I have a PCB prototype which uses an ESP8266 module, ESP-01, the first ESP model with a 4x2 male pin header, and I will interface the ESP with a microcontroller.

My main doubt is about GPIO0 pin of this module. GPIO0 is floating on my PCB. I measured the voltage on it and observed 3.3V. Ok, it should have an internal pull-up resistor on the module. But the question is: can I, WITH SURE, leave GPIO0 floating? (considering that I don't want to flash the ESP memory via UART). Does all ESP modules have internal pull-ups at these 2 pins?

The other GPIO of ESP-01, 'GPIO2', is connected to an I/O of the MCU and that I/O of the MCU is configured as output and driving logic 1 (3.3V) to the GPIO2 pin. Maybe it would be better to configure this I/O of the MCU as digital input... Do you have any observation about putting that I/O of the MCU as output?


  • 1
    \$\begingroup\$ If you are not planning to use those two GPIOs, providing two pull-up resistors is a good idea, as I read. \$\endgroup\$
    – User323693
    Aug 17, 2019 at 5:43

1 Answer 1


The internal pull-ups are not active at startup. The pins for io 0 and io 2 are boot configuration pins. The PCB decides the configuration, not the esp8266. The ROM bootloader of the esp8266 reads this pins and depending on the state of the pins it decides about boot process.

The io 0 must be HIGH for normal boot. With io 0 LOW the esp8266 boots into flashing mode. So you must provide external pull-up.

Read the Hardware design guidelines before designing a PCB.

  • \$\begingroup\$ Hi. So it is recommended to put an external pull-up connetected to GPIO0. But I am currently using the ESP01 without it and the module is working correctly. Just to say... of course I will connect a pull-up on GPIO0 on next PCB version. \$\endgroup\$
    – abomin3v3l
    Aug 21, 2019 at 16:43
  • 2
    \$\begingroup\$ @abomin3v3l, AIThinker esp-01S has external pull-up on io 0 \$\endgroup\$
    – Juraj
    Aug 21, 2019 at 17:18

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