thunders

A compile-time, fixed-packet one-way link layer for the Nordic 2.4 GHz RADIO, over two backends — a bare exclusive RADIO driver and an MPSL timeslot backend. Four heterogeneous boards interoperate on one wire format: nRF52833, nRF52840, nRF5340 (net core), nRF54LM20.

┌──────────────────┐  fixed 6-byte state   ┌──────────────────┐
│ Sender           │ ─────────────────────▶ │ Receiver         │
│  data slots      │                        │  one long window │
│  feedback listen │ ◀───────────────────── │  3-byte beacon   │
└──────────────────┘   per batch boundary   └──────────────────┘

The sender publishes fixed-length state packets on a compile-time slot plan (OneWayState<PAYLOAD, FEEDBACK_EVERY> + fixed_slot_plan); the receiver keeps the newest value and answers once per batch with a 3-byte feedback beacon — timing error, plus an addressed recall that pulls one sender into a reliable negotiation phase. Details: docs/architecture.md (protocol/timing), docs/negotiation.md (recall).

Crates

Crate	Role
thunders	Protocol core: one-way modes, const slot planner, negotiation codec, StaticTdma, the Phy trait. no_std, board-agnostic.
thunders-phy-nrf	nRF PHY: bare (exclusive RADIO, TXIDLE/RXIDLE restart per packet) and mpsl (RADIO inside MPSL timeslots). Registers via nrf-pac.
examples/{nrf52833,nrf52840,nrf5340,nrf54lm20}/{bare,mpsl}	Board firmware; sender/receiver roles are separate binaries/Cargo features.

MPSL is the stock alexmoon/nrf-sdc git dependency; embassy HAL is upstream embassy-rs/embassy via [patch.crates-io]. No vendored forks.

Measured rates (full bench over every example combination)

Link	TX	RX	Throughput	Integrity
MPSL one-way, default	LM20 3333/s	52840	3332 pkt/s	0 CRC-bad, 0 invalid
MPSL one-way, default	52833 2767/s	52840	2767 pkt/s	0 CRC-bad over 8.6M frames (~52 min); rare single-window invalid spikes (~0.003%)
MPSL one-way, phase-align	LM20 3306/s	52840	3306 pkt/s	0 CRC-bad over 83k frames; recall demo echo confirmed
MPSL one-way, phase-align	LM20 3331/s	52833	3331 pkt/s	0 CRC-bad, 0 invalid
MPSL one-way, hopping	LM20 3304/s	52840	3267 pkt/s	0 CRC-bad while hopping 8 channels
MPSL multi-sender	LM20 1666/s + 5340 1111/s	52840	1623/s + 898/s	both senders demultiplexed concurrently
MPSL multi-sender	52833 1384/s + 5340 1111/s	LM20	1373/s + 900/s	52833 as sender 0, 99% delivery
Bare one-way	LM20 14705/s	52840	14627 pkt/s	0% loss, feedback flowing
Bare one-way	LM20 15183/s	52833	15183 pkt/s	0% loss (1 frame lost in 380k)
Bare one-way, hopping	LM20 14723/s	52840	14625 pkt/s	0% loss
Bare TDMA, 2 senders	52840 5265/s + 5340 5221/s	LM20	5258/s + 5160/s	0 invalid; ~2% CRC-bad from collisions
Bare TDMA, 2 senders	52833 5264/s + 5340 5221/s	LM20	5263/s + 5183/s	52833 as TDMA master; same collision noise

Optional features (MPSL examples)

Feature	Effect
phase-align	Per-batch feedback event (receiver replies, sender locks). Enables recall.
hopping	Synchronized 8-channel hopping at the batch boundary (implies phase-align).
multi-sender	nrf52840 receiver demultiplexes up to 8 senders by RADIO logical address.
sender	nrf52833 full-rate one-way TX role (default build is the receiver).
multi-sender-bench / multi-receiver	The 2-TX/1-RX benchmark roles.
sender-1	nrf54lm20 transmits with the 0xC3 prefix (second-sender role).

The bare examples add a const-generic TDMA pair (--bin tdma-sender / --bin tdma-receiver, shared schedule in examples/tdma_config.rs).

Building & flashing

Prereqs: probe-rs, Rust targets thumbv7em-none-eabihf (nRF52833/52840), thumbv8m.main-none-eabi (nRF5340), thumbv8m.main-none-eabihf (nRF54LM20).

# receiver: nRF52840 (DAPLink)
cd examples/nrf52840/mpsl
cargo build --release --no-default-features --features phase-align
probe-rs run --chip nRF52840_xxAA --probe "0d28:0204-3:0700..." \
  target/thumbv7em-none-eabihf/release/thunders-mpsl

# sender: nRF54LM20 (J-Link; auto-detect fails, --chip is required)
cd examples/nrf54lm20/mpsl
cargo build --release --no-default-features --features phase-align
probe-rs run --verify --disable-double-buffering --chip nRF54LM20A \
  --probe 1366:1069 target/thumbv8m.main-none-eabihf/release/thunders-mpsl

The receiver's RTT log runs the recall demo (foreign prefix, addressed recall, echo, release). The nRF5340 net core is debug-locked: every flash needs --allow-erase-all.

nRF54L field notes (things the SVD won't tell you)

• TXPOWER is encoded: 0 dBm = 0x18. Raw 0x00 leaves the PA off (the radio runs its TX state machine but emits no RF).
• Event block at 0x200+: the nRF54L radio's events sit at 0x200-0x220 (the nRF5340's at 0x100-0x110).
• TXD/RXD DMA amounts: no SVD registers at 0xEE8/0xED4, but the writes are required on silicon (0-length PDUs / no RX otherwise).
• Errata 54L/49: the first on-air payload bits need a hidden register (0x5008C58C) set.
• Check flash integrity after post-flash boot faults: corrupted flash is one observed failure mode, not a diagnosis for every boot fault. With probe-rs's double-buffered flashing, dropped/duplicated words were observed on the LM20 (RRAMC buffered write) and the 52833 (NVMC). The next page buffer is downloaded over MEM-AP while the CPU runs the flash algorithm; bus contention was the suspected mechanism. Use --disable-double-buffering and --verify on these targets. Corrupted images can fault semi-randomly (blob assert, null call, bus fault); resets cannot repair stored flash content. Both LM20 examples retain precise HardFault diagnostics (CFSR/HFSR/BFAR and stacked PC/R0/LR over defmt) to help distinguish failures.
• Retracted: an early RRAMC.LOWPOWERCONFIG = Standby write turned out unnecessary — the boot asserts attributed to fetch latency were flash-corruption artifacts. Don't cargo-cult it back.