The PicoBuddy Computer project is an old-style Boot-To-Basic computer in the same vein as the plethora of machines from the late 70’s/early 80’s. It also set out to be to be a fun, possibly even educational, project for my kids to build.

The PicoBuddy Computer has been designed as a platform supporting the excellent PicoMite BASIC firmware for the Raspberry Pi Pico (https://geoffg.net/picomite.html).

The PicoBuddy Computer hardware project set out with the following design aims:

- Low-cost to build. The target was to be able to build a computer from a kit for the same price or less as the original ZX81 kit, £49.99. This meant using readily available components without relying on specialised or short life-expectancy parts. It also meant choosing as few different parts as possible so that instead of buying twenty different values of resistor, the builder would only need to buy multiples of the same component to benefit from price breaks where possible.

- Simple to build. The target was to make the build possible with only basic soldering tools and skills, and as uncomplicated as I could make it (e.g. using through-hole components wherever practical). The components had to be distinct for ease of assembly (e.g. resistors should avoid similar colour codes) and the orientation of polarised components should be strictly controlled on the PCB to aid visual inspection.

- Multiple applications. The computer should support as many options for expansion as could be managed within the limits of the price constraint. I kept in mind the BBC Micro, which was designed to maximise its “usefulness” in real-world and educational settings by having a huge number (for the time) of interfaces built-in.

- Multiple power options. The machine should be able to run from a variety of power sources. It also needed provide options for improved internal supplies e.g. to minimise noise and EMI and to remove supply “hiss” when generating audio.

- Flexible implementation. Whilst including the additional hardware options, the machine should not be reliant on them, so that it can be assembled with a minimal configuration (i.e. just a keyboard and VGA) as described in the PicoMite Basic firmware documentation.

- Simple installation. None of the additional hardware options should rely on alterations being made to the stock PicoMite Basic firmware. They should work either with the firmware out-of-the-box, or through the use of simple BASIC routines that can be included in user’s programs

The design files contain assembly details, some simple code samples and build drawings.

PicoBuddy Computer specifications

• Based on the Raspberry Pi Pico 2040 microcontoller board running PicoMiteVGA Basic
• Powered supply options:
• +5 V connected to the Pico USB
• +5 V connected to the 2.1 mm barrel jack (reverse and over-voltage protection).
• >5V connected to the 2.1 mm barrel jack (reverse and over-voltage protection) using onboard regulator.
• Selection of onboard SMPS or linear regulators to optimise efficiency/noise
• 2 x I2C busses with STEMMA QT connectors and/or 4-pin headers. Each bus can be independently set to 5 V or 3.3 V operation
• PS/2 keyboard input
• Micro--SD card slot
• 5 x digital I/O
• 3 x buffered analogue-to-digital inputs, plus buffered 3.0 V reference supply
• Stereo PWM output to headphones, or to 8 Ohm speakers via class A/B amplifiers
• VGA output, two colour palettes
• Onboard HD4487-compatible LCD interface
• Onboard 8 x digital I/O

Have fun!