DHT22 Fails read after Serial.println

Other Hardware Sensors
1

I noticed, when using a DHT22, the read must be done at least 5ms after any Serial.println.

The example "SimpleDHT - DHT22Default" will show this: the read sometimes fails multiple times.

Adding a delay after the serial before read2, will fix.

Serial.println("Sample DHT22...");
delay(5); // add this

You can have a better visualization of the error using the example "DHT22ErrCount": it works 100% as it is, but if you add a Serial.println(); before the read2, you'll see the success rate dropping to very low levels.

```
Serial.println("=================================");
if ((err = dht22.read2(pinDHT22, &temperature, &humidity, NULL)) != SimpleDHTErrSuccess) {
  Serial.print("Read DHT22 failed, err="); Serial.println(err);err_cnt++;
}
```

This way I got 66% error rate instead of 100%.

So, before any usage of the read2 function, add a delay(5) and you'll be safe.
I haven't checked other conditions, it may apply also to other functions or sensor (probably DHT11) .

2

Post code.

3

Here the full code, modified by adding this row:

    Serial.println("== READ2 MAY FAIL AFTER THIS ==");  // PRINT SERIAL BEFORE READ2

#include <SimpleDHT.h>

// for DHT22, 
//      VCC: 5V or 3V
//      GND: GND
//      DATA: 2
int pinDHT22 = 2;
SimpleDHT22 dht22;

void setup() {
  Serial.begin(115200);
}

void loop() {
  // start working...
  Serial.println("=================================");
  Serial.println("Sample DHT22 with error count");

  int cnt = 0;
  int err_cnt = 0;
  for (;;) {
    cnt++;

    float temperature = 0;
    float humidity = 0;
    int err = SimpleDHTErrSuccess;
   
    Serial.println("== READ2 MAY FAIL AFTER THIS ==");  // PRINT SERIAL BEFORE READ2

    if ((err = dht22.read2(pinDHT22, &temperature, &humidity, NULL)) != SimpleDHTErrSuccess) {
      Serial.print("Read DHT22 failed, err="); Serial.println(err);err_cnt++;
    }

    Serial.print((float)temperature); Serial.print(" *C, ");
    Serial.print((float)humidity); Serial.print(" RH%");
    Serial.print(", total: "); Serial.print(cnt);
    Serial.print(", err: "); Serial.print(err_cnt);
    Serial.print(", success rate: "); Serial.print((cnt - err_cnt) * 100.0 / (float)cnt); Serial.println("%");

    delay(2500);
  }
}

This way the example DHT22ErrCount will produce an error rate < 100%.

4

What if you try

5

This solves the problem too.

So do you think it happens because the serial is still transmitting, and by flushing we make sure the transmission is completed?

I also noticed, when using pin 2 or 4 it works as we've seen until now (it works unless you print without flushing/delay), with pin 3 or 5, instead, it fails a lot , even without print or with print +flush.

6

This topic was automatically closed 180 days after the last reply. New replies are no longer allowed.