diff options
Diffstat (limited to 'drivers/iio/humidity/dht11.c')
| -rw-r--r-- | drivers/iio/humidity/dht11.c | 77 |
1 files changed, 50 insertions, 27 deletions
diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c index cfc5a051a..20b500da9 100644 --- a/drivers/iio/humidity/dht11.c +++ b/drivers/iio/humidity/dht11.c @@ -50,12 +50,32 @@ #define DHT11_EDGES_PER_READ (2 * DHT11_BITS_PER_READ + \ DHT11_EDGES_PREAMBLE + 1) -/* Data transmission timing (nano seconds) */ +/* + * Data transmission timing: + * Data bits are encoded as pulse length (high time) on the data line. + * 0-bit: 22-30uS -- typically 26uS (AM2302) + * 1-bit: 68-75uS -- typically 70uS (AM2302) + * The acutal timings also depend on the properties of the cable, with + * longer cables typically making pulses shorter. + * + * Our decoding depends on the time resolution of the system: + * timeres > 34uS ... don't know what a 1-tick pulse is + * 34uS > timeres > 30uS ... no problem (30kHz and 32kHz clocks) + * 30uS > timeres > 23uS ... don't know what a 2-tick pulse is + * timeres < 23uS ... no problem + * + * Luckily clocks in the 33-44kHz range are quite uncommon, so we can + * support most systems if the threshold for decoding a pulse as 1-bit + * is chosen carefully. If somebody really wants to support clocks around + * 40kHz, where this driver is most unreliable, there are two options. + * a) select an implementation using busy loop polling on those systems + * b) use the checksum to do some probabilistic decoding + */ #define DHT11_START_TRANSMISSION 18 /* ms */ -#define DHT11_SENSOR_RESPONSE 80000 -#define DHT11_START_BIT 50000 -#define DHT11_DATA_BIT_LOW 27000 -#define DHT11_DATA_BIT_HIGH 70000 +#define DHT11_MIN_TIMERES 34000 /* ns */ +#define DHT11_THRESHOLD 49000 /* ns */ +#define DHT11_AMBIG_LOW 23000 /* ns */ +#define DHT11_AMBIG_HIGH 30000 /* ns */ struct dht11 { struct device *dev; @@ -76,43 +96,39 @@ struct dht11 { struct {s64 ts; int value; } edges[DHT11_EDGES_PER_READ]; }; -static unsigned char dht11_decode_byte(int *timing, int threshold) +static unsigned char dht11_decode_byte(char *bits) { unsigned char ret = 0; int i; for (i = 0; i < 8; ++i) { ret <<= 1; - if (timing[i] >= threshold) + if (bits[i]) ++ret; } return ret; } -static int dht11_decode(struct dht11 *dht11, int offset, int timeres) +static int dht11_decode(struct dht11 *dht11, int offset) { - int i, t, timing[DHT11_BITS_PER_READ], threshold; + int i, t; + char bits[DHT11_BITS_PER_READ]; unsigned char temp_int, temp_dec, hum_int, hum_dec, checksum; - threshold = DHT11_DATA_BIT_HIGH / timeres; - if (DHT11_DATA_BIT_LOW / timeres + 1 >= threshold) - pr_err("dht11: WARNING: decoding ambiguous\n"); - - /* scale down with timeres and check validity */ for (i = 0; i < DHT11_BITS_PER_READ; ++i) { t = dht11->edges[offset + 2 * i + 2].ts - dht11->edges[offset + 2 * i + 1].ts; if (!dht11->edges[offset + 2 * i + 1].value) return -EIO; /* lost synchronisation */ - timing[i] = t / timeres; + bits[i] = t > DHT11_THRESHOLD; } - hum_int = dht11_decode_byte(timing, threshold); - hum_dec = dht11_decode_byte(&timing[8], threshold); - temp_int = dht11_decode_byte(&timing[16], threshold); - temp_dec = dht11_decode_byte(&timing[24], threshold); - checksum = dht11_decode_byte(&timing[32], threshold); + hum_int = dht11_decode_byte(bits); + hum_dec = dht11_decode_byte(&bits[8]); + temp_int = dht11_decode_byte(&bits[16]); + temp_dec = dht11_decode_byte(&bits[24]); + checksum = dht11_decode_byte(&bits[32]); if (((hum_int + hum_dec + temp_int + temp_dec) & 0xff) != checksum) return -EIO; @@ -161,12 +177,12 @@ static int dht11_read_raw(struct iio_dev *iio_dev, int *val, int *val2, long m) { struct dht11 *dht11 = iio_priv(iio_dev); - int ret, timeres; + int ret, timeres, offset; mutex_lock(&dht11->lock); if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_boot_ns()) { timeres = ktime_get_resolution_ns(); - if (DHT11_DATA_BIT_HIGH < 2 * timeres) { + if (timeres > DHT11_MIN_TIMERES) { dev_err(dht11->dev, "timeresolution %dns too low\n", timeres); /* In theory a better clock could become available @@ -176,6 +192,10 @@ static int dht11_read_raw(struct iio_dev *iio_dev, ret = -EAGAIN; goto err; } + if (timeres > DHT11_AMBIG_LOW && timeres < DHT11_AMBIG_HIGH) + dev_warn(dht11->dev, + "timeresolution: %dns - decoding ambiguous\n", + timeres); reinit_completion(&dht11->completion); @@ -208,11 +228,14 @@ static int dht11_read_raw(struct iio_dev *iio_dev, if (ret < 0) goto err; - ret = dht11_decode(dht11, - dht11->num_edges == DHT11_EDGES_PER_READ ? - DHT11_EDGES_PREAMBLE : - DHT11_EDGES_PREAMBLE - 2, - timeres); + offset = DHT11_EDGES_PREAMBLE + + dht11->num_edges - DHT11_EDGES_PER_READ; + for (; offset >= 0; --offset) { + ret = dht11_decode(dht11, offset); + if (!ret) + break; + } + if (ret) goto err; } |