RPBP — 1-Wire subsystem (subsys = `0x09`)¶

Maxim/Dallas 1-Wire bus master implemented on a single PIO state machine on PIO1. Standard speed only (~70 µs per bit slot, ~1 ms reset). Overdrive speed is a future extension.

The PIO program emits one bit-time slot per FIFO command and pushes the sampled bus level back; the firmware composes 8-bit bytes (LSB-first per Maxim AN126) on top. Three RPBP opcodes — RESET, WRITE, READ — cover normal slave traffic; TRIPLET is the SEARCH-ROM primitive that lets a host enumerate slaves with one round-trip per ROM bit.

Hardware: a single GPIO with an external 4.7 kΩ pull-up to +3V3 (a weaker on-chip pull-up is enabled as a backup but is too soft for fast slaves). The PIO program emulates open-drain by toggling pindirs.

Access path: vendor RPBP. Linux exposes the bus via rpbridge.ko once an upstream kernel::w1::Master Rust wrapper exists; the mainline w1_master_driver shape is small enough that the adaptation is straightforward — see driver/TODO.md #M12.

Capabilities¶

buses.onewire[*] carries:

key	type	meaning
`idx`	uint	bus index (always `0` in v1)
`data_pin`	uint	GPIO carrying the data line
`speed`	text	`"standard"` (only option in v1)

Feature flag advertised when the build includes 1-Wire support: onewire.master.

Opcodes¶

`0x00` GET_INFO¶

Request: empty.

Response (4 bytes): [count u8][pin u8][speed u8][reserved u8].

count — 1 if a bus is online, 0 if init failed.
pin — GPIO number.
speed — 0 = standard.

`0x01` RESET¶

Request: [idx u8].

Response (1 byte): [presence u8]. 0 = no slaves, 1 = at least one slave asserted a presence pulse.

`0x02` WRITE¶

Request: [idx u8][bytes …]. Bytes are sent LSB-first per slot.

Total wire payload ≤ MTU (4 KiB) → max ~4 KiB per WRITE; chunk larger transfers across multiple WRITEs.
No reply payload — status only.

`0x03` READ¶

Request (3 bytes): [idx u8][len LE u16]. len ∈ [1, 256].

Response: the read bytes, LSB-first.

`0x04` TRIPLET¶

Request (2 bytes): [idx u8][dir u8]. dir is the search direction to write (0 or 1).

Response (1 byte): packed result —

Bit	Meaning
0	`id_bit` — the read true bit
1	`cmp_bit` — the read complement bit
2	`dir` — the bit that was actually written

Used by the SEARCH-ROM state machine (Maxim AN187): host issues TRIPLET 64 times per slave, picking dir based on the (id_bit, cmp_bit) pair to walk the binary tree of ROM IDs.

Examples¶

Enumerate the bus, read a DS18B20¶

RESET                                          → presence = 1
WRITE  bytes=[0xCC, 0x44]                      # SKIP_ROM, CONVERT_T
                                               # (wait ≥ 750 ms for conversion)
RESET                                          → presence = 1
WRITE  bytes=[0xCC, 0xBE]                      # SKIP_ROM, READ_SCRATCHPAD
READ   len=9                                   → 9 scratchpad bytes
                                               # bytes 0..1 = temperature LE i16

SEARCH-ROM (find a single slave)¶

RESET                                          → presence = 1
WRITE  bytes=[0xF0]                            # SEARCH_ROM
loop 64 times:
  TRIPLET dir=<from AN187 state machine>       → id_bit, cmp_bit, dir
                                               # accumulate dir into ROM ID

Error semantics¶

Status	Cause
`ENOENT`	`idx` ≠ 0
`EINVAL`	bad len (0 or > 256), short payload
`EAGAIN`	bus not initialised (subsystem disabled at build)
`EBUSY`	pin/PIO conflict at firmware boot
`ENOTSUP`	board built without `RPBRIDGE_HAVE_ONEWIRE`

Performance notes¶

A 9-byte READ (e.g., DS18B20 scratchpad) takes 9 × 8 × 70 µs = ~5 ms of bus time. A full SEARCH ROM with one slave on the bus takes 1 + 64 = 65 round-trips of ~70 µs each = ~5 ms bus time, plus the USB latency per opcode (~1 ms USB FS round-trip). For continuous temperature polling at ≥ 1 Hz this is comfortably within budget; for fast SEARCH on a bus with many slaves, batch slot count optimisation lives on the host side.

RPBP — 1-Wire subsystem (subsys = 0x09)¶