On 17/01/16 16:13, Harald Geyer wrote:
> The new algorithm uses a 'one size fits em all' threshold, which should
> be easier to understand and debug. I believe there are no regressions
> compared to the old adaptive threshold algorithm. I don't remember why
> I chose the old algorithm when I initially wrote the driver.
>
> Signed-off-by: Harald Geyer <harald@xxxxxxxxx>
Applied to the togreg branch of iio.git - initially pushed out as testing
for the autobuilders to play with it.
Thanks,
Jonathan
> ---
> changes since V1:
> * Simplify overly explicit code - no real change
>
> drivers/iio/humidity/dht11.c | 64 +++++++++++++++++++++++++++++---------------
> 1 file changed, 42 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
> index 1ca284a..96185f8 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;
> @@ -166,7 +182,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
> mutex_lock(&dht11->lock);
> if (dht11->timestamp + DHT11_DATA_VALID_TIME < ktime_get_real_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);
>
> @@ -211,7 +231,7 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
> offset = DHT11_EDGES_PREAMBLE +
> dht11->num_edges - DHT11_EDGES_PER_READ;
> for (; offset >= 0; --offset) {
> - ret = dht11_decode(dht11, offset, timeres);
> + ret = dht11_decode(dht11, offset);
> if (!ret)
> break;
> }
>
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