The DHT22 (AM2302) datasheet specifies that the LOW start pulse should not
exceed 20ms. However, observations with an oscilloscope of an RPi Model 2B
(rev 1.1) communicating with a DHT22 sensor showed that the driver was
consistently sending start pulses longer than 20ms:
Kernel 4.7.10-v7+ (n=132):
Minimum pulse length: 20.20ms
Maximum: 29.84ms
Mean: 24.96ms
StDev: 2.82ms
Sensor response rate: 100%
Read success rate: 76%
On kernel 4.8, the start pulse was so long that the sensor would not even
respond 97% of the time:
Kernel 4.8.16-v7+ (n=100):
Minimum pulse length: 30.4ms
Maximum: 74.4ms
Mean: 39.3ms
StDev: 10.2ms
Sensor response rate: 3%
Read success rate: 3%
The driver would return ETIMEDOUT and write log messages like this:
[ 51.430987] dht11 dht11@0: Only 1 signal edges detected
[ 66.311019] dht11 dht11@0: Only 0 signal edges detected
Replacing msleep(18) with usleep_range(18000, 20000) made the pulse length
sane again and restored responsiveness:
Kernel 4.8.16-v7+ with usleep_range (n=123):
Minimum pulse length: 18.16ms
Maximum: 20.20ms
Mean: 19.85ms
StDev: 0.51ms
Sensor response rate: 100%
Read success rate: 84%
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: John Brooks <john@xxxxxxxxxxxxx>
---
drivers/iio/humidity/dht11.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/drivers/iio/humidity/dht11.c b/drivers/iio/humidity/dht11.c
index 9c47bc9..2a22ad9 100644
--- a/drivers/iio/humidity/dht11.c
+++ b/drivers/iio/humidity/dht11.c
@@ -71,7 +71,8 @@
* 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_START_TRANSMISSION_MIN 18000 /* us */
+#define DHT11_START_TRANSMISSION_MAX 20000 /* us */
#define DHT11_MIN_TIMERES 34000 /* ns */
#define DHT11_THRESHOLD 49000 /* ns */
#define DHT11_AMBIG_LOW 23000 /* ns */
@@ -228,7 +229,8 @@ static int dht11_read_raw(struct iio_dev *iio_dev,
ret = gpio_direction_output(dht11->gpio, 0);
if (ret)
goto err;
- msleep(DHT11_START_TRANSMISSION);
+ usleep_range(DHT11_START_TRANSMISSION_MIN,
+ DHT11_START_TRANSMISSION_MAX);
ret = gpio_direction_input(dht11->gpio);
if (ret)
goto err;
--
2.7.4
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