I found an error in the DHT module, if the system load is high: I have many Python threads running and the DHT module constantly reports a timeout error. If system load is low, there is no error.

I have a fix for the problem: You should extend the atomic section to include the 18ms waiting. Simply move the mp_hal_quiet_timing_enter() before the mp_hal_delay_ms(18), see below. I see no problem to loose the 18ms because the DHT measurements should only be every 2-3 seconds.

The 18ms may be reduced for DHT22, because the data sheet needs at least 1ms delay. I use a 3ms delay with the DHT22 with no problems. But for DHT11 we need 18ms. So I would let the delay at 18ms.

The fix is easy, in the file:

micropython/drivers/dht/dht.c

ORIGINAL (line 51):

// issue start command
mp_hal_pin_od_high_dht(pin);
mp_hal_delay_ms(250);
mp_hal_pin_od_low(pin);
mp_hal_delay_ms(18);

mp_uint_t irq_state = mp_hal_quiet_timing_enter();

CHANGE TO:

// issue start command
mp_hal_pin_od_high_dht(pin);
mp_hal_delay_ms(250);
mp_hal_pin_od_low(pin);

// the atomic section should be before the delay of 18ms
// otherwise we have timeout errors on high system load
// Stefan Hammes, 2020-03-31
mp_uint_t irq_state = mp_hal_quiet_timing_enter();

mp_hal_delay_ms(18);

Or with diff:

$ diff dht.c-orig dht.c
55d54
<     mp_hal_delay_ms(18);
56a56,58
>     // the atomic section should be before the delay of 18ms
>     // otherwise we have timeout errors on high system load
>     // Stefan Hammes, 2020-03-31
57a60,61
>
>     mp_hal_delay_ms(18);

This gives correct DHT readings even with extremly high system load.

Please include this in the source code. Thanks.

Keep up the good work!

Stefan Hammes, Karlsruhe, Germany