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By Tim Blythman THE PICO COMPUTER A computer terminal using a Raspberry Pi Pico Turn a Raspberry Pi Pico, Pico W or Pico 2 board into a standalone computer with a USB keyboard and HDMI monitor. With the Pico Computer PCB, all the required circuitry fits in a compact and handy enclosure. I n April 2024, we presented the Digital Video Terminal (siliconchip.au/ Series/413) that can connect to a monitor via HDMI, a USB keyboard and a Raspberry Pi Pico or other device with a serial port. It provides a freestanding terminal console, ideal for working with many single-board computers. Its block diagram is shown in Fig.1. The Pico Computer Board can plug onto the Digital Video Terminal’s PCB, turning it into the Pico Computer with many features. For example, the Computer Board includes (among other features) an RTCC (real-time clock and calendar chip), a microSD card slot, an IR receiver and a 3.5mm stereo jack for audio. This turns the Terminal into a fully-fledged standalone Pico-based computer, fitting in the same compact footprint as the Terminal alone. Fig.2 shows the block diagram of the Computer Board integrated with a Digital Video Terminal. The new hardware is shown in the centre. The Computer Board replaces MOD2 of the Terminal and adds many extra features. Digital Video Terminal functions The earlier Digital Video Terminal is compact at just 105 × 80 × 25mm and Fig.1: the original Digital Video Terminal required a Pico (or similar device) to be connected externally via USB (shown at top centre) to access the keyboard and display facilities. 66 Silicon Chip Australia's electronics magazine siliconchip.com.au Features & Specifications ► Digital display and USB keyboard ► I2C header and onboard I2C real- time clock with battery ► Audio DAC with 3.5mm socket and header for output ► Options for onboard SPI flash (4MiB) and PSRAM (8MiB) ► microSD card slot ► USB host for devices like flash drives ► Optional PWM audio module ► Infrared remote receiver ► All Pico I/Os are broken out on a handy header ► Two user-controlled LEDs ► Digital Video Terminal for input and display ► Can use a Pico, Pico W or Pico 2 ► Fits in an Altronics H0192 instrument case includes three Pico microcontroller boards (MOD1-MOD3) that provide three distinct functions. MOD1 is the serial video display interface. It accepts serial data from MOD2 and interprets that according to the VT100 standard, generating video that is delivered via the HDMI socket for display on a modern monitor or TV. MOD3 is configured as a USB host supporting a USB keyboard. It receives keystrokes and sends them as VT100 data to MOD2. MOD2 works as a USB host that expects a USB-serial device to be connected. Devices like the Pico microcontroller board are recognised, as are various others. MOD2 simply channels data to and from the connected USB device, MOD1 and MOD3. The idea was to provide a video terminal with a keyboard that could interface with just about any USB/serial device. However, it occurred to us that MOD2 could be replaced by a Pico (or Pico W) board running different firmware and communicating directly with its keyboard and display controllers, turning it into a standalone computer. For example, a Pico loaded with the PicoMite firmware would turn the Terminal into a standalone BASIC computer with its own HDMI-compatible display and USB keyboard. We provided a few ideas in this vein in the Digital Video Terminal article. While many readers might be happy tinkering with such a machine, we thought it would be nice to flesh the concept out and provide plans to build such a computer. That is the idea behind the Pico Computer. Pico Computer The Pico Computer, like many of our similar projects, combines a microcontroller with a set of useful other devices. While they typically use an LCD panel as the display, in this case, it connects to a modern HDMI display device. It uses a similarly shaped PCB to the Digital Video Terminal that accepts a Pico microcontroller board. Thus, it can stack above a Terminal PCB and fit in the Altronics H0192 enclosure. While it is designed to be used with the Digital Video Terminal, it could have other applications. We are planning a project where it is used in a standalone capacity. It’s made from a mix of modules, through-hole parts and SMDs (surface-­ mounting devices). The SMDs are in SOIC or M3216 (imperial 1206) packages or larger, so it is straightforward to build as long as you have the correct tools and reasonable soldering skills. Circuit details The circuit of the Pico Computer Board is shown in Fig.3. Since it is intended to replace MOD2 in the Digital Video Terminal, we needed to provide a means to connect the two boards. A pair of 20-way headers labelled CON11, on the underside of the PCB, connects to the Digital Video Terminal where MOD2 would normally go. You can see that CON11 only connects a small subset of the available pins. There are numerous ground pins and the VBUS 5V rail so that the whole thing can be powered by a single USB connection. The Pico/Pico W/Pico2 (MOD11) connects to all the peripherals that were shown in Fig.2. Its GP0 and GP1 pins, used for the serial console, connect to CON11 to interface with the keyboard & HDMI-compatible display. Fig.2: replacing MOD2 in the Digital Video Terminal, the Pico Computer results in a single device that has all the features shown here. siliconchip.com.au Australia's electronics magazine December 2024  67 Fig.3: nearly all the Pico’s I/O pins connect to the numerous peripheral hardware devices, but most are optional. You can choose the accessories you need and access the remaining I/O pins via CON15. The 3V3EN line (pin 37) is also connected, allowing S2 on the Digital Video Terminal to reset this Pico. While pads for all 40 pins of CON11 are present, it will be sufficient in most cases to provide the three topmost pins on each side, connecting serial data, power and ground. The 3V3EN line also connects to S11, a tactile switch, which shorts this line to ground, resetting the Pico. Also available to power the shared 5V VBUS rail is CON18, a USB-C connector with two 5.1kW resistors between its CC lines and ground, as required by the USB standards. The Pico includes an onboard 3.3V buck regulator that can provide up to 68 Silicon Chip 800mA, which provides the 3.3V rail on the Computer Board. breaks out 3.3V, ground and the I2C lines for connection to external devices if desired. The pinout I2C and I2S matches a number of I2C-based modThe remaining circuitry is self-­ ules, one of which could be mounted contained within the Computer Board. directly to the Computer Board inside MOD11 pins GP2 (SDA) and GP3 the enclosure. (SCL) are configured for the I2C bus, There are also headers to suit with the two necessary 4.7kW pullup MOD12, a PCM5102A-based stereo resistors. audio DAC module. It takes power IC1 is a DS3231 or DS3231M real- from the 5V VBUS rail via a 10W resistime clock & calendar (RTCC) chip; tor, with 10μF of bypassing capaciit connects to coin cell holder BAT1, tance. This combination helps to supwhich provides battery backup for its press any noise that might otherwise timekeeping when the circuit is pow- reach the module. ered off. A 100nF capacitor bypasses The I2S digital audio data comes its 3.3V supply provided by MOD11. from the GP4, GP5 and GP6 digital outFour-way header CON14 also puts of MOD11, which are configured Australia's electronics magazine siliconchip.com.au as DIN (data), BCK (bit clock) and LRCK (left-right clock) respectively. MOD12’s configuration pins are tied to +3.3V or GND as required, and the resulting audio signals are fed to CON16, a three-way header, and CON17, a 3.5mm stereo jack socket. Data storage The Computer Board offers four options for data storage. Two SPI memory chips can be fitted as IC2 and IC3. They each have a 100nF bypass capacitor and connect to the Pico’s SPI1 peripheral. It uses the GP11 I/O pin as MOSI (master out/slave in), GP8 as MISO (master in/slave out) and GP10 as SCK (clock). IC2’s CS (chip select) pin is driven by GP7, while IC3’s is driven by GP9. There are numerous options available for these ICs, but we have chosen a 64Mbit (8MB) PSRAM chip for IC2. PSRAM stands for pseudo-static RAM; it is actually a dynamic RAM (DRAM) that has an integrated refresh controller, meaning it can be treated like static RAM (SRAM). This provides volatile storage, which is fast, but the data is lost when power is removed. We also used a W25Q32 32MBit (4MB) flash memory chip for IC3, which provides non-volatile storage. The AT25SF321B-SSHB-T is another compatible 32MBit flash chip that could be used. The interfaces are electrically identical between the flash and RAM chips, so the amounts of volatile and non-­ volatile memory can be changed to suit different applications. The other two storage options are removable. A microSD card slot (CON13) is connected to the other (SPI0) interface, which uses GP19 for MOSI, GP16 for MISO and GP18 for SCK, with GP21 wired as chip select. It has 100nF and 10μF bypass capacitors on its 3.3V rail. Digital I/Os GP26 and GP27 are wired to USB-A socket CON12 via 22W resistors, along with 5V (VBUS) and ground connections. These pins are configured in software to provide a USB host interface so a USB flash drive can be connected here, although the software could be changed to suit other USB devices. Below MOD11 are LED11 and LED12; they are be driven by the GP15 and GP20 digital outputs via 1kW series resistors. They are intended to show the status of the microSD card siliconchip.com.au and USB flash drive, but you could use them for any purpose. of the peripheral connections; they are also printed on the PCB silkscreen. Other parts Options These devices use up most of the I/O pins of the Pico, but we still had some room to fit an IR receiver (IR1), which is powered from 5V (VBUS) via a 100W resistor and 10μF capacitor for bypassing. The demodulated output connects to the Pico’s GP22 I/O pin via a 1kW resistor that limits the current into that pin if the IR receiver output is pulled up to 5V. A 28-pin header (CON15) breaks out all the Pico’s accessible I/O pins, as well as providing ground and power connections. The voltage of the power connection on this header is set by JP11 and can be either the nominally 5V VBUS rail or the regulated 3.3V rail. Table 1 provides a concise summary The Computer Board can be used with various software platforms that we’ll discuss in detail a bit later. For now, we’ll point out some important points that might be relevant as you come to construction. Not all software platforms will support all the hardware features; in particular, there is no universal support for I2S audio (eg, MMBasic does not). To this end, we have designed a small drop-in PCB module that allows the Computer Board to use PWM signals to generate audio instead. The construction and use of that module is discussed in the panel titled “A PWM audio module”. The circuit is quite similar to the PWM audio circuit used on the Pico BackPack from March Table 1: peripheral connections for the Pico Computer Board Feature I/O pins/peripherals Comments Notes Serial console GP0/GP1 (UART0) Can connect to MOD1 and MOD3 on Digital Video Terminal Check the jumpers on the Terminal I2C GP2/GP3 (I2C2) I2C RTC chip onboard, also broken out to 4-pin header. DS3231 or DS3231M IC I2S GP4/GP5/GP6 (PIO) Connects to onboard Not supported PCM5102A module with audio by the PicoMite fed to a 3.5mm socket on front panel and an internal 3-pin header PWM audio GP4/GP5 (PWM2) Custom module converts PWM signals to audio for 3.5mm socket on front panel and internal 3-pin header SPI memory GP7-11 (SPI1 and two CS pins) Connects to onboard IC2 (eg, PSRAM) and IC3 (eg, flash) microSD card GP16/GP18/GP19/ microSD card socket on the GP21 (SPI0 and one front panel CS pin) USB Host GP26/GP27 (PIO) USB-A socket on the front panel IR receiver GP22 On the front panel User LEDs GP15/GP20 Adjacent to microSD socket and USB socket, respectively I/O breakout GP0-GP22, GP26GP28, power, ground 28-pin R/A header accessible from rear panel. A separate link allows selection of 3.3V or 5V power. Power input VBUS Can be powered via the USB-C power-only socket, the microUSB socket on the Pico or via the Digital Video Terminal. Australia's electronics magazine Option of PWM audio or I2S but not both Not supported by the PicoMite 1kW series resistors (~3mA) December 2024  69 2022 (siliconchip.au/Article/15236). You may wish to use this instead of I2S audio, even if your software platform supports I2S. We suspect some readers might even find it a useful module for other projects. PicoMite BASIC does not appear to have a means of interfacing to a PIO USB host, so the CON12 USB interface will not be usable with MMBasic. You could keep the USB socket and leave off the two 22W resistors, freeing up the I/O pins and turning CON12 into a USB power-only charging port. None of the internal features are mandatory; you might even wish to simply use the Computer Board as a way of breaking out the Pico’s I/O pins at the CON15 header. JP11 must be fitted to provide power to CON15. If some features are omitted, other components can also be left off; generally, these will be the passives that are immediately adjacent to that part. For example, leaving off IC2 or IC3 K CON13 microSD CD 1kW 12 3 4 5 6 78 15 14 28 13 32 MOD11 8 7 35 6 36 5 37 4 + 38 6 PIN USB-C POWER SOCKET L 10 9 33 34 CR–1220 11 SCK BCK DIN LRCK GND VIN 31 5.1kW 5.1kW G R G 12 RP2040 MCU 3 MICRO USB–B PORT 39 40 2 1 CON18 K IR1 1kW 3 2 1 12 3 4 5 6 78 21 20 3.3V GND SCL SDA 4.7kW 26 15 14 28 13 29 31 12 RP2040 MCU 100nF IC3 18 27 30 1kW CON17 100nF 10mF SWCLK IC1 SWDIO Silicon 100nF 4C .7hip kW 1 19 CON11 RASPBERRY 17 25 PI Pico 16 24 CON14 S11 GND 22 23 A LED11 1kW CON12 22W 22W Pico Digital Video BackPack 07112234D 70 K CON13 microSD CD 4 100W 10mF A LED12 11 10 32 9 33 8 100nF IC2 CON16 L 4.7kW 26 27 29 BAT1 18 RASPBERRY 17 PI Pico 16 30 IC1 19 G R G 3.3V GND SCL SDA SWCLK 100nF 4.7kW 1 SWDIO 25 GND 24 CON14 S11 20 22 23 3.3V 21 Pico Digital Video BackPack 07112234D 3.3V 100nF 10mF CON12 22W 22W PCM5102A MOD12 V+ VBUS JP11 GP0/1: TX/RX GP2/3: SDA/SCL GP4/5/6: DIN/BCK/LR GP7/9: CS IC2/IC3 GP8/10/11: SPI1 GP16/18/19: SPI0 GP15/20: LED11/12 GP21: SD CS GP22: IR RX GP26/27: PIO USB Australia's electronics magazine 10W SCK BCK DIN LRCK GND VIN 1 3 2 4 100W 10mF A LED12 We’ll then detail the modifications that are needed for the Digital Video Terminal to allow it to connect to the Computer Board. In simple terms, this involves leaving off one of the Raspberry Pi Picos (MOD2) and all its associated parts, plus fitting headers to suit. Finally, we’ll assemble all these parts together into the enclosure. Combining the Computer Board with the Digital Video Terminal requires the tallest enclosure of that series, the Altronics H0192. Start by fitting the Computer Board PCB (coded 07112234 and measuring Construction 68 × 98mm) with the surface-mounting We’ll start by working through the components that are needed, using the assembly of the Pico Computer Board, PCB overlay diagram (Fig.4) as a guide. since there may be readers who wish We will mention all parts; simply skip to build it as a standalone device. By any you do not require. itself, it should comfortably fit in the We recommend you have on hand larger of the two Altronics cases that flux paste, solder-wicking braid, tweewe used for the Digital Video Termi- zers and a fine-tipped soldering iron nal (Altronics H0191). (or medium if you’re more experienced and prefer it). A magnifier and good illumination will be helpful, and K A GP0/1: TX/RXproper ventilation is mandatory so you LED11 GP2/3: SDA/Sdon’t CL inhale flux fumes. GP4/5/6: DIN/BCK/LR The microSD card socket (CON13) G P 7 / 9 : C S I C 2 / I C 3 C O N 1 7 1kW GP8/10/11: SPI1 and USB-C socket (CON18) are the CON16 100nF GP16/18/19: SPI0 most challenging to solder, so start IC3 GP15/20: LED11/12 GP21: SD CSwith them. Apply flux to the PCB pads V+ GP22: IR RX and rest the components in place. The 100nF V B U S PCM5102A GP26/27: PIO UScard B IC2 JP11 socket has locating pegs on its MOD12 underside, while the USB-C socket will not and will require a bit more 10W care in its placement. Clean the iron’s tip and apply some 10mF fresh solder, then tack one lead. Check that the connectors are flat and within their marked pads, adjusting if necessary, then solder the remaining leads. Use extra flux and the braid to draw away any excess solder or solder bridges. You could also add some extra solder to the shell of both connectors to give mechanical strength. Follow by installing the three ICs. IC1 can be in the 8-pin narrow SOIC package (DS3231M) or a wider 16-pin SOIC package (DS3231). In both cases, its pin 1 marking must be in the GP0 GP1 GP2 GP3 GP4 GP5 GP6 GP7 GP8 GP9 GP10 GP11 GP12 GP13 GP14 GP15 GP16 GP17 GP18 GP19 GP20 GP21 GP22 GP26 GP28 GP27 V+ GND CON15 IR1 1kW means that the corresponding 100nF capacitor can be omitted too. The two resistors and 10μF capacitor below IR1 are only needed if it is fitted. Similarly, the 100nF and 10μF capacitor next to the CON13 microSD card socket are only needed if it is installed. The 100nF capacitor near IC1 is only needed if it is fitted, although the two 4.7kW resistors should be kept if you intend to connect anything to the I2C bus at CON14. The two passives above MOD12 are needed only if an audio module is fitted. 10mF Figs.4 & 5: we have used slightly larger M3216 (imperial 1206) SMD parts in this design since there was plenty of room. Check these overlays and the photos to confirm how the parts are fitted, especially CON11, since it needs to align with the header on the PCB below. siliconchip.com.au upper-left corner. The narrower part should be fitted to the upper eight pins. Make sure not to mix this up with the other ICs, which will also be 8-pin SOIC parts. If you cannot see a pin 1 marker, there will also be a bevel along the edge belonging to pin 1; it is best viewed from the end of the IC. Apply flux, place the part, tack one lead and check that the part is flat and square before soldering the other pins. We recommend fitting a PSRAM chip for IC2 like we did. It will be noticeably narrower than the Winbond flash memory chip, but both will fit on either sets of pads for IC2 and IC3. Solder these like the other parts, observing the correct orientations. Follow with the seven capacitors. They won’t be marked, but the 10uF parts should be quite a bit thicker than the 100nF parts. Add flux paste, tack one lead, check the position and then solder the other lead. You can also go back and touch the iron on the first lead to refresh the solder joint. There are 11 resistors of various values; solder them as per Fig.4. Coin cell holder BAT1 is the last SMD part. Rest it in place, being sure that the opening faces the edge of the PCB. Tack one lead, adjust, then solder the other. It’s also worth adding a decent fillet to each side to give it mechanical strength. Use this opportunity to thoroughly clean away any excess flux from the PCB using your solvent of choice. Your flux’s data sheet might suggest a specific solvent, but isopropyl alcohol or Chemtools Kleanium G2 work well in most cases. Dry the board and examine it thoroughly for solder bridges, dry joints or pins that are not connected to the pad below. It will be easier to fix these now, before any other parts are fitted. Through-hole parts Now we can start on the throughhole parts. Some of these show through the front panel, so you can use the front panel PCB to check that they are correctly aligned, although they should both simply snap into place. Fit the USB-A socket CON12 now, checking that it is flat against the PCB before soldering its pins. Add a good amount of solder to the shell pins to give it strength. CON17, the 3.5mm socket, sits along this edge too, so solder it in place next. siliconchip.com.au Kits available for this project To build the project as shown in our photos, you should purchase the SC6917 and SC7374 kits from us and the enclosure from a retailer such as Altronics, plus accessories like the USB keyboard, HDMI monitor and appropriate USB cables. When ordering the kits, you may want to also get the optional PSRAM chip (SC7377) and PWM Audio Module kit (SC7376) if you don’t plan to use the included DAC module. The Pico Computer Board kit can be used standalone, although you will have to purchase an enclosure and arrange your own panels. In this case, you can order the SC7378 kit ($50 + P&P), which is the same as the SC7374 kit plus an unprogrammed Pico. Pico Computer Board Kit (SC7374, $40) ___________________ This kit contains the PCB (07112234) and almost all the parts needed to fully populate it (except the PSRAM & RTC chip, available separately, and coin cell). It also includes the new front panel and the hardware needed to connect the PCB to the Digital Video Terminal (which is already available as a kit, see blow). The SC7374 kit does not include a Raspberry Pi Pico because the SC6917 kit has three. The Pico Computer Board fitted with all parts except the I/O breakout headers at bottom right. Note the silkscreen guide at top right for the GPIO pinouts. The only parts needed on the underside of the Computer Board are pin headers to connect it to the Digital Video Terminal underneath. Leave these off when using the Computer Board on its own. Pico Digital Video Terminal Kit (SC6917, $65) ______________ Includes everything to build the Digital Video Terminal, except the case. The Raspberry Pi Picos are supplied unprogrammed. For building instructions, see the original article in the March & April 2024 issues (siliconchip.au/Series/413). PWM Audio Module Kit (SC7376, $10) _____________________ The PWM Audio Module is available as a kit with all parts listed in the panel, including the PCB. Australia's electronics magazine December 2024  71 The infrared receiver and LEDs also show through the front panel, so use the mounted components and front panel as a jig to align them correctly. Bend the LED leads right behind the body by 90°, being sure to bend the correct way to align the cathode with the K marks on the PCB. As well as being short for the German word “Kathode”, the letter K also looks like the cathode end of the LED symbol. Locate each component into the front panel and tack one lead wherever is convenient. Gently bend the leads slightly to achieve your desired placement. The IR receiver might need to be kinked forward slightly to show through the hole in the panel. Once you are happy with the front panel components, solder the remaining leads and trim the excess from the underside. Slot switch S11 into place and solder it as well. You will only need a pair of fourway headers to connect the Computer Board to the Digital Video Terminal, but we decided to use six-way headers since the corresponding stackable headers are commonly available in a six-way part. If you plan to use the Computer Board as a standalone board, these headers are not needed. Otherwise, fit them to the underside of the PCB, closest to the Pico’s USB socket. You should check this carefully in the photos and make sure they are mounted squarely. Another option is to run insulated wires for the handful of lines that are needed: ground, VBUS, GP0 and GP1. These are pins 1, 2, 3 and 40 on the Pico footprint. We’ve specified low-profile headers for mounting the Raspberry Pi Pico (MOD11), as it will be too tall for the intended enclosure if it is fitted on standard 8.5mm-tall socket headers. There is also the option of soldering it directly (with header pins to clear CON11) to the PCB, although you will not have access to the CON11 headers on the underside of the PCB after doing that. So, if you plan to hard-solder MOD11, ensure that you have the CON11 headers in place first. Then rest the header pins in place and tack a few pins on both the PCB and Pico, check their alignment, then solder all pins as needed. If you plan to fit low-profile headers, 72 Silicon Chip A PWM Audio Module The PicoMite has long supported PWM audio, but there doesn’t appear to be any support for I2S audio using a higher-quality external DAC. PWM audio involves supplying a pulse-width modulated signal of varying duty cycle. An external circuit filters and buffers the signal, converting it to an analog voltage so it can be fed to the headphones, an external amplifier or other devices. Many 8-bit microcontrollers will have no trouble generating PWM audio, so this module could also be an attractive option for anywhere that a cheap and simple audio output is needed. It’s cheap for a reason, though; the audio will not be as clean as that from an I2S DAC. Still, it will be good enough for many purposes. This module is designed to fit in the same footprint as the popular PCM5102A DAC modules. Instead of expecting I2S (serial) data, it receives a PWM signal on two of the pins, one for each channel. The circuit (Fig.a) is much the same as the one we used on the Pico BackPack from March 2022 (siliconchip.au/Article/15236), with a minor alteration to allow it to run from a single 5V supply. The external connectors match the pin locations of the PCM5102A module, allowing this module to be mechanically equivalent. 5V power comes in on the Vcc and GND pins and feeds directly to dual low-­ voltage rail-to-rail op amp IC1, bypassed by a 100nF capacitor. A low-pass filter comprising a 10kW resistor and 10μF capacitor provides a biasing rail, VF; its DC level is set to around 4V by other biasing components downstream. The two PWM signals come in on pins 4 and 5 of the six-pin header (L_IN & R_IN). Each are treated identically as they pass through the filter and buffer circuitry, so we will look at one channel only. Assuming an average 1.65V level (as expected for 50% duty cycle PWM from a 3.3V microcontroller), the biasing and filter circuit consisting of the 47kW and 22kW resistors and 1nF capacitor brings the level closer to the middle of the op amp’s range. They also attenuate the higher frequency elements of the squareedge PWM artefacts. Fig.a: this simple circuit filters and buffers two PWM signals from the Pico to provide a basic stereo audio output. You could also connect it to an Arduino or other microcontroller; the output should be able to drive headphones or a small speaker. as recommended, solder the corresponding pin headers to the Pico. You can then use them as a jig to ensure that the low-profile header sockets are square and aligned as you solder them to the PCB. MOD12 should be mounted directly Australia's electronics magazine to the PCB or on low-profile headers only. We opted to solder it directly to the PCB, even though that blocks off access to one of the mounting holes. The remaining headers are optional, and you can fit socket or pin headers to suit your purposes. The demonstration siliconchip.com.au There are just a few resistors and capacitors on the top side of the PCB. Take care with the orientation of IC1. We used standard pin headers, which can be soldered directly to the other PCB or plugged into socket headers. The op amp is configured for unity gain, so it simply buffers the signal that reaches its inputs, while the 10μF capacitor, 100W resistor and 100kW resistor provide AC coupling and biasing to ground. The output is suitable for driving headphones or an external amplifier. As long as the supply voltage suits the op amp and is not less than the incoming PWM amplitude, we expect the circuit will work fine. For example, a 5V PWM signal will work with a 5V supply. Those will with some expertise might tweak the component values to suit their application. Assembly The parts are SOIC and M3216 (imperial 1206), so you will need the standard surface-mount assembly tools (see the construction section in the main article). The top of the PCB is populated with pairs of components that are mirrored across channels, so each silkscreen marking corresponds to the two adjacent passive components. The PCB overlay diagrams shown in Fig.b depict the placement of the components. Work through them on the top side, taking care with the capacitors, since they will not be marked. Flip the PCB over and carefully align and solder the solitary IC, being sure to match the edge bevel to the silkscreen marking, then fit the remaining components on this side. Clean the PCB with an appropriate solvent and dry thoroughly. Solder headers to suit the application. The PWM Audio Module can now be fitted to the Computer Board PCB for testing. We have provided sample code in the PicoMite BASIC examples to use this module. Fig.b: assembly of the module is straightforward. The main thing to watch out for is to avoid mixing up the unmarked capacitors with different values. Parts List – PWM Audio Module 1 double-sided PCB coded 07112238, 32 × 17mm 1 3-way header, 2.54mm pitch (for audio output) 1 6-way header, 2.54mm pitch (power and signal inputs) 1 MCP6002 or similar low-voltage rail-to-rail op amp, SOIC-8 (IC1) 3 10μF M3216/1206 size 10V X7R SMD ceramic capacitors 1 100nF M3216/1206 size 50V X7R SMD ceramic capacitor 2 1nF M3216/1206 size 50V X7R, C0G or NP0 SMD ceramic capacitors 2 100kW SMD M3216/1206 size ¼W 1% resistors 4 47kW SMD M3216/1206 size ¼W 1% resistors 2 22kW SMD M3216/1206 size ¼W 1% resistors 3 10kW SMD M3216/1206 size ¼W 1% resistors 2 100W SMD M3216/1206 size ¼W 1% resistors software does not need to connect to any external circuitry apart from the likes of a microSD card, USB flash drive or headphones. You can even test the Computer Board without the Digital Video Terminal; as long as you have a serial siliconchip.com.au terminal program you can use to view the output and enter commands. Building the Digital Video Terminal If you are connecting the Computer Board to a Digital Video Terminal, a Australia's electronics magazine few minor changes are needed for the Terminal. It’s helpful to refer to the original Digital Video Terminal articles (March-April 2024; siliconchip. au/Series/413), although experienced constructors should get by following the silkscreen markings. The MOD1 and MOD3 Picos will need to be mounted on low-profile headers or directly to the Digital Video Terminal (07112231) PCB, since fullheight headers will be too tall and interfere with the Computer Board. You can load the firmware onto MOD1 and MOD3 before installing them. Be sure to load 0711223A.UF2 onto MOD1 and 0711223C.UF2 onto MOD3. To load the firmware, hold the BOOTSEL button on the Pico while connecting it to your computer, then copy the firmware UF2 file to the RPI-RP2 virtual drive that the Pico provides. Apart from omitting S2, MOD2, CON2, LED2, and the adjacent 1kW and two 22W resistors, most of the assembly proceeds without changes. Refer to our photo of the assembled Terminal; note that we’ve also left off some of the other components that we are not using. We used stackable headers to allow a 15mm spacing between the PCBs. You will need to have the headers fitted to the underside of the Computer Board PCB (07112234) to complete the alignment. Slot the stackable headers onto the headers on the underside of the Computer Board PCB, then temporarily fix the two PCBs together using 15mm spacers and machine screws. This will set the right spacing and align the boards squarely. The tips of the stackable headers should protrude through the matching MOD2 holes in the Terminal PCB. You can also check that the front panel PCB aligns with all the sockets that it presents on both boards. Solder the tips of the stackable headers to the Terminal PCB, then trim them to a neat length. Remove the screws and detach the two boards. To test the Terminal, power it from the USB-C socket (or one of the microUSB sockets if you have no USB-C cables). The LEDs onboard MOD1 and MOD3 should light up any time the board is powered; this shows they are running their firmware. Connecting a USB keyboard to CON3 should cause December 2024  73 LED3 to light up, while typing on the keyboard should make LED3 flicker. Plug your HDMI monitor or display into CON1 and check that LED1 lights and that you can see a flashing cursor in the top-left corner of the connected display. Place a single jumper on LK1, connecting pins 2 and 3 and matching the INT markings on the silkscreen. TERMINAL BACKPACK PICOMITE DEMO I2C2 DEVICE SCAN x0 x1 x2 x3 x4 x5 x6 x7 x8 0x .. 1x .. .. .. .. .. .. .. .. .. 2x .. .. .. .. .. .. .. .. .. 3x .. .. .. .. .. .. .. .. .. 4x .. .. .. .. .. .. .. .. .. 5x .. .. .. .. .. .. .. .. .. 6x .. .. .. .. .. .. .. .. 68 7x .. .. .. .. .. .. .. .. .. 1 DEVICES FOUND x9 .. .. .. .. .. .. .. If all is well, power off and detach the Digital Video Terminal, then plug the Computer Board into the Terminal and reconnect the keyboard and monitor. Demonstration software You can try the two software demos quite easily thanks to the Pico’s xA .. .. .. .. .. .. .. xB .. .. .. .. .. .. .. xC .. .. .. .. .. .. .. xD .. .. .. .. .. .. .. xE .. .. .. .. .. .. .. xF .. .. .. .. .. .. .. Screen 1: the PicoMite BASIC example (seen here in the TeraTerm serial terminal IC2 ID:&H00000C0D. IC2 IS ESP PSRAM IC3 ID:&HEF401600. IC3 IS WINBOND 25Q32 program) scans for devices and READY displays what it finds. Various A:/>ir commands can be used to interact WAITING FOR IR SIGNAL with the PicoMite’s internal file PRESS ANY KEY TO EXIT system or that of a microSD card. Received device = 255 key = 162 Starting Pico Digital Video Terminal BackPack SD OK A: SD card root has 32 files totalling 171393622 bytes. USB MSC OK B: USB MSC card root has 50 files totalling 175713 bytes. RTC found RTC started OK Time is 14:19:46 on 2/9/2024 Screen 2: This display is produced IC2 ID is 0xD. by the Pico Computer’s Arduino IC3 ID is 0xEF401600. demo software on the Digital Video Audio started OK Terminal. It provides a status report I2C scan: and also provides commands to 0x68 I2C scan done. access the included hardware. A:/> ir Waiting for IR signals Press any key to exit Protocol=NEC Address=0x0 Command=0x16 Raw-Data=0xE916FF00 32 bits LSB first Unknown IR Signal Protocol=NEC Address=0x0 Command=0x5E Raw-Data=0xA15EFF00 32 bits LSB first Protocol=NEC Address=0x0 Command=0x5E Repeat gap=40000us A:/> tone Playing tone. Done. A:/> ▇ MPY: soft reboot MicroPython v1.23.0 on 2024-06-02; Raspberry Pi Pico with RP2040 Type “help()” for more information. >>> #Serial console >>> import uos >>> from machine import UART, Pin >>> repl_uart = UART(0, baudrate=115200, tx=Pin(0), rx=Pin(1)) >>> uos.dupterm(repl_uart, 0) >>> >>> #I2C >>> from machine import Pin, I2C >>> i2c = I2C(1, scl=Pin(3), sda=Pin(2), freq=100000) >>> i2c.scan() [104] >>> ▇ Screen 3: These commands can be used with MicroPython to configure it for use with an integrated Digital Video Terminal. They redirect the console to a serial terminal as well as the virtual USB serial port. It’s also straightforward to run an I2C scan, showing the RTC chip at address 104 (0x68) 74 Silicon Chip Australia's electronics magazine bootloader. Connect the Pico Computer to a computer using the microUSB socket on the Pico. If you do not see the RPI-RP2 virtual drive, hold its BOOTSEL button while pressing and releasing S11. You can then copy the UF2 file to the virtual drive. You can view the operation of the software either from a serial terminal program connected to the Pico’s virtual USB-serial port, or using the keyboard and monitor connections of the Terminal. The USB-serial port name or number might change due to the way that different operating systems handle these things. The source code (and other code such as libraries and BASIC OPTIONs) is also available in the software download package at siliconchip. au/Shop/6/528 To try out the external features, you will need to connect appropriate devices, like a USB flash drive or microSD card. These should be FAT formatted (FAT16 or FAT32, although the latter is more standard these days) and inserted before powering on the hardware. Connect some headphones to CON17 to try out the audio. We recommend not connecting an amplifier until you are sure that the audio is working properly. The sections below will detail which features are supported and what to expect. There are several firmware (UF2) files in the UF2 folder of the software downloads, including the three files for the Digital Video Terminal and “flash_ nuke.UF2”, which can be used to completely wipe a Pico’s flash memory. PicoMite BASIC The PicoMite BASIC (MMBasic) example demonstrates most of the available peripherals. There are several OPTIONs that can be configured from the BASIC prompt, plus a demonstration program that has its own interactive command prompt. The “Terminal BackPack BASIC. UF2” file is configured with PicoMite BASIC, the required OPTIONs and the BASIC program. You can load it using the RPI-RP2 bootloader and immediately try it out using a keyboard and monitor attached to the Pico Computer. Once the PicoMite firmware is loaded, it should flash the Pico’s onboard LED. Alternatively, the PicoMite BASIC UF2 file, OPTIONS.BAS and BASIC_ siliconchip.com.au DEMO.BAS files can be individually loaded and edited as needed. Note that the AUDIO option (for PWM audio) is configured; you will want to disable that if you have the I2S DAC module fitted instead. The demo starts by running an I2C scan and the RTC chip should be found at address 0x68, assuming it is fitted. The memory chips are also interrogated for their IDs. Screen 1 shows the boot sequence followed by the IR command, which displays codes received by IR1. The available commands can be listed by entering the HELP command. The TONE command will play audio (if the PWM audio module is fitted), while accessing B: drive allows you to examine the microSD card contents. The A: drive is an internal drive stored in the Pico’s flash memory. The PicoMiteV5.08.00.UF2 file is the same file we installed before applying the necessary OPTIONs and loading the program file. It is the current version available from https://geoffg. net/picomite.html at the time of writing this. Arduino demo The Arduino demo is loaded in similar fashion with the “Terminal BackPack Arduino.UF2” file. The source code for this, along with the libraries we used, can be found in the Arduino folder. There are other libraries that are included with the Pico Arduino board profile. More information about the board profile and its integrated libraries (including those for I2C, SPI and I2S) can be found at https://github.com/ earlephilhower/arduino-pico There is also a PWMAudio library, which should work with the PWM audio module. The Arduino demo is similar to that for PicoMite BASIC, but offers a different set of features. The A: drive refers to the microSD card, while the B: drive is the USB flash memory. The interface is meant to resemble a command prompt, although the commands are not comprehensive, but rather intended to be a simple demonstration of the hardware features. The HELP command lists the available commands. Screen 2 shows the output of this demo. The first 14 lines are automatically produced after it boots up, while the remaining lines show the IR and siliconchip.com.au Parts List – Pico Computer 1 modified Digital Video Terminal (see below) 1 Pico Computer Board (see below) 1 black front panel PCB coded 07112235, 37 × 99mm 2 4-way, 6-way, 8-way or 10-way stackable headers (CON11) 2 4-way, 6-way, 8-way or 10-way headers (CON11) 1 105 × 80 × 40mm Hammond RM2005LTBK or Multicomp MP004809 ABS instrument case [Altronics H0192] 1 micro-USB cable for power, communication and programming 4 15mm-long M3 untapped spacers 4 20mm-long M3 panhead machine screws Pico Computer Board parts 1 double-sided PCB coded 07112234, 68 × 98mm 1 Raspberry Pi Pico microcontroller module (MOD11) 2 20-way low-profile socket headers (for MOD11) [Adafruit 5585] 2 20-way low-profile header strips (for MOD11) 1 PCM5102A DAC module (MOD12) OR 1 PWM audio module (see panel) 1 1220 surface-mounting coin cell holder (BAT1) [BAT-HLD-012-SMT] 1 CR1220 Lithium coin cell (BAT1) 1 USB Type-A through-hole right-angle socket (CON12) 1 SMD microSD card socket (CON13) [Altronics P5717] 1 4-way 2.54mm pitch socket header or pin header (CON14; optional, for I2C breakout) 1 2×14-way 2.54mm pitch right-angle header (CON15; optional, for I/O breakout) 1 3-way 2.54mm pitch pin header (CON16; optional, for audio) 1 3.5mm stereo jack socket (CON17) [Altronics P0094] 1 USB-C power-only SMD socket (CON18) [GCT USB4135 or similar] 1 3-way 2.54mm pitch pin header and jumper shunt (JP11; optional, for I/O breakout) 1 6mm through-hole tactile pushbutton switch (S11) 4 small self-adhesive rubber feet Semiconductors (all optional) 1 DS3231 (wide SOIC-16) or DS3231M (SOIC-8) real-time clock & calendar (IC1) 1 64Mbit SPI PSRAM, SOIC-8 (IC2) [SC7377, Adafruit 4677, ESP-PSRAM64H or similar] 1 32Mbit SPI flash memory, SOIC-8 (IC3) [Winbond W25Q32JVS, AT25SF321B-SSHB-T or similar] 1 3-pin infrared receiver (IR1) 2 3mm green through-hole LEDs (LED11, LED12) Capacitors (all SMD X7R, M3216/1206 size) 3 10μF 10V 4 100nF 50V Resistors (all ¼W SMD M3216/1206 size, 1%) 2 5.1kW 2 4.7kW 3 1kW 1 100W 2 22W 1 10W Parts for modified Digital Video Terminal 1 double-sided PCB coded 07112231, 98 × 68mm 1 Raspberry Pi Pico programmed with 0711223A.UF2 (MOD1) 1 Raspberry Pi Pico programmed with 0711223C.UF2 (MOD3) 1 HDMI-compatible socket (CON1) [Stewart SS-53000-001] 1 USB-A through-hole right-angle socket (CON2) 1 USB-C power-only SMD socket (CON4) [GCT USB4135 or similar] 3 6mm through-hole tactile switches (S1-S3) 4 2-pin headers, 2.54mm pitch (JP1-JP4) 1 4-pin header, 2.54mm pitch (LK1) 5 jumper shunts (JP1-JP4 & LK1) 4 20-way pin headers, 2.54mm pitch (for MOD1 & MOD3) 4 20-way low-profile header sockets (optional; for MOD1 & MOD3) Semiconductors 2 2N7002 SMD N-channel Mosfets, SOT-23 (Q1, Q2) 2 green 3mm through-hole LEDs (LED1, LED3) Resistors (all ¼W SMD M2012/0805 size, 1%) 6 10kW 2 5.1kW 2 1kW 8 270W Australia's electronics magazine 2 22W December 2024  75 TONE commands. The TONE command assumes an I2S DAC is fitted and will not work with the PWM module. During operation, the LED on the Pico should be lit, as should that on the I2S DAC module. MicroPython and C SDK When used with the Pico Computer Board, the Digital Video Terminal only needs to be partially populated, as shown here. Set LK1 to the INT position. All of these photos have been shown enlarged for clarity. Since we do not use MicroPython much, we have not deeply investigated its usage with the Pico Computer, although we were able to work out some basics such as duplicating the USB-serial terminal to the connected Terminal hardware and running an I2C scan. There is a MicroPython folder in the software downloads with some brief notes and sample code to get you started. That includes links to recommended libraries, along with a copy of the MicroPython UF2 file we tested. Screen 3 shows the Digital Video Terminal being configured, followed by an I2C scan identifying the RTC chip at address 104 decimal (0x68 in hexadecimal). Note that you will have to run the first command to configure the terminal from the USB serial port, since that is what makes the hardware serial port available. Subsequently, the USB keyboard and HDMI monitor attached to the Terminal can be used to interact directly with MicroPython. We have not created any demonstrations using the C SDK. There are not many integrated high-level libraries for the peripherals on the Pico Computer, so we have focused our attention on the Arduino code (which is based on the C SDK anyway). Using a Pico W or Pico 2 Take your Digital Video Terminal to the next level by adding a real-time clock, multiple storage facilities and stereo audio. The Pico Computer is the perfect basis for a custom computer project. Although we have not tested them, the examples presented here should work fine with a Pico W in place of the Pico. However, note that the LED on the Pico W is driven differently, so it will probably not light up. None of the Pico Computer peripherals depend on the WiFi or Bluetooth features the Pico W provides. At the time of writing, the Pico 2 has just became available, with a much faster processor and twice as much RAM. We performed some quick tests by recompiling the Arduino code for the Pico 2 and uploading it to the Pico Computer. Everything seemed to work as expected, so if you’re after a more powerful computer, the Pico 2 may be for you! Australia's electronics magazine siliconchip.com.au 76 Silicon Chip Completion Disconnect all the cables and fit a 1220 cell in BAT1 if you have fitted an RTC chip. Slot in the front panel PCB and secure the two boards into the base of the enclosure by threading machine screws through the top PCB, spacer, then bottom PCB and into the enclosure’s lower half. If the I2S DAC module is permanently affixed and blocking that hole, a short screw can be used to affix the lower PCB directly to the enclosure. We have not designed a rear panel, since there are a few options for which sockets to use. If you don’t need access to any of the Pico USB connections or the rear USB-C socket, no rear panel holes are needed and the Pico Computer can be powered from the front panel USB-C socket. If you need access to the CON15 I/O breakout header, you might decide to leave the rear panel off completely. In that case, you should glue a small piece of plastic to the enclosure to restrict access to the coin cell. You will need to have the case fitted to ensure that the coin cell is inaccessible. The Pico Computer is not a toy, so it should be kept away from children in any case. Since the included panels are translucent, you could easily mark them by eye and then cut the required holes out. One option is to drill 3mm holes at each end of the desired cut-out, then This shows how the two PCBs are stacked. We recommend mounting the modules on low-profile headers or directly to their respective PCBs. 15mm spacers separate the two PCBs. You could use a different height but 15mm is required to match our front panel PCB. use a sharp knife or files to remove the remainder of the plastic. Now you can affix the top half of the enclosure using the included screws and reattach any necessary cables and accessories. Standalone use If you are using the Computer Board PCB without the Digital Video Terminal, it can mount directly to the base of any enclosures in the series we are using (Altronics H0190, H0191 or H0192). Which you choose depending on the height of the assembled PCB. Figs.6 and 7 are panel cutting diagrams for this scenario. We expect that readers doing this will have a specific project in mind that might create other panel requirements, so we have not created a panel PCB for this use case. Note that the heights of the LEDs and IR receiver could vary, depending on how exactly you solder them. The heights shown match the PCB panel. Conclusion Fig.6 & 7: these panel cutting diagrams are for using the Pico Computer PCB on its own without connecting it to the Digital Video Terminal PCB. If using them combined, either move all the cut-outs up by 16.6mm (15mm for the spacers and 1.6mm for the PCB thickness), or use our PCB-based front panel and save yourself the effort. siliconchip.com.au Australia's electronics magazine The Pico Computer is a great way of adding numerous features to the Pico Digital Video Terminal. It’s also a handy combination of accessories that work well with the Pico, meaning that it will have numerous applications on its own. Combined with the Digital Video Terminal, the Pico Computer has the potential to become a very interesting standalone computing device. Those skilled in programming may be interested in porting an emulator to the hardware or even writing a standalone operating system. We plan to produce another project using this hardware, and we look forward to seeing what devices other readers create. SC December 2024  77