8000 Add option in DHT driver to do the measure without blocking interrupts by EddieParis · Pull Request #6946 · micropython/micropython · GitHub
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Add option in DHT driver to do the measure without blocking interrupts #6946

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Add option in DHT driver to do the measure without blocking interrupts #6946

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drivers/dht: Add option to do the measure without blocking interrupts.
EddieParis Feb 21, 2021
e6dbe2a
drivers/dht/dht.c: Modifications after pull requests comments.
EddieParis Feb 24, 2021
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19 changes: 19 additions & 0 deletions docs/esp8266/tutorial/dht.rst
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Expand Up @@ -63,3 +63,22 @@ To make newer I2C sensors work in backwards compatible 1-wire mode, you must
connect both pins 3 and 4 to GND. This disables the I2C interface.

DHT22 sensors are now sold under the name AM2302 and are otherwise identical.

NOTE: if your are using a PWM channel in parallel of your measure, there can
be some glitches on the PWM channel at the measure time. This is because
the DHT drivers locks interrupts for accurate protocol timing handling,
and the PWM driver relies on these interrupts. The constructor of the DHT
objects has an extra argument irq_lock which can be set to False.
The risk is low, but in case of timing issues a DHTChecksumError can be
raised, catch it and retry. Depending on your use case, consider limiting
the number of retries.

>>> import dht
>>> import machine
>>> d = dht.DHT22(machine.Pin(4), False)
>>> while True:
>>> try:
>>> d.measure()
>>> break
>>> except dht.DHTChecksumError:
>>> pass
20 changes: 15 additions & 5 deletions drivers/dht/dht.c
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Expand Up @@ -37,7 +37,11 @@
#define mp_hal_pin_od_high_dht mp_hal_pin_od_high
#endif

STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in) {
STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in, mp_obj_t irq_lock) {
mp_uint_t irq_state = 0;

const bool do_lock = mp_obj_is_true(irq_lock);

mp_hal_pin_obj_t pin = mp_hal_get_pin_obj(pin_in);
mp_hal_pin_open_drain(pin);

Expand All @@ -54,7 +58,9 @@ STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in) {
mp_hal_pin_od_low(pin);
mp_hal_delay_ms(18);

mp_uint_t irq_state = mp_hal_quiet_timing_enter();
if (do_lock) {
irq_state = mp_hal_quiet_timing_enter();
}

// release the line so the device can respond
mp_hal_pin_od_high_dht(pin);
Expand Down Expand Up @@ -84,11 +90,15 @@ STATIC mp_obj_t dht_readinto(mp_obj_t pin_in, mp_obj_t buf_in) {
buf[i / 8] = (buf[i / 8] << 1) | (ticks > 48);
}

mp_hal_quiet_timing_exit(irq_state);
if (do_lock) {
mp_hal_quiet_timing_exit(irq_state);
}
return mp_const_none;

timeout:
mp_hal_quiet_timing_exit(irq_state);
if (do_lock) {
mp_hal_quiet_timing_exit(irq_state);
}
mp_raise_OSError(MP_ETIMEDOUT);
}
MP_DEFINE_CONST_FUN_OBJ_2(dht_readinto_obj, dht_readinto);
MP_DEFINE_CONST_FUN_OBJ_3(dht_readinto_obj, dht_readinto);
11 changes: 8 additions & 3 deletions drivers/dht/dht.py
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Expand Up @@ -7,16 +7,21 @@
from pyb import dht_readinto


class DHTChecksumError(Exception):
pass


class DHTBase:
def __init__(self, pin):
def __init__(self, pin, irq_block=True):
self.pin = pin
self.buf = bytearray(5)
self.irq_block = irq_block

def measure(self):
buf = self.buf
dht_readinto(self.pin, buf)
dht_readinto(self.pin, buf, self.irq_block)
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We could consider putting a retry loop here in case self.irq_block is false to free the users from doing that. I would avoid the infinite loop, though.. so maybe some fixed amount of retries should be introduced?

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@EddieParis EddieParis Feb 24, 2021 •
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I prefer to let the user decide to manage that as he prefers. In my case, I have other things to do so I can't lock main loop for too long, so I retry later. DHT is already really slow, not worth adding more delays.

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Seems valid argument, indeed. I was just afraid that anybody has to implement this loop while using this argument anyways.

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With a DHT sensor you have to implement a try:except: retry.. loop anyway because they are not very stable.

58B3
if (buf[0] + buf[1] + buf[2] + buf[3]) & 0xFF != buf[4]:
raise Exception("checksum error")
raise DHTChecksumError("Measure failed")


class DHT11(DHTBase):
Expand Down
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