Keyboard

sbc3: small computing environment

Self contained board that can be easily programmed on target for fun little games or GUIs. Inspired by the environment presented by TI calculators and TI-BASIC.

Directory structure

sbc3
|_ art: various images for putting on PCB and for on-screen graphics
|_ cubemx: STM32 CubeMX projects for pinmapping
|_ docs: datasheets etc
|  |_ cpu_hardware: hardware support and layout for STM32s
|  |_ hardware: various datasheets for parts used
|  \_ sofware: CPU ref manuals etc
|_ img: images for docs
|_ keyboard_v1: kicad project for keyboard
|_ main_board_v1: kicad project for main board
|_ rust_firmware: Firmware and screen driver in Rust (abandoned for now)
\_ software: (mostly c++) software and firmware
   |_ cmake: cmake scripts
   |_ firmware: firmware for main_board_v1
   |  |_ app: main application sources
   |  \_ (other dirs): supporting libraries unchanged from CubeMX/FreeRTOS upstream
   \_ vm: virtual machine
      |_ analysis: jupyter notebook for vm stat calculations
      |_ engine: main vm sources (portable)
      |_ pc_port: raylib port of VM for desktop
      |_ programs: sbc3 assembly programs
      |_ tests: vm unit tests (lol)
      |_ vscode-extension-sbcs-syntax: syntax support in VSCode for SBC3 assembly
      \_ as2.py: sbc3 vm bytecode assembler

Components

main_board_v1

System diagram

PCB

Manufactured

• PCB and assembly done by JLCPCB
• Extended parts used:
  • TLV320DAC3101IRHBR Audio DAC
  • 475900001 Micro-USB connector
  • LD1117S33TR 3V3 Linear Regulator
    • Honestly can't remember why I went for this one instead of the JLC preferred part AMS1117-3.3. Maybe max current
  • LD1117S18TR 1V8 Linear Regulator (for DAC)
  • STM32F405VGT6 MCU

Changes and ideas for v2

• Ability to disconnect 3v3 from rs485 connectors
• Isolation transformers on rs485 for long connections
• External flash or eeprom or FRAM for filesystem
• "Cartridges" with flash
• Label all the headers pin for pin to avoid looking on the schematic every time you're connecting the SWD
• Fix BOOT0/BOOT1? Or just 0ohm unpopulated for de-bricking?
• Proper enclosure
• ESD diode protection on all inputs
• External qspi flash (w25q128jv)
• Wire OE on LED shift registers to a PWMable pin! So you can easily pwm the brightness
• 18650 batteries + holder?
• smaller form factor with parts hidden underscreen and less rs485 or more compact layout (look for resistor/cap sizing for 0402 wattage)
• ACTUALLY WIRE UP THE BUTTONS AND ENCODER THIS TIME!
  • Of all the things to screw up on the first PCB run, I didn't expect to completely forget to connect the button and rotary nets
• Tiny JST connectors really are shit! Hot pluggable trs circuit? Enable/disabable power chip? Need to get this right before making a bunch of peripheral boards.

keyboard_v1

System diagram

PCB

Not yet manufactured, since still thinking about re-doing the comms (definitely not going to use the current JST connectors as theyre much too finicky).

virtual machine

Also included is a stack based virtual bytecode machine (see software/vm). The bytecode can call out to pre-defined system modules, including graphics routines. Below is an example of the main board STM32 and a Linux desktop running the same bytecode program.

STM32

Linux desktop

As the bytecode is stack based, a thin forth layer has been built for it, which is currently assembled by a python script. See the source program for the above gifs here.