ESP8266 Community Forum

- Mon Dec 14, 2015 3:16 pm #36419 I recognized that compiling at 80Mhz instead of 160Mhz causes about 75% less misreads.
So it looks like that CPU cycle is indead basic the problem

- Tue Dec 15, 2015 10:33 am #36484 Getting a better sensor is a solution but the DHT11 and DHT22 have low prices. I2C slave sensor that stretch the clock are a good solution.
Now the esp8266 has pretty good interrupt on a voltage change on a pin. The DHT devices are open drain so they will actively pull down and passively pull up. The DHT devices use pulse width length to transfer logic 0's and logic 1's. The commercial code focused on Arduiono pcb's doesn't use interrupt on change. The esp8266 is very able in this regard. The issues with the DHT so far appear to be timing issues and power issues. The esp8266 needs lots of power when transmitting and also needs cpu cycles. The assumption can be that the esp8266 can't guarantee correct timing in all situations. The DHT timing seems tolerant enough at a first glance but perhaps the esp8266 can make it unreliable. What if the esp8266 had an interrupt method to measure pulse length to mimic CCP features many PIC MCU's have?

- Tue Dec 15, 2015 4:40 pm #36521 Hi,
I use the AM2321, it's cheap, has i2c and onewire and the accuracy is better than DHT11.

- Fri Jan 01, 2016 4:47 pm #37647 There seems to be a variance in the timing of the ESP8266 core cycles and this is reflected on the First read, read wait then read again appears to provide a much more stable output with the exclusion of the third parameter provided to the library....
The advantage of this is the "Pin Multiplexer" internal to the ESP8266 will be in a settled state.....

The data from this channel was very erratic but this seems to have sorted most variances, I still have a dodgy sensor...
http://api.thingspeak.com/apps/matlab_visualizations/41029

Code: Select all//////////////////////////////
// DHT11 / AMS2301 is at GPIO2
//////////////////////////////
#define DHTPIN 2

// Uncomment whatever type you're using!
#define DHTTYPE DHT11   // DHT 11
//#define DHTTYPE DHT22   // DHT 22  (AM2302)
//#define DHTTYPE DHT21   // DHT 21 (AM2301)

// init DHT; 3rd parameter = 16 works for ESP8266@80MHz
DHT dht(DHTPIN, DHTTYPE); ;
     long int spdcount = ESP.getCycleCount();
     delay(1);
     long int spdcount1 = ESP.getCycleCount();
     long int speedcnt = spdcount1-spdcount;
//Use this to get the "current timing"

delay(50);
   pfTemp = dht.readTemperature(); 
   pfHum = dht.readHumidity();
delay(2010);
   pfTemp = dht.readTemperature(); 
   pfHum = dht.readHumidity();
   float a = 17.67;
   float b = 243.5;
   float alpha = (a * pfTemp)/(b + pfTemp) + log(pfHum/100);
   pfDew = (b * alpha)/(a - alpha);