These are short notes on loading baremetal software on the Raspberry Pi Zero. I started down this path due to wanting to learn Rust, which lead to Tock which is an embedded operating system in Rust. Rather than buy yet another piece of hardware I though I’d use one of my 5 (!) Raspberry Pi Zeros as a development board instead. It’s just another microcontroller, just one that runs at 1 GHz and has a ton of RAM…
ESPHome is a framework for building custom home automation that runs on an ESP8266 or ESP32. It’s pretty cool - you select and configure components by writing a YAML file, which then drives host side Python snippets to configure and bind the device side code, which is then built and pushed using PlatformIO.
Some nice touches:
It integrates with Home Assistant and has automatic discovery It can drive displays, including rendering TTF files host side to give nice fonts device side It looks reasonably composable so, for example, you can have multiple Bluetooth broadcast parsers I used this to bridge between Ruuvi tags and my Home Assistant instance.
My son wanted to get an alarm clock for his room which preferably plays MP3s. MP3 players are surprisingly expensive at 40 CHF for a 4 GiB model so I picked up a used Moto E 2nd gen phone for 35 GBP, loaded a stripped down version of Lineage OS on it, and designed a 3D printed a stand:
I’m quite happy with how it turned out. The phone is just an alarm clock and MP3 player as the other apps are either not installed or (like the Play Store) behind Lineage’s protected app feature.
I got distracted and had a hack on using a STL Link v2 clone as a development board. There’s a lot to like:
A Cortex-M3 STM32F103C8T processor with 64 KiB of flash and 20 KiB of RAM 4 I/O lines and a LED to blink Decent support in STM32duino with a DFU bootloader Comes in a case and plugs directly into a USB port ~$2/each The I/O is strange and limited but it’s enough to drive a RGB LED via PWM, drive a 40 RGB LED pHAT over SPI, and drive a 320x240 LCD over fast bitbanged SPI.
I’m hacking on adding SDHC over SPI block device support to the Zephyr Project RTOS.
I’m currently getting 224 KiB/s on an Arduino Zero with a 4 MHz bus and 1 KiB read size, which is an OK-ish 46 % of the top bus capacity.
Here’s where the time goes:
4550 us - total time to read 2x 512 byte blocks from the card 80 us - time spent in ELM FS (impressive!