dht v0.1.0

DHT11 and DHT22 frame decoders.

Application scenarios

• Room temperature and humidity on a display.
• Greenhouse or enclosure monitoring.
• A low-cost sensor on a product without a calibrated sensor.

Contract

typedef struct {
    uint8_t humidity;      /* %RH */
    int8_t temperature;    /* degrees C */
} dht11_reading_t;

typedef struct {
    uint16_t humidity_tenths;   /* 0.1 %RH */
    int16_t temperature_tenths; /* 0.1 degrees C */
} dht22_reading_t;

uint8_t dht11_decode(const uint16_t *high_us, uint8_t count, dht11_reading_t *out);
uint8_t dht22_decode(const uint16_t *high_us, uint8_t count, dht22_reading_t *out);

• high_us holds the high-pulse widths of the 40 data bits, in order.
• A pulse of at least 50 us is a one; a shorter pulse is a zero.
• count must be 40.
• Returns 1 on a valid checksum, 0 otherwise.

Parameters

• Capture with a 1 us timer or a pin-change interrupt.
• DHT11: humidity and temperature are integers; the fractional bytes are zero.
• DHT22: 16-bit values in tenths; the temperature sign is bit 15.

Memory and timing

• No state; the caller owns the 40-entry capture array (80 bytes).
• The sensor needs a start pulse and about 4 ms of frame time; keep interrupts short.

Integration

uint16_t highs[40];
dht11_reading_t reading;
if (dht11_decode(highs, 40, &reading)) {
    show(reading.temperature, reading.humidity);
}

Adapters

• The adapter drives the line low for at least 18 ms, releases it, and captures

the 40 high pulses.

• Reference: Adafruit DHT sensor library (MIT) and the DHT11/DHT22 datasheets.
• This decoder is original; it does not copy the library.

When not to use

• A sensor with a standard bus (I2C/SPI) is available.
• The cable is long or the environment is electrically noisy.

Verification

make test covers DHT11 values, DHT22 negative temperatures, a bad checksum,

and a short frame.

uint8_t humidity;
int8_t temperature;

uint16_t humidity_tenths;
int16_t temperature_tenths;