DHT22 sensor data pin reading low

[arshdeep]

Hi Team Nordic,

According to the AM2302 (DHT22) datasheet, the data pin will be pulled to high state in idle state. But in my case, when i am reading the data pin before even sending any signal to the sensor, i am reading 0 on the data pin. I am using p0.07 (pin 9 ) as data pin. VCC -> 5V, GND -> GND.

Am i not reading the pin correctly or is it a communication protocol error that i have to figure out?

Please find attached the image with output. 

Thanks, 

[awneil]

arshdeep said:

I am using p0.07 (pin 9 ) as data pin. VCC -> 5V, GND -> GND.

Trying to describe circuit connections in words is a waste of time. Please post a schematic (aka "circuit diagram").

You haven't said what Nordic part you're using, but the Absolute Maximum voltage at any IO pin on the nRF52832 is 3.9V - I expect others are similar ...

[arshdeep]

I am using nrf52832 DK .

[awneil]

arshdeep said:

I am using nrf52832 DK

So the Absolute Maximum voltage at any IO is 3.9V, then!

What does the DHT-22 datasheet say about  the voltage it outputs to its DATA pin ... ?

[arshdeep]

Hi,

I did not find anything about output voltage in datasheet, just that it sends 1/high signal . But I think the output voltage will be same as of supplied power. So, if I'm supplying 5V , It must be sending 5V output and the pins as you mentioned can only read up to 3.9V. This could be the problem.

And my VDD pins read 2.8V which is not sufficient to drive the sensor. It needs at least 3.3V.

I fired up an oscilloscope as well and can see a high signal as default.

Thanks

[hmolesworth]

I answered this earlier but it seems it got lost .. the DHT22 requires a pull-up to nRF Vdd (note not 5 volt) on the Data line, as do all 1-wire devices. The nominal value is 1k, but you can try simply changing _NOPULL to _PULLUP to see if that works at low data rates. You might also use high Drive for '0' and '1' when in output mode if the is any significant parasitic capacitance or long leads to the DHT22.

[awneil]

hmolesworth said:

the DHT22 requires a pull-up

That's kind of hinted-at in the datasheet, but there is no clear specification at all of the output characteristics: https://cdn-shop.adafruit.com/datasheets/DHT22.pdf 

Sadly, that is typical of these cheap chinese components.

However, these are widely used in the "maker" community - so there's no shortage of tutorials and examples on using them.

My first hit on googling "DHT22" is https://www.adafruit.com/product/385 - which says,

"Comes with a 4.7K - 10K resistor, which you will want to use as a pullup from the data pin to VCC"

Which is another clue!

[arshdeep]

Yes that's true. The sensor is indeed very popular and I am sure I will find out soon what is going wrong with it and will post the answer here .

 Thanks 

[awneil]

Well, if it needs a pull-up and you haven't provided one, then it obviously won't work!

In that case, would expect to always read zero from it!

[arshdeep]

Hi,

Now this is strange.

I am reading 2.94V voltage on oscilloscope from my data pin of sensor but my program is still reading 0.

Am i  missing something in my code?

#include "sdk_common.h"
#include "dht22.h"
#include <string.h>
#include "nrf_gpio.h"
#include "boards.h"
#include "nrf_log.h"


void read_dht11_dat()
 { 
	uint8_t laststate	= 1;
	uint8_t counter		= 0;
	uint8_t j		= 0, i;
	float	f; /* fahrenheit */
 
	
         nrf_gpio_cfg_input(DHTPIN,  NRF_GPIO_PIN_PULLUP) ;
         laststate = nrf_gpio_pin_read( DHTPIN );
         printf( "%d \n" ,laststate );
        
        
        
        dht11_dat[0] = dht11_dat[1] = dht11_dat[2] = dht11_dat[3] = dht11_dat[4] = 0;
        
           nrf_gpio_cfg_output(DHTPIN);
           nrf_gpio_pin_write(DHTPIN, 0);
           nrf_delay_ms(10);

        

         nrf_gpio_pin_write(DHTPIN, 1);
         nrf_delay_us(40);

         nrf_gpio_cfg_input(DHTPIN,  NRF_GPIO_PIN_PULLUP );

         for ( i = 0; i < MAXTIMINGS; i++ )
	{
		counter = 0;
		while ( nrf_gpio_pin_read(DHTPIN) == laststate)
		{
			counter++;
			nrf_delay_us(1);
			if ( counter == 255 )
			{
				break;
			}
		}
		laststate = nrf_gpio_pin_read( DHTPIN );
 
		if ( counter == 255 )
			break;
 
		/* ignore first 3 transitions */
		if ( (i >= 4) && (i % 2 == 0) )
		{
			/* shove each bit into the storage bytes */
			dht11_dat[j / 8] <<= 1;
			if ( counter > 16 )
				dht11_dat[j / 8] |= 1;
			j++;
		}
	}

        if ( (j >= 40) &&
	     (dht11_dat[4] == ( (dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3]) & 0xFF) ) )
	{
		f = dht11_dat[2] * 9. / 5. + 32;
		printf( "Humidity = %d.%d %% Temperature = %d.%d *C (%.1f *F)\n",
			dht11_dat[0], dht11_dat[1], dht11_dat[2], dht11_dat[3], f );
	}else  {
		printf( "Data not good, skip\n" );
	}
    
    }