ds18b20 v0.1.0

Read a DS18B20 temperature over a 1-Wire port.

Application scenarios

• Board or enclosure temperature on a single sensor.
• A probe on a long cable where I2C is inconvenient.
• A low-cost temperature reading with a CRC check.

Contract

typedef struct {
    void *context;
    uint8_t (*reset)(void *context);
    void (*write_bit)(void *context, uint8_t bit);
    uint8_t (*read_bit)(void *context);
    void (*delay_ms)(void *context, uint16_t ms);
} onewire_port_t;

uint8_t ds18b20_read(onewire_port_t port, int16_t *temperature_q4);

• reset returns 1 when a presence pulse was seen.
• The port owns the 1-Wire timing: 480 us reset, 60 us slots.
• The result is the raw DS18B20 temperature in 1/16 C (Q4).
• The scratchpad CRC is checked; a bad CRC returns 0.
• No allocation; one device on the bus.

Parameters

• Conversion takes up to 750 ms at 12-bit resolution; the port's

delay_ms must provide that wait.

• For multiple sensors, add a ROM search layer; this unit uses Skip ROM.

Memory and timing

• 8 bytes of state plus the caller's 9-byte scratchpad.
• 72 bit slots per byte read, plus the conversion wait.

Integration

onewire_port_t bus = board_onewire_port();
int16_t raw;
if (ds18b20_read(bus, &raw)) {
    int32_t milli_c = (int32_t)raw * 1000 / 16;
}

Adapters

• Keep GPIO direction changes, delays, and interrupt masking in the adapter.
• Reference: OneWire (MIT) and the DS18B20 datasheet.
• Parasite power needs a strong pull-up; this core assumes normal power.

When not to use

• A multi-drop bus without a ROM search layer.
• A sensor that needs a different family code or resolution configuration.

Verification

make test covers a valid 25.0625 C scratchpad, CRC failure, no presence, and the command sequence.