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Second-Generation Colour Maximite 2 The Colour Maximite 2 computer, published about a year ago, has been a huge hit, with thousands built. Now we present the second generation of the Colour Maximite 2. This does not make the first generation obsolete; it is an evolution with several improvements that will be appreciated by enthusiasts who are pushing the boundaries. Part 1: introduction F or readers who missed the introduction of the Colour Maximite 2 (July & August 2020; siliconchip. com.au/Series/348), this computer is inspired by the personal computers of the early 1980s. Computers such as the Apple II, Commodore 64 and the Tandy TRS-80. But thanks to modern technology, it’s way more powerful and costs much less. Like those computers, the Colour Maximite 2 has a built-in BASIC interpreter and boots up instantly, straight into the BASIC prompt. You can immediately enter a command or a program and start doing something useful. The emphasis of the Colour Maximite 2 is on ease of use and having fun. Plug it in, and within seconds, you can be entering a program to draw on the screen, calculate astronomical movements or play music. It is ideal for learning to program, educating children and just exploring what you can do with this small and cheap computer you program yourself. While the concept of the Colour Maximite 2 (CMM2) is borrowed from the computers of the 80s, the technology used is very modern. The CPU is an ARM Cortex-M7 32-bit RISC 26  Silicon Chip Words and MMBasic by Geoff Graham Design and firmware by Peter Mather processor running at 480MHz, hundreds of times faster than the 8-bit CPUs of the 80s. This chip also includes integrated memory, communication systems and its own video controller, capable of generating a VGA output at resolutions of up to 1920x1080 pixels with some resolutions supporting 24-bit ‘true colour’. New features The second-generation CMM2 offers three main improvements over the original Colour Maximite 2. The first is that the random access memory has been boosted to 32MB compared to the original 8MB, and it is also much faster. The revised circuit is shown in Figs.1 & 2. This increased memory capacity and speed have enabled several new features, including a 1920x1080 pixel VGA mode, 24-bit colour and more RAM for BASIC programs to use for arrays, I/O buffers, etc. The second improvement is the video DAC (digital-to-analog converter) which now uses eight bits for each colour channel. That means that this version can generate 24-bit colour, Australia’s electronics magazine supporting over 16 million different colours. This is known as ‘true colour’ and is the same colour range used by PCs. So photographs can now be displayed without noticeable colour banding (eg, in the sky). The third feature is the use of a fourlayer PCB with all components placed on the top side of the PCB. With the first generation, we were able to get away with mainly using through-hole parts, but as we are now mounting the CPU directly on the board (rather than via a module), that is not a viable option. As a result, most vendors will offer this design partially or fully assembled rather than a simple kit of parts. There are a few other minor new features in the Generation 2 design, which we will cover later. These include two Wii game controller connectors, the ability to connect a mouse easily, an optional high-accuracy real-time clock and the ability to mount an ESP-01 WiFi module on the PCB. Circuit description The circuit consists mostly of connectors and ICs surrounding the main processor, IC3, so we’ll just mention siliconchip.com.au some of the more noteworthy aspects of the circuit. The full circuit is shown in Figs.1 & 2. The 24-bit colour VGA output is generated using 24 digital outputs from IC3 arranged in three groups: one for red, one for green and one for blue. Each group drives an R-2R ladder DAC made from discrete resistors. The effect of this is that the 7th output in a group has half the effect on the output voltage as the 8th, the 6th half that of the 7th and so on down the ladder. Almost all components require a 3.3V supply. As the incoming power is 5V DC, the power supply is very simple, consisting mainly of linear regulator REG1 plus many bypassing and filter capacitors. IC4, the RAM chip, connects to IC3 via a 16-bit data bus and 13-bit address bus, plus 10 control lines. Assembly options The new four-layer PCB with mainly SMD components mounted onboard makes scratch-building the CMM2 Gen2 a bit more challenging than the earlier version. If you’re keen to build it yourself, you can still do that, although you might find sourcing the processor somewhat tricky given the severe shortages affecting the semiconductor industry at the time of publishing this article. But it is an option for those who are confident in their SMD assembly skills (or keen for a challenge!). Another option would be to use a PCB fabrication company to populate and solder the surface-mounted components for you, using their pick-and-place machines and reflow ovens. They can do this reasonably cheaply in small quantities. But it will probably be cheaper and easier for you to buy one of the kits that come with a mostly pre-populated PCB, as described below. If you want to solder your own Colour Maximite 2 but are not confident that you can handle the SMDs, especially the 144-pin main CPU, consider building the first-generation design. It primarily uses through-hole parts and offers many of the same features as this revised version. Upgrades to the original In the following discussion, we will describe the second-generation design, including its new features. However, many of its features also apply to the siliconchip.com.au Features & Specifications 480MHz ARM Cortex-M7 32-bit CPU with 2MB of flash and 1MB of RAM Additional 32MB off-chip RAM, used for BASIC variable storage and video pages Colour VGA output with 15 software-selectable resolutions from 240x216 pixels to 1920x1080 pixels, in both standard 4:3 and widescreen 16:9 ratios Four colour modes from 8-bit (256 colours) to 24-bit (16 million colours) Full-featured BASIC interpreter with support for strings, double-precision floating-point and 64-bit integers, long variable names, arrays with up to five dimensions and ‘unlimited’ user-defined subroutines and functions BASIC programs can be up to 516KB (typically 25,000 lines or more) and run at 200,000+ lines per second 24MB storage memory for BASIC programs Seven selectable fonts, user-designed fonts, line drawing, circles, squares and full control over all pixels. Can load image files formatted as BMP, GIF, JPG or PNG from the SD card, positioned anywhere on the screen and scaled and rotated USB keyboard support for US, UK, French, Spanish or German layouts and wireless keyboards with a USB dongle PS/2 mouse support for dual-mode USB mice with a PS/2 adaptor – an optional chip provides support for standard USB mice. SD card support up to 128GB for storing programs and files (FAT16, FAT32 or exFAT) Built-in graphical file manager makes it easier to manage files and directories, along with mouse support Stereo audio output; can play WAV, FLAC and MP3 files, computergenerated music (MOD format), robot speech, synthesised sound effects and sinewave tones Battery-backed real-time clock (RTC) will keep the time even when powered down 28 I/O lines which can be configured as analog inputs, digital inputs/ outputs, for frequency measurement etc; pin layout is compatible with Raspberry Pi HATs Support for communications protocols including serial, I2C, SPI and 1-wire USB socket for connecting to a personal computer (Windows, Mac or Linux) as a terminal or for file transfer Special features for animated games including multiple video layers with selectable levels of transparency, multiple video pages with high-speed copying between pages, BLIT (copy a block of video), SPRITE (animated sprites) and support for Wii game controllers Built-in full-screen editor with colour coded text, up to 255 character line lengths, clipboard for copy and paste, advanced search and replace and mouse support Powered from USB 5V, drawing less than 300mA Firmware upgrades via USB with no special hardware required Compatibility mode for running programs written for the original Colour Maximite Australia’s electronics magazine August 2021  27 REG1 AMS1117-3.3 4 4 2 D– 6 5 3 D+ C ON2 7 8 Vcc V3 RTS DTR R232 DCD IC8 CH340G UD– UD+ RI DSR RXD XI TXD XO GND CTS 12pF 14 10 W 13 10 3 2 +3.3V 9 X3 1 100nF 19 38 37 39 15 21 20 23 24 25 26 4 VDD ST 8MHz OUT XO 3 MODE JP7 GND 3 40 100nF 1mF 10mF 11 + 3 .3 V 16 17 18 12pF 12 1 6x 100nF X1 32768Hz 9 43 49 VDDQ 1 14 27 VDD 53 DQ15 51 DQ14 50 DQ13 48 DQ12 NC 47 DQ11 45 WE DQ10 44 DQ9 CAS 42 RAS DQ8 13 DQ7 CS 11 DQ6 10 DQ5 8 DQ4 CKE IC4 7 CLK MT48LC16M16A2 DQ3 5 DQ2 4 DQ1 DQMH 2 DQ0 DQML 36 A12 35 A11 22 BA1 A10 34 BA0 A9 33 A8 32 A7 A0 31 A6 A1 30 A5 A2 29 A3 A4 VSSQ VSS 6 12 46 52 28 41 54 2 TO NUNCHUK 1 (SEE FIG.2) TO A3 ON CON1 (SEE FIG.2) TO A2 ON CON1 (SEE FIG.2) AUDIO C ON4 SC Ó2021 28 4.7kW 4.7kW 2.2mF COLOUR MAXIMITE 2 GEN2  Silicon Chip 2.2mF 45 10 79 118 117 39 38 157 156 111 120 121 164 152 26 27 29 30 172 159 149 136 103 127 82 91 62 49 72 PE2 PB12 PD3 PD2 PC12 PH4 PB10 PC9 PC8 VDDA VREF+ PG14 PG13 PG7 PA9 PA10 PB6 PG9 MAIN CIRCUIT Australia’s electronics magazine 13 1 92 145 144 141 139 140 89 110 57 32 PC10 PC11 PH12 PG6 PB1 PC0 87 PH10 86 PH9 85 PH8 44 PH3 43 PH2 176 PI7 175 PI6 174 PI5 173 PI4 154 PG11 150 PD6 155 PG12 3 PE4 11 VERT SYNC PI9 12 HORIZ SYNC PI10 PC15/OSC32_OUT PF8 PF9 PH0/OSC_IN PH1/OSC_OUT 151 PD7 45 PA2 80 PB11 98 PD10 97 PD9 96 PD8 78 PE15 77 PE14 76 PE13 75 PE12 74 PE11 73 PE10 70 PE9 69 PE8 68 PE7 143 PD1 142 PD0 105 PD15 104 PD14 106 PG2 67 PG1 66 PG0 65 PF15 64 PF14 63 PF13 60 PF12 21 PF5 20 PF4 19 PF3 18 PF2 17 PF1 16 P F0 108 PG4 109 PG5 169 PE0 170 PE1 112 PG8 84 PH7 58 PB2 101 PD13 100 PD12 99 PD11 83 PH6 59 PF11 160 PG15 46 PH5 123 PA12 122 PA11 50 PA4 51 PA5 125 VCAP 81 VCAP VSSA 80 31 171 +3.3V 2 4 6 COM2:Tx 8 1 16 15 USB TYPE B PWR/CONSOLE 2x 10k W + 3 .3 V PI11 PD4 PA14 PA13 PC14/OSC32_IN 166 107 IC3 STM32H743IIG 1 3 5 7 5 6 100nF PA14 PA13 GND RST C ON8 SDA COM2:Rx 133 132 131 130 153 128 5 4 165 I2C #2 SDA 168 I2C #1 SDA 2 167 I C #1 SCL 33 COUNT1 129 COM2:Rx 34 COUNT2 35 COUNT3 163 SPI1 MOSI PB5 162 SPI1 MISO PB4 161 SPI1 CLK PB3 PWM1C 56 PB0 116 PWM2B PC7 GPIO 134 PI3 SPI2 MISO 94 PB14 41 COM1:DE PA1 SPI2 CLK 93 PB13 95 SPI2 MOSI PB15 115 PWM2A PC6 7 GPIO PI8 2 88 I C #2 SCL PH11 55 GPIO PC5 54 COUNT4 PC4 53 PWM1B PA7 138 FAST COUNT PA15 COM2:Tx 40 PA0 PWM1A 52 PA6 COM1:Rx 47 PA3 COM1:Tx 42 PA2 119 PA8 2 PE3 PI2 PI1 PI0 PH15 PG10 PH13 PE6 PE5 PB7 PB9 PB8 PC1 PH14 PC2_C PC3_C VSS 158 SDA 7 148 RESET PA13 5 SCL SCL 135 4 SQW/INT 126 3 GND 4 100nF RST BOOT0 PG3 PB11 RST PDR_ON 113 PA14 CR1220 BATTERY 6 VBAT VDD VBAT PC13 PF6 PF7 PF10 PD5 102 3 2 Vcc VBAT IC7 DS3231MZ 8 32kHz 8 24 25 28 147 146 137 124 9 71 1 3.3V 1 6 90 100nF ST-LINK 2 61 +3.3V POWER 4.7mF 14x 100nF 48 1m F 22 100mF GND 23 10mF S1 36 PWR +3.3V + 3 .3 V OUT IN 14 +5V 15 JP1 37 TO CON3 PINS 37 & 39 JP3 JP4 TO B3 ON CON1 (SEE FIG.2) TO B2 ON CON1 (SEE FIG.2) siliconchip.com.au BOOT0 + 3 .3 V + 3 .3 V 2.2W ESP_3.3V 100nF 10kW 10kW 100mF 1k W RESET S2 100nF RST 10k W A l JP2 1kW 4.7kW 3 2 A POWER SD CARD K l K 1 IC2 100nF Vcc DS18B20 DQ DIGITAL THERMOMETER 1 TSOP4838 IR SENSOR 3 l GND PROG/RUN IRD1 2 2. 2 W LED1 10 m F 100nF CON6 SD CARD SKT CARD PRESENT CD 240W 240W 120 W 240W 240W 240 W 240W 240 W DATA TO CARD 240W CLOCK TO CARD DATA FROM CARD 120 W 120 W 120 W 120W 120 W 120 W CARD WRITE PROTECT 240W 240W VERT SYNC 240W 120 W HORIZ SYNC 9 1 2 3 4 5 6 7 8 CARD ENABLE 240W 240W 240 W 240 W CON5 VGA CONNECTOR 75 W 240W 240W WP VIDEO – RED 6 11 7 12 8 13 9 14 10 15 1 VIDEO – GREEN 2 120 W 120 W 120 W 120W 120 W 75W 120 W 240W VIDEO – BLUE 3 75W CON9 4 5 (HEADER FOR CONNECTING ESP-01 WIFI MODULE) 240 W 240W 120 W 240 W 120 W 240W 240 W 240W 240W 240 W IC3 PI10 120W 120 W 120W 120W 240W 2 I C #2 SDA IC3 PI9 2 I C #2 SDA 2 I C #1 SDA 2 I C #1 SCL 120 W 10kW VERT SYNC HORIZ SYNC TO NUNCHUK 2 (SEE FIG.2) +3.3V 10k W COUNT1 COM2:Rx COUNT2 COUNT3 SPI1 MOSI SPI1 MISO SPI1 CLK 10kW PWM1C PWM2B GPIO SPI2 MISO +3.3V COM1:DE 2 28 30 32 34 36 38 40 PWM2A GPIO 2 4 6 8 10 12 14 16 18 20 22 24 26 SPI2 CLK SPI2 MOSI GPIO COUNT4 PWM1B FAST COUNT 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 I C #2 SCL COM2:Tx PWM1A COM1:Rx COM1:Tx 2 I C #2 SDA 2 I C #2 SCL IC5_PIN18 IC5_PIN17 CON3 EXTERNAL I/O +5V 10kW COUNT4 PWM1B FAST COUNT COM2:Tx PWM1A COM1:Rx COM1:Tx Fig.1: the Colour Maximite 2 is centred around the ARM Cortex-M7 processor, IC3. This, along with 32MB (512Mb) RAM chip IC4, does most of the work. The rest of the circuit is mainly concerned with supplying power to those chips and connecting the processor to the outside world. The keyboard & mouse interface didn’t fit in this diagram, so it is shown separately in Fig.2. siliconchip.com.au Australia’s electronics magazine August 2021  29 first generation version via firmware upgrades, for example, mouse support and the 1280x720 pixel display resolution. So, if you have the original Colour Maximite 2, you should upgrade the firmware to the latest version to get these features. The firmware download is available from the Silicon Chip website or the Author’s website at http:// geoffg.net/maximite.html Note that the same firmware file will load and run on both the first- and second-generation designs. The firmware automatically detects the hardware that it is running on and configures itself accordingly. Basic operation summary To use the Colour Maximite 2, all you need to do is plug in a monitor, keyboard and power supply. The computer generates a VGA output with one of 15 different video modes, some widescreen and some that work best with older 4:3 aspect ratio monitors. The highest resolution is 1920x1080 pixels; the 1280x720 resolution works well with a widescreen monitor, and the text is easy to read. The CMM2 defaults to an 800x600 resolution which suits all monitors, but this can be changed using the OPTION DEFAULT MODE command. The keyboard interface accommodates most USB keyboards, including keyboards that use a wireless dongle. Keyboards that have been tested and work well include the Logitech K120, K270, K400+ or K800 models, HP SK2885, Lenovo KU-0225 and Microsoft 600. We have discovered that some keyboards will not work correctly for an unknown reason related to the USB protocol stack. This is rare, but if you run into keyboard problems, try one of the above-listed types. The Logitech K120 works well, is cheap (under $20) and is readily available. The power supply can be any USB source of 5V. The typical power draw of the computer with just a keyboard and monitor is 250mA; however, when you connect external circuitry to the rear I/O connector, this can increase. If you are using a USB charger as a power supply you need to be careful, as many of these struggle when they are anywhere near their limit. So make sure that it is rated for at least 500mA, and preferably at least 1A. Also be careful if you are using a laptop as the 30  Silicon Chip The front view showing the infrared receiver (for remote control), two Wii controller ports for the Wii Nunchuck or the Wii Classic game controllers, status LEDs for power and SD card activity, the SD card slot, the audio output socket and the power switch. power source, as they often limit the current delivered via their USB ports to conserve their battery capacity. With the first generation Colour Maximite 2, we found that most problems encountered by our readers could be traced to the power source, closely followed by the quality of the USB cable used for the power. These are the first things you should investigate if you have strange problems such as failure to boot, errors with the SD card, intermittent crashes, keyboard problems etc. You will need an SD card to hold your programs. The Colour Maximite 2 supports cards up to 128GB formatted with FAT16, FAT32 or exFAT. Generally, you do not need a very large capacity, so an 8GB or 16GB card formatted with FAT32 will provide more than enough space and will be quite cheap (under $10). With a power supply, keyboard and monitor attached, flipping on the power switch will result in the Colour Maximite 2 booting up in under a second, straight into the MMBasic interpreter. It will display the BASIC prompt (a greater than symbol, “>”), and you are ready to enter your first command or program. MMBasic BASIC is a programming language that has been around for a long time. Initially developed in the 1960s by Dartmouth College (USA) for teaching Australia’s electronics magazine programming, it is easy to use and learn. It became popular in the late 70s and early 80s as the default language for early personal computers. MMBasic is the name given to the BASIC interpreter running on the Colour Maximite 2. It is an interpreter, which means that the computer will decode each line of the program as it encounters it. This is different from compiled languages such as C and C++, where code is converted to native machine instructions before execution. Compilers use a series of programs (a compiler, linker and loader) to take your program and convert it into machine code. This is then used to create a program file that the computer can run. This process results in a higher execution speed than an interpreter, but creates certain restrictions in how programs can be written. It also means you need to wait each time you make a change for the code to compile before you can launch it and see what effect that change has had. Many programs don’t need the absolute fastest execution speed, especially with modern processors being quite fast. So interpreted languages are popular with non-professional programmers (and even with professionals for specific tasks). MMBasic in the Colour Maximite 2 is the same as the BASIC interpreter that runs on our popular Micromite series of embedded controller chips. siliconchip.com.au Along the back panel, you can see the VGA output socket, the external I/O socket for a ribbon cable, a Type-B USB connector for power and serial console access and two stacked Type-A USB connectors. The top connector is for the keyboard, while the lower connector is for a mouse. MMBasic has over 500 built-in commands and functions. It is also designed to emulate Microsoft BASIC, the premier programming language for personal computers in the 80s. This means that you can get Micromite programs or programs written for Microsoft BASIC working on the CMM2 with minimal changes. You can type in almost any command at the MMBasic prompt, and the interpreter will execute it immediately. For example, if you type PRINT 2 + 2 and press Enter, it displays “4” on the screen as you would expect. This immediate feedback is one of the benefits of running an interpreter, and it allows you to easily test the commands and functions in the BASIC programming language. ARM Cortex-M7 processor The hardware and firmware of the Colour Maximite 2 are fully covered in the Colour Maximite 2 User Manual, so we will not go into all the details here (there are a lot of details). The computer is centred around the ARM Cortex-M7 processor. This is the large central IC that you can see in the photographs. Along with the 32MB (512Mb) RAM chip, this does most of the work; the other components and connectors supply power and connect the processor to the outside world. When we designed the original Colour Maximite 2, the STMicroelectronics STM32H743IIT6 chip came in siliconchip.com.au two speed types – the older 400MHz version and a later 480MHz version. STMicro was transitioning from the slower to the faster variant, but annoyingly, they used the same part number for both versions. This made it difficult for suppliers and you, the end user, to know what variant you were going to receive. It seems that now the supply chain has flushed out most of the 400MHz chips, as over the past few months, all the chips that we have seen are the 480MHz version. So you can be reasonably sure that the second generation Colour Maximite 2 will run at this speed. But, that is not guaranteed. Regardless, either speed is very fast, and the firmware will automatically support whatever chip is supplied. If you are curious, you can determine the variant you have by using the following command to report the speed: PRINT MM.INFO(CPUSPEED) Oscillator module The ARM processor is clocked by an external 8MHz oscillator module. This signal is multiplied within the processor to generate all the various clocks required, including the instruction clock, USB clock, VGA timing etc. In the original Colour Maximite 2, we used the crystal on the Waveshare CPU module and the oscillator built into the ARM processor to generate this clock. But it turned out that this Australia’s electronics magazine arrangement created a slight jitter in the signal, which sometimes caused a corresponding instability in the VGA video output. Generally, this was not a problem at the standard 800x600 pixel resolution. But as higher resolutions were enabled via firmware upgrades, it became more of a problem. With the second generation design capable of generating a 1920x1080 pixel video output, the requirement for a more stable clock became critical. This is why an external (to the ARM processor) 8MHz oscillator module has been specified. This oscillator is very stable and supports the extended resolutions that many users would like to use. If you have the original Colour Maximite 2 and would like to use the high resolutions provided by the later firmware upgrades, we recommend that you also upgrade the hardware to an external oscillator. The first generation PCB was designed for this possibility, and the second article in this series will have the details of how to perform the upgrade. Usually, video images are stored in the RAM within the ARM Cortex-M7. But for high-resolution images, we needed more RAM than it has. This, in part, is the reason for the external 32MB RAM chip. Using this, the firmware can assemble much larger images. This RAM is also used to support 24-bit graphics modes and provide more memory for BASIC programs. The internal clock/calendar used by MMBasic is derived from the real-time clock built into the ARM Cortex-M7 processor. This is effective, but if you want a much better level of accuracy, you can add an optional DS3231MZ+ to the PCB, and it will typically only lose or gain a second or two in a week. The command to enable this optional feature is: OPTION DS3231 Whichever real-time clock is used, it is powered by the coin battery on the PCB. This is also used to keep alive some static memory within the ARM Cortex-M7 that stores option settings and saved variables so that they are not lost on power down. The current drawn from this battery is tiny, so it should last for many years. The main board now has provision for mounting an ESP-01 WiFi module. August 2021  31 This module uses the ESP8266 chip, a self-contained SOC (system on a chip) that includes the TCP/IP protocol stack, 2.4GHz transmitter/receiver and other features to allow the Colour Maximite 2 to access a WiFi network. Currently, MMBasic does not have Internet features built in, but you can access the ESP8266 using a standard serial interface and the AT commands built into the ESP8266. With special firmware running on the ESP8266, you can extend the BASIC console over WiFi so that you can remotely connect to the Colour Maximite 2 to upload, edit and run programs. Front panel arrangement The two Wii Controller ports dominate the front panel. These will accept either the Wii Nunchuck or the Wii Classic game controllers; MMBasic can work with either. Many games written for the Colour Maximite 2 use such a controller, so they are a useful addition if you plan on playing some games. MMBasic can support up to three controllers, with the third via the rear I/O connector. Positioned between the two game controller connectors are two LEDs. The bottom green LED illuminates when power is applied. The top red LED initially illuminates to indicate that the firmware has successfully found and enumerated the USB keyboard. This is a handy indicator if you are experiencing problems with your keyboard. Following this, the red LED is used to indicate SD card activity. It will illuminate while the SD card is being accessed, and this is a warning: do not remove the card while the LED is lit. The SD card acts as the computer’s “disk drive”, where programs and data are stored. Next to it is the audio socket. The tip is the left channel, the ring is the right channel and the sleeve is ground – the standard configuration. The output is a high-impedance signal at about 1V RMS, suitable for feeding to an amplifier or amplified speakers. Programs can generate audio in many formats, ranging from simple sinewave tones through to playing WAV, FLAC, and MP3 files. Next along the front panel is the power switch, which has a special feature: it can be set to be on when the toggle is down (for Australian and New Zealand readers), or the opposite for 32  Silicon Chip our North American cousins. This is configured via the three jumper pins beside the switch. When the centre pin and the pin to the rear are connected, down will be on. The reverse is true if the centre pin and the pin towards the front panel are connected. Rear panel features Along the back panel (starting from the left) is the VGA connector for your monitor. As described previously, this supports a wide range of resolutions and colour depths. We have been asked why the Colour Maximite 2 does not support HDMI, and the answer is that the ARM CortexM7 does not support this protocol. So an expensive and complicated HDMI controller chip would be required. There are also difficulties associated with HDMI licensing, so this feature was just not practical. VGA to HDMI converters are readily available and inexpensive, so if you want to connect the CMM2 to a monitor via HDMI, that is the best way to do it. These converters cost a lot less than it would cost us to implement onboard HDMI. Next on the back panel is the 40-pin external I/O connector. This supports 28 inputs or outputs, with 12 of these able to be configured as analog inputs. Many of the I/O pins can also be used as PWM outputs or to handle serial communications, including asynchronous serial, I2C, SPI and more. The pin allocations on this connector are inverted compared to the first generation Colour Maximite 2. This is because while the I/O signal allocation on the first generation was compatible with the Raspberry Pi, the pin numbering was inverted, which sometimes caused problems. So the Generation 2 version corrects this by exactly matching the Raspberry Pi configuration, including pin numbering. With the first generation, you had to cut a new key slot in the shroud if you wanted to plug in a device intended for the Raspberry Pi. With the second generation design, this is no longer necessary. Continuing across the back panel, the Type-B USB connector provides both power and a serial console. We covered the power requirements of the Colour Maximite 2 earlier, but the serial console feature needs a little explanation. Australia’s electronics magazine The console is where you enter commands and programs into the computer. Typically, this is done using a keyboard and VGA monitor, but the serial console allows you to connect a PC or laptop to the Colour Maximite 2’s console via a serial interface over USB. You can do everything that can be done via the keyboard and monitor (except graphics) via this interface. You can enter and edit programs, set options, run programs etc. A CH340C USB-to-serial bridge is used. This converts the serial I/O from the ARM Cortex-M7 to USB using the CDC (communication device class) protocol over USB. Support for this chip and the CDC protocol is included in Windows 10 and other operating systems. The first generation Colour Maximite 2 used a different chip for the same task, but the CH340C is cheaper and more readily available, so we have switched to that. Mouse interface The last connector on the back panel consists of two stacked USB Type-A sockets. These are for the keyboard (top connector) and a mouse (lower connector). The second-generation design supports two types of mouse interfaces. The first is a USB-only mouse, which requires a Hobbytronics mouse controller chip (www.hobbytronics.co.uk/ usb-host-soic) to be installed as IC5, along with its supporting components. You can then plug the mouse into the USB mouse socket (the lower socket). This feature is optional, and the circuit for it is shown in Fig.2. Typically, suppliers of the second generation Colour Maximite 2 will not include this chip as it is easier to use a dual USB-PS/2 mouse, which is the second type of mouse interface supported. Most wired mice will automatically switch between USB and PS/2 modes, and many come with a USB to PS/2 adaptor. This adaptor simply signals the mouse to switch to PS/2 mode via a pull-up resistor inside the adaptor. The adaptor also provides the physical PS/2 connector. Even if your wired mouse did not come with this adaptor, it is very likely that it will work as a PS/2 mouse – so it is worth giving it a try. A typical example is the Microsoft Basic Optical Mouse, which is low in cost (under $20), widely available and works well siliconchip.com.au Fig.2: the keyboard & mouse connector, along with the optional USB mouse interface chip, IC5. You generally won’t need this chip as most wired USB mice will work in PS/2 mode, regardless of whether they came with a PS/2 adaptor. as a PS/2 mouse with the Colour Maximite 2. To use a dual USB-PS/2 mouse, IC5 must not be populated, and all four solder jumpers marked PS/2-CLK and PS/2-DAT on the PCB (JP3-JP6) must be joined with solder blobs. You can then plug the mouse into the USB mouse socket (the lower socket). A USB to PS/2 adaptor is not required, as the Colour Maximite 2 will force the mouse into PS/2 mode, even though it is plugged into a USB socket. MMBasic has built-in support for a mouse via the MOUSE() function. The program can query the mouse cursor’s position and detect clicks or double clicks of the mouse buttons using this. Both the file manager and the editor built into MMBasic can also use the mouse for most functions that you might expect. For example, you can use the mouse to point and select a file or line; you can select text with the mouse, scroll using the scroll wheel, double click to open/run a file and so on. While the second-generation design makes it easy to plug a dual-function mouse into the USB socket (without an adaptor), the first generation design running the latest firmware also supports a PS/2 mouse via the rear I/O connector. The only difference is that you will need to solder some wires from the mouse’s connector to the I/O connector, as illustrated in Fig.3. siliconchip.com.au The mouse CLOCK (pin 5) line connects to pin 33 of the I/O connector, and the DATA (pin 1) connects to pin 32. Both must have a 4.7kW pull-up resistor to +5V. This can be assembled on a small piece of perforated stripboard. Where to get it The Colour Maximite 2 is available from several suppliers around the world. Many of these will supply it with all the SMDs already soldered, as building it from scratch requires good soldering skills. Vendors selling kits for the second generation Colour Maximite 2 include Rictech Ltd in New Zealand (www. rictech.nz) and Micromite.org in the UK (https://micromite.org). Both will send kits anywhere in the world. They offer partially assembled kits and, by that, we mean that the PCB is populated with all the small surfacemount components already soldered in place. The larger components (connectors, SD card socket, battery holder, etc) are supplied loose for you to solder yourself. This only takes half an hour or so. These suppliers might also offer fully assembled and tested versions, pre-cut front/rear panels and a suitable case for the completed computer – check the supplier’s website for the details. You will also need USB cables and a 5V supply, as these are generally not included. For brave readers, a construction kit is available from the Author’s website (http://geoffg.net/maximite.html) and this contains the parts list, PCB Fig.3: If you have a first-generation Colour Maximite 2, you can add a PS/2 mouse to it by wiring it to the rear I/O connector as shown here. For the mouse to be recognised by MMBasic, you must upgrade the firmware to version 5.07 or later. Australia’s electronics magazine August 2021  33 Parts List – Colour Maximite Gen2 1 partially assembled PCB module – see below 1 USB 5V power supply or computer with powered USB socket 1 DS18B20+ temperature sensor (IC2; optional) 1 3-pin infrared receiver (IRD1; optional) 1 USB Type-A to Type-B cable (for power) 1 dual horizontal USB Type-A PCB socket (CON1; Amphenol FC1 723098034BLF) ∎ 1 USB2 Type-B right-angle PCB socket (CON2; Amphenol FC1 61729-0010BLF) ∎ 1 40-way DIL right-angle box header, 2.54mm pitch (CON3; Hirose HIF3F-40PA2.54DS(71)) ∎ 1 3.5mm stereo jack socket (CON4; Switchcraft 35RASMT4BHNTRX) ∎ 1 15-pin right-angle HD D-sub PCB socket (CON5) [RS 481-443, element14 2401183/2857990, Digi-Key AE11036-ND, Mouser 523-7HDE15SDH4RHNVGA] 1 SD card socket (CON6; Hirose DM1AA-SF-PEJ(21) or DM1AA-SF-PEJ(82)) ∎ 1 3-pin header (CON7; optional – for serial comms) 1 6-pin header (CON8; optional – for ST-LINK programmer) 1 4x2-pin header (CON9; optional – for connecting an ESP-01 WiFi module) 1 right-angle vertical PCB-mount SPDT toggle switch (S1) [Altronics S1320, RS 734-7107, element14 9473297, Digi-Key EG2364-ND, Mouser 34ASP27T7M2QT] 1 button cell holder for CR1220 (BAT1; Harwin S8411-45R) ∎ 1 CR1220 lithium button cell (BAT1) 1 3mm dual green/red LED assembly (LEDs1a & 1b; Dialight 553-0112F) ∎ 1 3-pin header with jumper shunt (JP2) 1 short length of 0.7mm diameter tinned copper wire, or a component lead offcut (PWR) 1 plastic instrument case, 140 x 110 x 35mm [Jaycar HB5970, Altronics H0472, element14 1526699] 4 5mm untapped spacers ∎ available from [RS, element14, Digi-Key and Mouser] Partially assembled PCB module parts 1 four-layer PCB with plated through holes coded 07108211, 128mm x 107mm 1 32768Hz 12.5pF SMD crystal, 3.2 x 1.5mm two-pin package (X1) [eg, Seiko Epson Q13FC1350000400] 1 8MHz 3.3V SMD crystal oscillator module, 7 x 4mm four-pin package (X3) [eg, Seiko Epson X1G004481001400] 1 5.1 x 5.1mm SMD four-pin tactile switch (S2) [eg, XKB Connectivity TS-1187A-C-C-B] Semiconductors 1 STM32H743IIT6 32-bit microcontroller, LQFP-176 (IC3) 1 Micron MT48LC16M16A2P-6A IT:G 256Mb (32MB) SDRAM, TSOP(II)-54 (IC4) 1 DS3231MZ real-time clock, SOIC-8 (IC7; optional) 1 CH340C USB/serial converter, SOIC-16 (IC8) 1 AMS1117-3.3 3.3V low-dropout linear regulator, SOT-223 (REG1) Capacitors (all SMD 50V X7R ceramic, M3216/1206 size unless otherwise stated) 2 100μF 6.3V SMD tantalum, SMB, SMC or SMD case 4 10μF 16V SMD tantalum, SMA case 1 4.7μF 2 2.2μF 2 1μF 32 100nF 2 6pF C0G/NP0 ceramic, M2012/0805 size 3 1kW 1 10W Resistors (all SMD 1/8W 1% thick film, M2012/0805 size) 5 4.7kW 27 240W 21 120W 3 75W 2 2.2W Optional parts for USB Mouse 1 Hobbytronics USBHOST-SOIC, SOIC-28 (IC5) – www.hobbytronics.co.uk 1 16MHz 10ppm 9pF SMD crystal, 3.2 x 2.5mm four-pin package (X2) [eg, Yangxing Tech X322516MLB4SI] 1 SMD LED, M2012/0805 size (LED2) 2 18pF 50V C0G/NP0 ceramic capacitors, M2012/0805 size 34  Silicon Chip Difficulty obtaining parts When you are purchasing a Colour Maximite 2, be aware that large scale semiconductors such as the ARM Cortex-M7 are in short supply worldwide. It has got so bad that some car manufacturers such as Ford have had to shut down plants and lay off workers because they cannot get the semiconductors needed to finish the vehicles. We have seen this effect with both the ARM Cortex-M7 and the 32MB RAM chip used in the Generation 2 design. So you might experience longer delivery times for your Colour Maximite 2 kit than you would typically expect. This is caused by events outside the control of the supplier, and patience is the only answer. Next month Resistors (all SMD 1/4W 1% thick film, M3216/1206 size) 13 10kW fabrication files, schematic, and the pick-and-place assembly files. You can also get the PCB for the revised Colour Maximite 2 from the Silicon Chip Online Shop, but you will have to gather all the other bits yourself. Note that we still sell a kit for the original CMM2, which includes almost everything you need; it just lacks the case. The design and firmware for the Colour Maximite 2 are in the public domain (free to anyone), and two other vendors have created their own versions of the Generation 2 design. These are compatible with the standard firmware and offer additional features such as a sea-of-holes PCB prototyping area, more Wii Controller ports, etc. These vendors are CircuitGizmos in the USA (https://circuitgizmos.com) and PS Labs in Poland (http://maximite. pslabs.pl). Both of these will ship worldwide, and you should check their websites for the various features of their versions, including the supplied cases. Australia’s electronics magazine In the follow-up article next month, we’ll have PCB layout details, for if you’re planning on building it yourself, debugging/testing, or you just want to see what connection is where. The final construction details will follow that, plus information on loading the firmware into the STM32 chip, a short guide on writing BASIC programs on the CMM2 and some links to external resources that you will find helpful. SC siliconchip.com.au