Makes end-to-end verification of the bus possible with a spare RP2040 and
no dedicated RS485 dongle.
bridge/ turns a second Pico into a transparent USB-to-RS485 bridge. It is
deliberately protocol-agnostic: bytes from USB go out on the pair, bytes
from the pair go back up USB, and it knows nothing about frames,
addresses or CRCs. A protocol-aware bridge could have a bug that happens
to agree with the node's own, which would defeat the point of using it as
a test instrument. It also needs no register-level code — it never has to
delimit a frame, and the one thing it does need, knowing that the final
stop bit has cleared the shifter before releasing DE, rp2040-hal exposes
as uart_is_busy().
tools/wiredsensor.py is an independent implementation of the wire format,
written from the specification in README.md rather than sharing code with
wiredsensor-core. Shared code would let a framing or CRC bug cancel out
and every test pass regardless. Cross-checked: it reproduces the
CRC-16/MODBUS catalogue vector and builds byte-identical frames to the
Rust side for all five documented examples.
Its `test` subcommand covers what host tests structurally cannot, all of
it timing-dependent: that malformed and mis-addressed frames leave the
node silent rather than colliding with whoever was actually addressed,
that the driver-enable turnaround leaves replies intact, and that no
state leaks between back-to-back frames.
Uses one wiring diagram for both boards, since the bridge mirrors the
node's GPIO0/1/2 assignment. Frames must fit one 64-byte USB packet, as
each host write becomes exactly one DE-bracketed transmission; every real
command is at most 21 bytes.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
RTIC 2 firmware for an RP2040 acting as an RS485 slave, reading a
Sensirion SHT31 over I2C and answering a custom binary protocol.
Split into two crates so the protocol is testable without hardware:
- wiredsensor-core: framing, CRC-16/MODBUS, command dispatch and SHT3x
data-sheet math. Pure computation, no I/O, no peripherals. Covered by
42 host tests including the CRC-16/MODBUS catalogue vector, the SHT3x
data-sheet CRC-8 example, and an exhaustive single-bit-corruption
sweep over every byte of a frame.
- wiredsensor-fw: the RTIC application, PL011 register driver and SHT31
I2C driver.
Requests are answered entirely from a cached reading. An SHT31
high-repeatability conversion takes up to 15 ms, well past the
turnaround a master expects, so the sensor is polled at the lowest
priority and the bus path never touches I2C. RTIC's priority ceilings
make "a conversion cannot delay a reply" a checked property rather than
an argument.
Two PL011 details drive the low-level approach, since rp2040-hal exposes
neither: the receive-timeout interrupt for prompt frame delimiting, and
the BUSY flag, which is the only way to know the final stop bit has left
the shift register before releasing DE. The HAL performs the fiddly
baud-divisor setup once, then the raw peripheral is reclaimed via free().
Note that RTIM cannot be the sole frame delimiter: it only fires while
the RX FIFO is non-empty, so a frame landing exactly on the FIFO
watermark is drained by the watermark interrupt and never times out. The
authoritative delimiter is a monotonic timer measured from the last
received byte.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Oliver Walter