Start listening to the sensor right after releasing the start signal … #70
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DHT.cpp
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| // set the pin LOW within 20 to 40 microseconds. | ||
| // The (F_CPU / 16000000) is a ratio to keep the timeout to approximately | ||
| // the same period if processor used has different frequency. | ||
| unsigned long timesup = (F_CPU / 16000000) * 700; |
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Your code is broken for any MCU with F_CPU < 16000000. timesup will be 0 because, for example, (15000000 / 16000000) == 0. I would recommend lowering the denominator to 1000000 (or lower, depending on the minimum supported F_CPU) and increasing the multiplier.
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Excellent comment! Thanks @ianhattendorf. I will try to fix this in a near future.
…start signal time depending on sensor type
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Hi all,
Here is my pull request for the changes I have done relate to how the library sends the start signal to the sensor and the hope is it will solve and that it will have the effect of a closed #48. It also relates to PR #51 and PR #58.
Firstly, there is an unnecessary wait of 250ms before the start signal. This is the reason I started to look at fixing the init sequence. I remember reading that it could be needed by those not using a pull up, in 85a02c7. ( If that were the case, the pin is set HIGH in begin(), it could be set again before exiting the read method. The pin state should stay HIGH as no other read can be made in the following two seconds, so more than enough time to cover that 250ms that I would like to have removed. We would also need to make sure the first read is done after proper period if sensor needs time to wake up upon start. )
Second, from what I conclude after a read of the datasheet and from what I have read on several forums, the MCU should only pull the line LOW for a period of time and then leave the line to the sensor. There is a possible variable amount of time before the sensor will signal its LOW and HIGH for ~80us each while preparing the data. This part
was changed for a busy-wait loop to ensure the library gets it at a correct time and not have the guess the wait period for all possible sensors. This has made the delayMicroseconds() calls redundant. That should also satisfy the change that was originally suggested by Wolfgang and referenced in 5ed9818.
The changes' goal is to ensure the MCU and sensor do not talk at the same time. A few comments were also added to support the code changes.
Thank you,
Pascal