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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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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.