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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@vger.kernel.org Signed-off-by: John Brooks <john@fastquake.com> Reviewed-by: Harald Geyer <harald@ccbib.org> Signed-off-by: Jonathan Cameron <jic23@kernel.org> |
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.. | ||
accel | ||
adc | ||
amplifiers | ||
buffer | ||
chemical | ||
common | ||
counter | ||
dac | ||
dummy | ||
frequency | ||
gyro | ||
health | ||
humidity | ||
imu | ||
light | ||
magnetometer | ||
orientation | ||
potentiometer | ||
potentiostat | ||
pressure | ||
proximity | ||
temperature | ||
trigger | ||
iio_core_trigger.h | ||
iio_core.h | ||
industrialio-buffer.c | ||
industrialio-configfs.c | ||
industrialio-core.c | ||
industrialio-event.c | ||
industrialio-sw-device.c | ||
industrialio-sw-trigger.c | ||
industrialio-trigger.c | ||
industrialio-triggered-event.c | ||
inkern.c | ||
Kconfig | ||
Makefile |