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Bootloader for Ben Eater's 8-bit Computer

Because I'm sick of DIP switches.

When powered on, this connects to the Wi-Fi, pulls down a program, loads it into RAM, hits reset and (optionally) starts the clock.

This code makes a couple assumptions:

  • All address and data DIP switches are set to one.
  • RAM program mode is disabled.
  • The clock is in manual pulse mode, or that you're using Dawid Buchwald's modified clock selector circuit. The auto-start logic only works with the modified selector circuit.

Status LEDs show completion of boot, Wi-Fi association, download and program load steps.

One of my goals was to preserve the manual program mode. If that's not important to you, you could replace the DIP switches with a pair of 74LS164 shift registers and output serially from the ESP32. That would greatly simplify the wiring (six connections from the ESP32 vs 16) and let you use a more compact breakout board.

Hardware

I used the HiLegto ESP-WROOM-32 board. It exposes all of the ESP32's GPIO pins, and is narrow enough to fit on a breadboard with one row of holes on each side. Besides that, there's nothing special about this particular board. The code should work anywhere. But be aware that the mapping of GPIO pin to breakout board pin isn't standard. If you use another board you won't be able to copy my wiring.

Software

Program files can be up to 16 bytes in length, big-endian. If the file is less than 16 bytes, the remaining memory addresses will be initialized to zero.

The stupid-simple 8sm assembler will generate files in the right format. See fibonacci.8sm for an example input file.

You can host the program files on any web server your bootloader can reach. I use a simple static file server on my laptop for development. This example assumes that you set PROGRAM_URL to http://<your IP>:8000/a.out.

./8sm fibonacci.8sm
ruby -run -ehttpd . -p8000

Installation

Update bootloader.py to specify your SSID, Wi-Fi password and program URL.

This is what I did to load it on my board. I'm using a Mac. I assume this is equally easy on Linux or Windows but I don't know the details.

wget https://micropython.org/resources/firmware/esp32-idf4-20200902-v1.13.bin
esptool.py --port /dev/tty.usbserial-0001 -c esp32 erase_flash
esptool.py --port /dev/tty.usbserial-0001 -c esp32 write_flash -z 0x1000 esp32-idf4-20200902-v1.13.bin

sudo pip3 install git+https://github.com/natemueller/ampy.git@96936f946a32fba8d89c5607fb60e2cef83f4f1a
ampy --port /dev/tty.usbserial-0001 put bootloader.py main.py

Note that I had to use a patched version of ampy to talk to the MicroPython interpreter on the ESP-WROOM-32. See scientifichackers/ampy#19. Depending on your OS or breakout board you may not need to worry about this.

Wiring

wiring image

Basically, wire the GPIO pins in parallel with the RAM module DIP switches, program button, reset button and the clock mode toggle switch. See the wiring photos if you want to see the details. You can see the GPIO pin numbers in the bootloader source.