ctor Features & Specifications • • Self-contained high-performance microcomputer. Microchip 32-bit processor running at 80MHz with 512KB flash memory and    128KB RAM. • Colour VGA output, 480 x 432 pixels with eight colours (black, red,    green, blue, cyan, yellow, purple and white). • • • Standard PS/2-style keyboard input. Stereo audio synthesiser for music and sound effects. Battery backed real-time clock (optional). • 20 external I/O lines which can be configured as analog inputs, digital     inputs/outputs, frequency measurement, etc. • Arduino compatible connector with an additional 20 I/O lines that     are independent of the original 20 I/Os. • • SD card support – up to 32GB for storing programs and files. USB port for connecting to a personal computer (Windows, Mac or Linux)     as a terminal or for file transfer. • • • Extensive communications protocols including serial, I2C, SPI and 1-wire. 2-channel audio/analog/PWM analog output. Graphic capability includes selectable fonts, user designed fonts, drawing     lines, circles, squares and control over any pixel with any colour. • Special commands for animated games. BLIT will copy a block of the     video screen at high speed and SPRITE will create animated sprites     that can be moved on the screen without disturbing the background. • • • • Firmware upgrades via USB. Full-featured BASIC interpreter and operating system. Instant on (ready for you to type in and test programs). Powered from USB, 9V plugpack or battery. Note: new features for the Colour Maximite shown in bold. discovering prime numbers to hunting the Wumpus. It will take you back to the days when the Tandy TRS-80 and Commodore 64 were hi-tech. A second use for the Colour Maximite is in controlling the outside world. As stated, with its 40 I/O pins, you can use the Colour Maximite as an embedded controller to monitor voltages and signals and then respond by operating motors, turning on lights or driving an LCD. The Arduino connector makes it easy to add your own customised circuits or relay drivers to interface to the real world. There are already many Maximites out there that are used by readers to monitor GPS receivers, control windmill power systems and many other tasks both complex and simple. With double the number of I/O pins compared to its predecessor, the Colour Maximite can be used for even more complex projects. The Colour Maximite also supports a wide range of communications protocols – serial, I2C, SPI and 1-wire. siliconchip.com.au These are mostly used for communicating between semiconductor chips so, using the Colour Maximite, you can easily measure temperatures, log data or interface with a test instrument. The Colour Maximite should also encourage another group of readers who enjoy writing and playing animated games. Colour is especially useful for games and the Colour Maximite has a special high-speed colour mode designed for that use. Version 4.0 of MMBasic also has some special commands (BLIT and SPRITE) that are designed to move video images around the screen – just what you need for an animated game. Colour, colour The technique for producing colour was developed by Dr Kilian Singer, a Maximite fan at the University of Mainz in Germany. He built a prototype Maximite on a breadboard, started experimenting with it . . . and came up with colour. The technique he used is deceptive- ly simple. The monochrome Maximite generates its image by tying the three colour inputs on the monitor together and drives them with a single SPI channel (we will delve more into that technique later). To generate colour, we just use three separate SPI outputs to independently drive the three colour inputs to the monitor. Sounds simple – so why didn’t we do that in the first place and give the original Maximite colour? The answer is that the original Maximite did not have the speed or memory capacity. But with successive releases of the Maximite firmware, we have optimised its speed and memory usage to the point where this technique is now viable. It’s not a completely free lunch though. Generating colour uses more of the processor’s time so, with eight colours, programs will run about 25% slower and there’s less free memory than when generating a single colour. Most readers won’t notice this so it is a worthwhile trade-off. And in any September 2012  23 but you need to be aware that many require a specialised driver to be included in your program (which must be written in the C language). For this reason, we recommend that the Colour Maximite’s Arduino connector be used with simple prototyping boards, relay boards and others that do not require special software to drive them and can work with 3.3V. There are plenty of these out there and they are very cheap. With the Arduino-compatible con­nector, you can construct special circuits on a prototyping board and permanently add them to your Colour Maximite. This feature can be used for adding signal amplifiers, relay drivers and many other simple applications. Stereo audio output The Colour Maximite interfaces to a VGA monitor and a standard PS/2 key­ board. You can see how small the unit is compared to these external parts. The monitor is shown here displaying the start-up screen, with MMBasic loaded and ready to run programs. case, you can easily switch into monochrome mode within your programs and get the speed and memory of the monochrome version if required. ger (14 x 14mm) but we have plenty of space on the PCB so that’s not a problem. New PIC32 chip With the 100-pin chip, you also get a lot more I/O pins so we decided to put them to good use. First, we decided to add an Arduinocompatible connector. This provides an additional 20 I/O pins that can be controlled from within MMBasic and are independent of the standard 20 Maximite-compatible I/O pins on the back panel. The Arduino connector is mounted on the top of the PCB and is primarily intended for use with Arduinocompatible prototyping boards which can be purchased cheaply from many retailers and on the internet. These boards are called “shields” in “Arduino speak” and can be stacked on top of each other. There are many Arduino shields available (we covered the Arduino world in the January 2012 issue) One of the issues with generating colour is that it requires three separate SPI outputs. We also need an additional SPI output for the SD card, so that’s four in total. The 64-pin PIC32 chip used in the original Maximite only has three SPI devices so, for the Colour Maximite, we needed to move to the 100-pin variant which has four SPI outputs. Most 100-pin chips have fine leads with a very small gap between them and that makes it extremely hard to hand solder the chip to a PCB. That’s the reason we didn’t use the 100-pin chip in the original Maximite design (the cost is similar). Fortunately, Microchip now produce a 100-pin version of the PIC32 with the same pin spacing as the 64pin chip. The chip’s package is big24  Silicon Chip Arduino connector Another new feature in the Colour Maximite is a stereo audio output. This can be used to play music, generate tones and create program-controlled analog voltages. The PLAYMOD command will start a music synthesiser which is built into the latest version of MMBasic. This command will read a sequence of instructions from a file in the MOD format and generate high-quality stereo music which will play in the background. You can use this feature to liven up games, add sound effects to your programs and provide some interesting warning signals. The TONE command also uses the stereo audio output and will generate a sinewave at the output with selectable frequencies for the left and right channels. This is intended for generating attention-catching sounds but because the frequency is very accurate, it can be also be used in many other applications, eg, creating DTMF tones or testing the frequency response of loudspeakers. Another new command in MMBasic is the PWM command which uses the audio output connector to deliver analog voltage signals. There are two outputs and these can be set to any voltage from 0-3.3V under program control. They could be used to drive a voltage-controlled power supply, a motor controller or anything else that needs an analog voltage input. Battery-backed clock Yet another new feature in the Colour Maximite is the provision of a siliconchip.com.au battery-backed clock. This runs independently of the PIC32 and will keep the correct time when you remove power from the computer. Not all readers need this feature, so we have made it optional – refer to the accompanying panel “Adding The Battery-Backed Clock Option” for details. BASIC language A great feature of the Maximite is that it comes complete with its own BASIC programming language, called MMBasic. This language is generally compatible with Microsoft BASIC and allows you to type your program directly into the Maximite. You can then test and run your program while getting instant feedback. This is a full-featured implementation of the BASIC language and it supports floating point, arrays with multiple dimensions, long variable names and many other powerful features. A key feature of MMBasic is its support for the SD-card interface in the Colour Maximite. You can store programs and data on the card and because the Colour Maximite uses the standard FAT16 or FAT32 file system, you can pop the card into your desktop computer to access and edit data created by your programs. The most important benefit of BASIC is that it is very easy to learn and use. This makes it ideal for people who would like an easy introduction to programming and want to just play around without being forced into a complicated programming environment. Since its introduction with the original Maximite, MMBasic has been continuously improved. It now includes a full-screen editor and your programs do not need to use line numbers. You can also use modern programming structures like subroutines and functions with parameter lists to make your programs more readable and easier to maintain. With the Colour Maximite, we have introduced version 4.0 of MMBasic. The most obvious addition to this version is support for colour and there’s also support for a stereo synthesiser. Other additions include two commands especially suited for programming games. These commands are (1) BLIT which will rapidly copy an area of the video output to another location siliconchip.com.au Colour Maximite: Parts List 1 PCB, code 07109121, 130 x 102mm 1 plastic case, 140 x 110 x 35mm (Jaycar HB-5970, Altronics H0472, Element14 1526699) 1 SD memory card connector (Hirose DM1A, Element14 1764372, 4UCON 19607) 1 USB Type-B socket, PCB-mount (Jaycar PS-0920, Altronics P1307, Element14 1696537) 1 DE-15 (or HD-15) high-density 15-pin female D connector (AMP 1-1734530-1, MULTICOMP SPC15430, Element14 1557991 or 1564252) 1 8MHz crystal (HC-49 low profile) 1 IDC 26-pin boxed header, 90° PCB-mount 1 6-pin mini DIN female connector socket, PCB-mount 1 2.1mm DC power socket, PCBmount 1 3.5mm stereo phono socket, panel-mount 1 micro-tactile pushbutton switch 2 3-pin SIL headers, 0.1-inch (male) 2 shorting blocks 1 3-pin polarised header plug, 0.1-inch, PCB-mount 1 3-pin polarised header socket 2 8-pin header sockets (Jaycar HM3207) 2 6-pin header sockets (Jaycar HM3207) 1 type 6073 mini TO-220 heatsink (Jaycar HH8502, Altronics H0630) 4 No.4 x 9mm self-tapping screws (to secure PCB) 1 M3 x 6mm machine screw 1 M3 nut 1 M3 star washer 1 100mm-length 3-way ribbon cable Semiconductors 1 PIC32MX695F512L-80I/PF or PIC32MX795F512L-80I/PF microcontroller (IC1) (Microchip Direct, Element14 1778489 or 1778491) on the screen; and (2) SPRITE which enables you to create animated graphics that can move across the screen without disturbing the underlying graphics. 1 7805 5V voltage regulator, TO220 package (REG1) 1 TC1262-3.3VAB 3.3V voltage regulator, T0-220 package (REG2) (Microchip Direct, Element14 1852182) 1 1N4004 silicon diode (D1) 2 1N5819 Schottky diodes (D2-D3) 3 1N4148 silicon diodes (D4-D6) 1 green 3mm LED (LED1) 1 orange 3mm LED (LED2) Capacitors 1 10µF 6.3V ceramic, SMD 0805 package (Element14 1844286) 2 10µF 25V tantalum 1 330nF MKT 7 100nF monolithic ceramic 2 47nF MKT or polyester 2 22pF ceramic Resistors (0.25W 5%) 1 10kΩ 2 47Ω 2 4.7kΩ 1 10Ω 5 1kΩ 1 2.2Ω 3 120Ω Parts For Battery-Backed Clock 1 coin cell holder (see futurlec.com, Element14 2064715) 1 CR2032 lithium cell (coin type) or 1 x CR2032 PCB-mount lithium cell (Jaycar CR-2032VC, Element14 189267001) 1 32.768kHz watch crystal Semiconductors 1 DS1307 real time clock (RTC), 8-pin PDIP package ( see futurlec.com, Element14 1188042) Capacitors 1 100nF ceramic or MKT Resistors (0.25W, 5%) 2 10kΩ Note: a PCB (with screen-printing & solder mask) and a programmed PIC32 micro are available from the SILICON CHIP Partshop. Version 4.0 of MMBasic is not exclusive to the Colour Maximite. There is also a version for the original Maximite and it is worth updating to it if you are running an earlier version. Of September 2012  25 Left: inside the Colour Maximite’s case. There’s not a lot to it as all the “smarts” are inside the 100-pin PIC32 chip in the centre of the PCB. The unit has a USB interface and supports SD cards with capacities up to 32GB for storing programs and files. course, you will not get colour output but the other features will all work, including the music synthesiser (with mono output). How it works Take a look now at Fig.1 which shows the complete circuit diagram of the Colour Maximite. It’s really quite simple and basically consists of a PIC32 chip with a lot of connectors attached. The most complicated part of the Colour Maximite is the VGA driver and that only uses one resistor and one diode for each colour. VGA colour video consists of separate red, green and blue signals. If you mix red and blue you get purple, while mixing red and green gives yellow and so on. If you count black and white as colours, this means that we can get eight colours just by switching the red, green and blue signals either fully on or fully off. In the Colour Maximite, each colour uses an identical scheme to generate the video so we’ll just look at one colour channel to see how it’s done. Fig.2 shows the basic details. Within MMBasic, the video data is stored as a bitmap in general RAM. This means that when something is to be displayed, the firmware must convert it into individual pixels before writing these to the video buffer. This applies regardless as to whether it is a character or a graphic symbol. The process of turning this bitmap 26  Silicon Chip into a video stream starts with the horizontal sync pulse. This is created by a timer within the PIC32 that is configured to generate a 3.813μs horizontal sync pulse every 31.778μs (about 31kHz). Once configured, this timer runs continuously without further intervention. An interrupt is triggered on the leading edge of each horizontal sync pulse, which causes the processor to stop whatever it is doing at the time and run a special section of its program. In this routine, the processor calculates the memory address of the next line of pixels and instructs the DMA (Direct Memory Access) system to copy that part of memory to the SPI (Serial Peripheral Interface). The SPI is a circuit in the PIC32 that takes a byte of data and outputs it one bit at a time with specific timing. It is this stream of 1s and 0s that forms the video signal. A logic 1 (or high voltage) is an illuminated pixel while a logic 0 is a pixel that is off. The resistor and diode at the output clamp the video signal to 0.7V to conform to the VGA standard. As the scan line progresses across the screen, the DMA sequentially reads the data in memory and pushes it to the SPI device which in turn sends the data bit by bit to its output. The beauty of this scheme is that, other than setting up the DMA and SPI at the beginning of the scan line, the processor is free to undertake other duties (like running your BASIC pro- gram) while the video is generated in the background. Framing input There’s just one problem with this scheme. Because of the way the CPU works, you cannot precisely guarantee when it will start the video stream – it may be a little earlier or later than the previous scan line. While these differences are small (only a few tens of nanoseconds), they will show up as a jitter in the video output. To eliminate this, we take the horizontal sync pulse generated by the timer and feed it back to the SPI as a “framing input”. This can be seen in Fig.1 where pins 69, 47 & 14 are the framing inputs (one for each colour) and 77 is the horizontal sync output. Framing is a special feature of the PIC32’s SPI and it forces the SPI to wait for the trailing edge of the sync pulse before starting the data stream. The benefit of this feature is that the video stream will always be precisely Fig.1 (right): the circuit is based on a PIC32 microcontroller. It manages everything, including the colour VGA, keyboard, SD card and I/O and it does this while running a BASIC program at high speed. Regulators REG1 & REG2 respectively produce a +5V rail for the keyboard and a +3.3V rail to power the PIC32 and the SD card. Note: this diagram does not include the battery-backed clock option which is shown separately in Fig.3. siliconchip.com.au D1 1N4004 EXT POWER A REG1 7805 D2 1N5819 K PWR SW OR LINK REG2 TC1262-3.3 GND 10 F TANT REG1, REG2 +3.3V OUT IN J1 GND 330nF CON1 OUT IN K A 6x 100nF 10 F TANT +5V GND 10 VIN D3 1N5819 POWER 30 85 A 55 Vdd RE0 Vcap RG8 10 F USB TYPE B 1k 1 4 56 3 57 CON2 73 BOOTLOAD SOUND OR PWM OUT 4.7k L/#1 78 76 1k CON9 Vbus RC14 D– RC13 84 3 4 2 63 KYBD DATA 1 X1 8MHz CON4 (FRONT VIEW) +3.3V +5V 22pF 64 22pF RF13 RD13 RF4 RD7 OSC1 RD6 26 24 22 21 PIN 1 20 19 18 17 16 15 14 13 PIN PIN PIN PIN 2 3 4 5 21 22 23 10 9 8 7 PIN 7 PIN 8 42 PIN 9 PIN 10 44 6 5 4 3 2 1 CON6 PIN 11 PIN 12 PIN 13 PIN PIN PIN PIN PIN 14 15 16 17 18 PIN 19 PIN 20 SCL SDA MMBASIC PIN NUMBERS 41 43 18 67 RB5 PGED2 RB4 RB3 PGEC2 6 RB9 RB8 RB12 RB13 RD8 RB14 RB15 RD11 7 9 10 47 12 11 HORIZ SYNC 13 VERT SYNC 14 60 15 82 CON3 CARD PRESENT CD 38 CARD ENABLE 50 DATA TO CARD 39 CLOCK TO CARD 49 DATA FROM CARD 83 9 1 2 3 4 5 6 7 8 CARD WRITE PROTECT LED2  A K 47 RG15 RE5 27 RESET 32 68 71 Vss 31 36 45 65 D0 D4 D5 D6 D7 79 91 92 97 100 CON7 SCL SDA RF0 88 RF1 89 RG1 90 RG0 RE4 J3 J2 RD12 81 RD4 RG13 RA0 ICSP HDR* 1 MCLR 2 Vcc 3 GND 4 PGD 5 PGC 6 NC 10k 33 D1 D2 D3 RA6 CON5 +3.3V 26 RA7 95 RG14 96 RG12 RC1 RC2 8 RC3 15 SD CARD SOCKET SD ACTIVITY 13 87 RE8 7 17 K SELECT COMPOSITE 74 35 RA15 RA14 19 RE9 1 K 8 RB11 34 RB10 66 3 K WP RB2 RB1 25 RB0 PIN 6 5 77 IC1 PIC32MX795F512L-80I/PT 94 RE1 OR PIC32MX695F512L-80I/PT 24 11 12 4 6 72 23 20 3x 120 OSC2 MCLR 25 VIDEO – BLUE 3 A D6 A D5 A D4 2.2 RD5 80 VGA RD1 RF5 KYBD CLOCK 1 VIDEO – GREEN 2 RD3 RA1 5  K RD14 14 RG9 PS2 KEYBD 6 12 47 VIDEO – RED RD2 69 RD9 D+ 1k 47nF 4.7k 47nF 1k 93 A RD0 53 RF8 RA4 1k R/#2 +5V 54 2 S1 LED1 2 16 37 46 62 86 AVdd VUSB OUT +3.3V 100nF K GND IN A5 A4 A3 A2 A1 A0 VIN RESET D8 +5V +3.3V D9 D10 D11 D12 D13 CON8 ARDUINO CONNECTORS 75 D4, D5, D6: 1N4148 A K LEDS SC 2012 COLOUR MAXIMITE COMPUTER siliconchip.com.au 1N4004, 1N5819 A K K A 1 IC1 September 2012  27 DMA SPI VIDEO OUTPUT FRAMING INPUT TIMER RAM PIC32 (4 s PULSE AT 31kHz) HORIZONTAL SYNC Fig.2: the video is generated entirely inside the PIC32 chip. It starts with the MMBasic software which converts the data to a bitmap and then writes that bitmap to a portion of memory reserved as the video buffer. This buffer is then streamed by the DMA controller to the SPI output which in turn feeds it to the video output one bit at a time, with each bit representing one pixel. synchronised with the sync pulse – and therefore will display without any jitter. As far as we know, the PIC32 is the only microcontroller that has the resources needed for the Colour Maximite and has framed SPI. That is why we used the PIC32 and not some other processor (there are many similar processors on offer). The video bits are clocked out at a frequency of 20MHz. This means that the DMA circuit must read the video buffer and pump the data to the SPI at that same frequency. And because there are three colours with three DMA/SPI channels running in parallel, the total data rate is 60 million bits per second. Fortunately, the PIC32 has a sophisticated system to connect memory and peripherals (called the switching matrix) but the overhead in handling this volume of data is one of the reasons why the Colour Maximite runs a lit- tle slower when in the 8-colour mode compared to the monochrome mode. Power supply The Colour Maximite will happily run on a DC supply from 7-16V (9V recommended) with a current drain of about 160mA. It will also run on 5V via the USB connector. When running from a 7-16V rail, a simple 3-terminal regulator (REG1) is used to derive the 5V supply. Because the power can also come via the USB connector, diodes are used to select the source for the rest of the computer. A second 3-terminal regulator (REG2), a Microchip TC1262, is used to drop the 5V rail to 3.3V for the PIC32 micro and the SD card. While this regulator looks like any other 3-terminal regulator, this particular type was chosen for two key characteristics. First, it has a low drop-out voltage, which is essential because the USB supply can be as low as 4.5V and this This rear view shows (from left to right) the VGA socket, the DC power socket, the multi-way I/O connector, the keyboard socket and the audio and USB sockets. Power comes from either a USB port or a 9V plugpack (or battery). 28  Silicon Chip combined with the 0.3V drop across the diode gives the regulator less than 0.9V to work with. Second, it has a very accurate output voltage. The specifications state ±0.5% but on the samples we checked, the output was within 0.05% of 3.3V. This is important because this voltage is used as the reference when the PIC32 measures a voltage on one of its input pins. The more accurate the supply voltage, the more accurate the reading will be. There is a third regulator which you cannot see. This is internal to the PIC32 and provides 1.8V for the MIPS CPU which is at the core of the PIC32 chip. This voltage appears at pin 85 of the PIC32, where a 10µF ceramic capacitor is used to suppress noise and stabilise the regulator. It is difficult to find a through-hole component of this value so a surface-mount capacitor is used in this location. There are also quite a few 100nF capacitors sprinkled around the PCB and connected across the 3.3V supply rail. They are there to suppress the switching noise generated by the PIC32 as it races through your program. External I/O Many Arduino systems make the I2C signals available on connector pins A4 & A5. We have therefore provided two jumpers (J2 & J3) so that you can select between the normal I/O function for these pins (A4 & A5) or the I2C signals (SCL and SCA). Note that these I2C signals are also on external I/O pins 12 & 13 on the back panel, so effectively these jumpers join these I/O pins together when in the I2C position. The keyboard, SD card, USB and external I/O connectors all connect direct to the PIC32. Care is required when using the external I/O pins, as there is no special protection for these inputs. The PIC32 is a tough little chip but it can still be damaged by a highvoltage static spark or a voltage outside its operating range. As a result, it’s a good idea to always touch a ground point before handling the I/O pins. Similarly, always double-check your connections before applying power. The sound output circuit shown in Fig.1 is intended for driving an audio amplifier but you can easily modify the output to drive headphones or even an efficient speaker. The audio generated by MMBasic is output as a pulse width siliconchip.com.au Adding The Battery-Backed Clock Option +5V 100nF 10k 8 3 X2 32.768kHz 1 3V LiMn CELL IC2 DS1307 2 10k 5 SDA 59 6 SCL 58 PIC32 RA3 RA2 4 Fig.3: the add-on circuit for the battery-backed clock option. It’s based on a Maxim DS1307 real-time clock (RTC) chip and a 32.768kHz watch crystal. O NE OF THE most popular requests for the original Maximite was the addition of a battery-backed real-time clock. The Maximite already had an accurate method of keeping time but the problem was that the time was lost when ever the power was turned off. This meant that it had to be reset (if you needed the correct time) each time the Maximite was powered up. For most people, this wasn’t an issue. If you are writing a game or calculating prime numbers, the time is irrelevant. However, for some uses (eg, data logging), being Moon Lander Check out this video and see what Fabrice Muller (one of our beta testers in France) has done with with the Colour Maximite: http://youtu.be/knq5vX65Xy4 Everything, including the sound, is generated by the Colour Maxi­mite! modulated (PWM) signal on pin 76 of the PIC32 for the right channel and pin 78 for the left. PWM can be used to generate an analog output from a digital source. It uses a high-frequency sequence of pulses (about 100kHz) and MMBasic can modulate the output by varying the duty cycle. When the signal is averaged (using a low-pass filter), we end up with an analog voltage that’s controlled by your BASIC program. So before we feed the signal to an amplifier, we need a low-pass filter. This is done using a 1kΩ resistor and 47nF capacitor in each channel, which together form a simple low-pass filter with a -3dB point of about 4kHz. Following this, an attenuator consiliconchip.com.au able to keep the correct time is important. For this reason, we have made space on the PCB for a battery-backed clock. When you add these extra clock components, MMBasic will automatically recognise the presence of the clock and retrieve the time from it each time the system boots. Conversely, if you leave the components out, MMBasic will carry on as before and use the PIC’s internal clock which is reset to zero on power up. Circuit details The circuit shown in Fig.3 is simple sisting of 4.7kΩ and 1kΩ resistors reduces the output to about 0.5V p-p (peak-to-peak). This attenuator also isolates the filter from variations in the load presented by the amplifier. An alternative is to drive a set of earphones. The low-pass filter isn’t required in this case, as the headphone coils will do the averaging for you. This means that you can omit the capacitor in the low-pass filter and the attenuator. However, to avoid damaging your hearing, the 1kΩ resistor should be increased to 4.7kΩ or higher. If you can live with a low volume, you can even drive an efficient speaker direct. In this configuration, you need to reduce the 1kΩ resistor to 22Ω, replace the 4.7kΩ resistor with a link and leave out the 47nF capacitor. The sound level will depend on the efficiency of the speaker but don’t expect it to be loud. Analog outputs As previously stated, the sound output can also be used to generate two independent analog voltage outputs. MMBasic has a command specifically for this (the PWM command) and when you use the outputs in this mode, enough. It’s based on a Maxim DS1307 real-time clock (IC2) and this uses a 32.768kHz watch crystal to accurately track the time and the date, including adjustments for leap years. The DS1307 communicates via the I2C protocol with the PIC32. MMBasic uses this to interrogate the chip on power-up and then uses the PIC’s own crystal-locked clock to keep track of time from then on. The reason for this is that it’s much faster to retrieve the time from internal registers rather than request it over the relatively slow I2C bus. A great feature of the DS1307 is that it monitors its supply voltage on pin 8 and automatically switches to battery power when the supply voltage drops below a threshold. At the same time, it disables the I2C bus to prevent random noise from being interpreted as a command to change the time. We’ve specified a CR2032 lithium cell in this circuit because it’s cheap and readily available but it’s really overkill. The DS1307 consumes about 0.25μA when running off the cell and given the 140mAh capacity of the CR2032, this translates into a cell life of about 64 years! This means that the limiting factor will be the shelf-life of the CR2032 which is more than 10 years. you should not use any of the audio commands. In this case, you probably want a much lower filter frequency to give a smoother output so you should replace the values shown on the schematic with something more suited to your application. This can be easily calculated using the formula: RC = 1 ÷ 2πF where R and C are the values in the low-pass filter and “F” is the roll-off frequency. Typical values would be 4.7kΩ and 330nF, which would give a fast enough response for you to quickly change the output while eliminating most of the PWM switching. If you want to use these outputs to generate a PWM square wave rather than an analog voltage, you can use the same method as described above for connecting a speaker; ie, use a 22Ω resistor and a wire link. Next month: construction! Well, that’s all we have space for this month. In Pt.2, we will describe how to put the Colour Maximite together and give you an introduction to programSC ming in colour. September 2012  29