The analog section on the ESP32 is a far cry from the ESP8266’s single ADC input pin. There are three UARTs, three SPIs, two I2Ss, and two I2C busses. All of these participate in the I/O matrix ...

Battery voltage is also measured via a voltage divider connected to the chip’s ADC pin. At regular intervals throughout the day, the ESP8266 polls the ATtiny13 to pull the stored sensor pulses ...

Its small form factor allows you to keep it on a keychain and so you can bring your ESP8266 everywhere; hence the name. It has a small connector that allows you to access 5 GPIOS, and one ADC pin.