sditch - (micro)SD (sw)itch - boot storage mux control script
This is a script designed to run on a Raspberry Pi to control a microSD card MUX, switching a microSD card between a USB card reader/writer and a single-board computer (SBC such as another Raspberry Pi or similar). This allows the microSD card to be (re)imaged or copied, and the SBC to be booted from that card, without physical access -- good for automated testing (and remote access during a pandemic!)
The actual MUX hardware consists of a pair of sn74sbt3257 chips interfaced to the control computer through GPIO lines. One GPIO controls the !OE (not output enable, i.e., output disable) line and the other controls the S (select) line on the mux chips. The A inputs on the chips are connected to the microSD card socket, and the B1/B2 outputs are connected to two microSD outputs (we used the SparkFun microSD card sniffers to provide the microSD card connectors to plug into the two devices; we may produce a single thin PCB in the future, and will publish the board design if we do so).
The script is also designed to manage a PDU controlling the SBC; it's set for a Raritan PDU at this point but this can be readily adjusted.
This script may also be used without the mux board, by using USB-based storage and a USB automatic switch. The SBC is plugged into the higher-priority USB port on the switch, and the controlling computer is plugged into the lower-priority port, so that when the SBC is turned off the USB storage connection reverts to the control system and vice versa.
These are the commands accepted by the script:
- status - status of the mux (enabled/disabled)
- muxoff - mux off
- sbc - storage connected to Pi
- reader - storage connected to reader
- [re]boot - storage connect to Pi, Pi started
- poweron - ...same as boot
- poweroff - shutdown and poweroff the Pi
- shutdown - ...same as poweroff
- read file - storage contents placed in file (.gz)
- write file - storage written from file (.gz)
- writeon file - ...write followed by poweron
- mount - storage partitions mounted under /mnt/sd
- serial - start minicom for serial console monitoring
- video - start a video stream for attached CSI camera (useful for watching a video monitor)