ESP8266 - Reading the Vcc

This is a quick summary of how to read the Vcc of ESP8266.

Arduino Sketch

1 2 3 4 5 6 7 8 9 10 11 12 13

// Source: https://github.com/esp8266/Arduino/issues/3800 ADC_MODE(ADC_VCC); void setup() { Serial.begin(115200); } void loop() { int vcc = ESP.getVcc(); Serial.println(vcc); delay(1000); }

Serial Monitor Output - Running on NodeMCU

Serial Monitor Output - Running on ESP-12 module (not NodeMCU)

The difference in readings is caused by the NodeMCU board has A0 connected to a voltage divider (see below schematic). In order to use getVdd, A0 needs to be floating.

NodeMCU Schematic

Here is a bigger, more clear version of the schematic.
https://raw.githubusercontent.com/nodemcu/nodemcu-devkit/master/Documents/NODEMCU_DEVKIT_SCH.png

Pure ESP-12 Schematic

The A0 pin (ADC) is floating in the schematic below.

Ref.: https://www.esp8266.com/viewtopic.php?f=160&t=14561

References:

vcc = ESP.getVcc() inaccurate in 2.4.0-rc2 version?
https://github.com/esp8266/Arduino/issues/3800

ESP.getVcc() doesn't read exact values
https://github.com/esp8266/Arduino/issues/721