111 lines
3.7 KiB
Markdown
111 lines
3.7 KiB
Markdown
# Pico with Embassy
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Read temperature values and write them to an MQTT topic.
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The Raspberry Pi Pico is well supported by [embassy](https://embassy.dev), apart
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from the bluetooth stack (_2024/11/18:
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[might not be true anymore](https://github.com/embassy-rs/embassy/blob/main/examples/rp/src/bin/bluetooth.rs)_).
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As we do not use bluetooth for communication we can ignore that.
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## Flashing
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There are two ways of getting the code onto the microcontroller.
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### elf2uf2
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Compile the binary as normal, convert it into a uf2 firmware which is flashable
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to the pico with only an USB connection. Disadvantage of this apporach is that
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it is more annoying to do and does not lend itself to debugging.
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- Clone the [elf2uf2](https://github.com/rej696/elf2uf2) repository and follow
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its instructions to compile the tool
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- Compile the controller code with `cargo build --release`
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- Convert the resulting binary with something like
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`elf2uf2 target/thumbv6m-none-eabi/release/controller ./controller.uf2`
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- Hold the bootselect button of the pico when plugging it in
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- Copy the uf2 file to the mass storage device
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### CMSIS-DAP
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Use the [CMSIS-DAP](https://arm-software.github.io/CMSIS_5/DAP/html/index.html)
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protocol for flashing and debugging.
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The
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[raspberry pi debug probe](https://www.raspberrypi.com/documentation/microcontrollers/debug-probe.html)
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works well but anything implementing that protocol is fine.
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 _Example setup with a Pico WH
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(Pico W works as well, just a bit more annoying)_
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- Setup debug probe
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- Install [probe-rs](https://github.com/probe-rs/probe-rs/)
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- Run `cargo run` and it should upload and logs be visible
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## Configuration
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Configuration is done by sending commands across a serial connection. Only one
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command is implemented for now: Set-config with two parameters.
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A sample set-config command looks like this (not encoded yet): `SC ssid MyNet`.
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It consists of three parts:
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- `SC`: Command prefix, always the same.
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- `ssid`: Configuration key.
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- `MyNet`: Configuration value.
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The following keys are recognized:
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- `ssid`: Name of the network to connect to.
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- `ssid_pw`: Password to connect to the network.
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- `mqtt`: URL of the MQTT broker (must not use https).
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- `client_id`: ID of the device (used for identifaction in MQTT).
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A message needs to be encoded into its byte representation looking on a high
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level as follows:
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```
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|<parameters>|<parameter-lengths>|<prefix>|<parameters>|
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```
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- `Parameters` is an unsigned byte signifying the amount of parameters in the
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message.
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- `Parameter-Lengths`: Length of each parameter.
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- `Prefix` is always two bytes long and encoded as UTF-8 (meaning it is
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basically ASCII).
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- `Parameters`: every parameter encodes is own length in its first byte,
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followed by the actual data.
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Taking the above ssid example this would lead to this (the prefix counts as its
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own parameter):
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```
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0x03 0x02 0x04 0x05 0x53 0x43 0x73 0x73 0x69 0x64 0x4D 0x59 0x4E 0x65 0x74
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```
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Now the COBS encoding is applied before sending it across the wire:
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```
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0x10 0x03 0x02 0x04 0x05 0x53 0x43 0x73 0x73 0x69 0x64 0x4d 0x79 0x4e 0x65 0x74
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```
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In this case (as in most) it merely adds two additional bytes (the last 0x00
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byte is implied).
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Sending this this command on linux, assuming the serial connection is accessible
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on `/dev/ttyACM1`:
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```
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echo -en "\x10\x03\x02\x04\x05\x53\x43\x73\x73\x69\x64\x4d\x79\x4e\x65\x74\x00" > /dev/ttyACM1
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```
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If listening to the serial connection, command acknowledgements like `OK` or
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`ERROR` can be seen.
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In order to decode a message, apply the above steps in reverse order.
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All configuration is read on startup of the microcontroller.
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## Outlook
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- Pull the flash/config handling into separate library for testability
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