Why do I have to open Serial Monitor to display what is in loop function?

Question

Why do I have to open Serial Monitor to display non-NaN values retrieved from DHT22,
what am I doing wrong in the code below?

#include <DHT.h>
#include <Wire.h>
#include <Adafruit_SSD1306.h>
#include <Adafruit_GFX.h>
// OLED display TWI address
#define OLED_ADDR 0x3C
// reset pin not used on 4-pin OLED module
Adafruit_SSD1306 display(-1); // -1 = no reset pin
// 128 x 64 pixel display
#if (SSD1306_LCDHEIGHT != 64)
#error("Height incorrect, please fix Adafruit_SSD1306.h!");
#endif
#define DHTPIN D3
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);
void setup() {
Serial.begin(115200);
delay(100);
 dht.begin();
 pinMode (DHTPIN, OUTPUT);
 /// initialize and clear display
 display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDR);
 display.clearDisplay();
 // display.drawRect(1, 1, display.width()-1, display.height()-1, WHITE); // draws the outer rectangular boundary on the screen
 display.display();
 // display a pixel in each corner of the screen
 display.drawPixel(0, 0, WHITE);
 display.drawPixel(127, 0, WHITE);
 display.drawPixel(0, 63, WHITE);
 display.drawPixel(127, 63, WHITE);
 // display a line of text
 display.setTextSize(2);
 display.setTextColor(WHITE);
 display.setCursor(20, 18);
 display.print("Welcome");
 display.setCursor(25, 40);
 display.print("San");
 // update display with all of the above graphics
 display.display();
}
void loop() {
 // put your main code here, to run repeatedly:
 float h = dht.readHumidity();
 float t = dht.readTemperature();
 display.clearDisplay();
 display.setTextSize(1);
 display.setTextColor(WHITE);
 display.setCursor(0, 1);
 display.print("Showing Temperature");
 // display Temperature
 display.setTextSize(2);
 display.setTextColor(WHITE);
 display.setCursor(1, 18);
 display.print("T : ");
 display.print(t);
 display.setTextSize(1);
 display.print((char)247);
 display.setTextSize(2);
 display.print("C");
 display.setCursor(1, 40);
 display.print("H : ");
 display.print(h);
 display.print(" %");
 // update display with all of the above graphics
 display.display();
}

After uploading code it does will display NaN but when I open serial monitor it works fine displaying actual values from DHT22 :o

below is the fritzing of the hookup, I do not have witty cloud in fritzing, so I have used wemos D1Mini instead. on wemos d1 mini 3v3 is VCC on witty cloud dev board.

enter image description here

It looks like loop function doesn't execute by itself unless I open the serial monitor, why is it so? please help

• Mikael Patel: What happens if you remove/comment out "Serial.begin(115200);"?
• AltAir: Please show your wiring schematic. Do you use pull-up resistors for RX and TX?
• per1234: Also asked at: forum.arduino.cc/index.php?topic=512284 If you're going to do that then please be considerate enough to add links to the other places you cross posted. This will let us avoid wasting time due to duplicate effort and also help others who have the same questions and find your post to discover all the relevant information.
• Ciasto piekarz: if i do not have serial monitor open loop function doesnt work. if I have serial monitor open the OLED updates, if i close serial monitor window the text on doest updates from the loop function. why is loop function dependent on serial monitor window being open ?
• Ciasto piekarz: @MikaelPatel I have tried commenting that too didnt help, long time back I had similar issue and commented the serial.begin and it worked but with witty cloud dev board it didnt.

Answer 1

Hard to tell from your fritzing which actual ESP8266 pin you are using, but you require a pull up resistor on the data line of the DHT22.

My guess is that you are using one of the serial UART pins for the data connection, and opening the serial monitor activates the output of the onboard UART chip which then acts as your missing pullup resistor.

• Ciasto piekarz: Used D7 i.e. GPIO13 on witty cloud.

Answer 2

Serial monitor sends a RESET signal during its initialization - maybe this is the problem. Sometimes these Witty boards need an implicit RESET after power up / programming.