Cannot use GPIO2 (D4) as digital input?

[nagylzs]

If I connect GPIO2 (D4) to GND, then I'm not able to boot my ESP8266. If I connect a serial terminal and reset the device, then it starts sending garbage continuously instead of booting up.

• I have tested this on a WeMos Mini, a NodeMcu 0.9 board and a normal sized WeMos board.

• I have completely reflashed all of them before testing, and they behave the same.

• In order to rule out all possible errors, I have used the bare dev boards (e.g. only the USB cable and the GND -> GPIO2 line was connected, nothing else.)

The most interesting thing is that this pin works fine after the device has booted. I can configure it as digital I/O pin and read 0/1 values from it. But if I connect GND on it before booting, then it simply won't boot.

I was digging through information on GPIO pins and found this:

https://community.blynk.cc/t/esp8266-gp ... es-8/22872

There are certain restrictions about GPIO pins on ESP8266, but in particular GPIO2 (D4) is said to be a "safe pin, first choice". That page also says that GPIO2 is initially LOW. On two of my dev boards, GPIO2 is connected to a LED and it is active LOW (e.g. if I connect GPIO2 to GND, then the LED lights up.) It indicates that GPIO2 is either initially HIGH, or (more probably) it is initially an input pin with a possible pull up or pull down resistor. In any case, it does not seem to be an initially LOW (output) pin. I suspected that this pin might be an initially HIGH output pin, so I have also tried to put a 2.2k resistor between GND and GPIO2 but it did not help either (e.g. won't boot).

Here are some questions:

• Is there something in MicroPython that prevents GPIO2 pin from using as an (initially LOW) input PIN? Was it somehow configured in the firmware as an output pin (initially)?

• Or maybe this is a "feature" of my dev boards, and not the ESP module itself?

• Is it a known phenomenon (for either dev boards or bare ESP modules)?

My biggest problem is that I'm out of GPIO pins and I wanted to connect a sensor to GPIO2. The output of the sensor is initially (and usually) LOW. It makes it impossible to boot my device.

[nagylzs]

Oh I can answer my own question. I have mixed up GIPO4 with D4 (GPIO2). I see now that this pin must be HIGH at boot time. Sorry for the silly question. It was a good lesson...

[nagylzs]

But now I wonder if there is a similar description for the ESP32?

[Roberthh]

The esp32 data sheet at the espressif web site.