Simple Serial Link Protocol - SiLink

This repository implements a simple, low-level wire protocol for serial links, targeted at micropython runtimes. The protocol covers both serialization and framing. It's original intended use case is to transfer IMU and depth sensor data through a UART using rp2040 microcontroller's micropython firmware.

Design Goals

• Simple, yet convenient API.
• Low computational overhead.
• Transmission error detection.

Usage example

import silink
from silink.messages import DepthSensor

# Message Encoding.
ds_msg = DepthSensor(
   b'DS', pressure=1.0, temperature=25)
encoded_msg = silink.encode(ds_msg)
print(f"Encoded msg: {encoded_msg}")


# Message decoding in practical setting.
buff = b'noise_or_junk_data'
buff += encoded_msg
buff += b'noise_or_partial_msg'

decoded_msg, rest_of_buff = silink.decode(buff)
print(f"Decoded msg: {decoded_msg}")

# Trim the potentially junk filled buffer upto some
# size so that it does not consume all memory.
buff = rest_of_buff[:-200]

Defining new messages

Messsages are defined in the silink.messages module. Each message is declared as a namedtuple, with the first field being a bytestring of size two - it's typecode.

Each message needs to have an entry in the silink.messages.MESSAGE_FORMATS dictionary. The key is the typecode of the message. The value is a three field tuple of format - (<struct-format>, <class>, <defaults>).

• <struct-format> is a str describing the byte-packing structure of the message, as described in struct module of micropython standard library. Note some datatypes supported by cpython are missing in micropython.
• <class> is the class of the message.
• <defaults> is a tuple containing default field values. This is a workaround of micropython's namedtuple-s missing default fields.

Testing

To run the tests, execute -

tools/run_tests.sh

Limitations

• Creation of new messages require the programmer to know not only the class, but also the 2-byte type code. This is an unintended side effect of micropython's namedtuple not supporting default fields.
• Despite the aim at efficiency, current implementation does quite a few memory allocations during operation. This could be improved.

Contributors

• Titon Barua
• Ibrahim Salman