scd4x v0.1.0

Sensirion SCD40/SCD41 CO2, temperature, and humidity sensor.

Application scenarios

• Indoor air quality.
• A ventilation controller.
• A greenhouse or classroom CO2 monitor.

Contract

uint8_t scd4x_init(scd4x_t *sensor, scd4x_port_t port, uint8_t i2c_address);
uint8_t scd4x_read(scd4x_t *sensor, uint16_t *co2_ppm,
                   int32_t *milli_celsius, uint32_t *milli_percent);

• init stops any running measurement, then starts periodic measurement.
• read polls data-ready, reads nine bytes, and validates all three CRCs.
• CO2 is ppm; temperature is millidegrees Celsius; humidity is

millipercent RH.

• Returns 1 on success, 0 on a bus error, a bad CRC, or a timeout.

Parameters

• I2C address 0x62.
• CO2 range 400..5000 ppm; temperature -10..60 C; humidity 0..100 %RH.
• The first measurement needs about 5 s; later ones arrive every 5 s.

Memory and timing

• 8 bytes; one 9-byte read.
• A data-ready poll waits up to 5 s in 100 ms steps.

Integration

scd4x_t sensor;
uint16_t co2;
int32_t temperature;
uint32_t humidity;
scd4x_init(&sensor, board_i2c_port(), 0x62);
if (scd4x_read(&sensor, &co2, &temperature, &humidity)) {
    show(co2, temperature / 1000, humidity / 1000);
}

Adapters

• The adapter writes a two-byte command and reads the response.
• Reference: LibDriver scd4x (MIT) and the Sensirion SCD4x datasheet.
• This implementation is original.

When not to use

• A fast gas measurement; the CO2 measurement period is seconds.
• A condensing environment without a protective membrane.

Verification

make test covers the command sequence, all three CRCs, conversion, and a

bad CRC.

void *context;
uint8_t (*write)(void *context, uint8_t address, const uint8_t *data, uint16_t length);
uint8_t (*read)(void *context, uint8_t address, uint8_t *data, uint16_t length);
void (*delay_ms)(void *context, uint16_t milliseconds);

scd4x_port_t port;
uint8_t i2c_address;