Wed Apr 05, 2017 3:42 pm
I did not deep dive it, but I think I have a similar problem. Using the latest rPi3B /w the latest raspbian, installed eclipse and bound in the Pi4J libraries. I wrote a Java code that pulls data off a sensor (DHT-11) via a timed bit sequence. Since nothing ever worked right, I abandoned all further programming and inspected the data read in on one pin via gpio.
Basically, I initiate a data transfer by telling the sensor H->L, then it starts spewing out bits. Supposedly, the bits are preceded by a 50us low. A "1" is a 70us high, a "0" is a 27us high.
All I do these days is record 5,000 read-in events - format them - and stare/analyze them. I come to the following conclusions:
- only 50% of the time, everything is well behaved meaning all bits can be read, and all times in the sequence have the same us/readcycle (calculated by 50microsecond / number of "0" counted for every bit)
- every other time I read the block, I get a much longer us/readcycle. Some of these are ok as I can recalculate everything based on the 50us/numberof0s, but why the heck is it suddenly slowing down ?!
- a lot of times, the timing changes somewhere within the bitsequence block. Meaning it starts out well behaved with 1.6us/cycle and then suddenly slows to 3.8us/cycle, and then goes back to 1.6us/cycle. Unfortunately, these events mess up the bit timing correction. As a result, I get maybe a 60-75% hitrate on the validity of the bitsequence read.
- the worst failures are where the system slows down during the 50us low, but then speeds back up while I am trying to read the 70 or 27us of the bit.
What can I do to ensure that this one particular tiny little reading command does not get interrupted by anything ?
Is there anything I can tell Java in the code before and after that command ?
Is there a method/library that would read in data on a pin, but also timestamp it ? With enough speed to make out 27us vs 70us ?
by the way, I tried the same methodology in python and inspected the data blocks. Same issue there. Yes it is a little faster, but it still changes reading speed randomly.
any help would be greatly appreciated !
Thanks.
T
