ESP8266 Community Forum

- Thu Sep 17, 2020 6:45 am #88810 The 36k pull ups I referred to are inside the processor chip ESP8266EX and I believe are the same as the pullups used if a pin is programmed to have an internal pull up. I.e. these pullups are activated during the boot cycle. Modules like the ESP-12F which bring out these pins to the external connector do not have any extra pull ups and will boot without extra pull-ups on GPIO00 and GPIO02.

Although not directly involved in boot mode selection it is documented that TXD should not be pulled low by any external circuitry as this can prevent boot.

- Thu Sep 17, 2020 9:39 am #88812 I have found out that fact. This pin has some clock output by default.

The internal pull-ups are quite weak, I don't think something like 36 kOhms, more like 100 kOhms or more. Which is prone to noise.

- Thu Sep 17, 2020 10:42 am #88814 I have measured the pullups during the boot phase by using a 100K pull down which is insufficient to lower the logic level below 1. At boot time it gives a pin level of 2.56V relative to a 3.25V supply which corresponds to a internal pull up resistance of 27K. I have found this value to be fairly consistent across different batches of units.

GPIO02 does indeed have a burst of a clock output around 36KHz but this starts after the boot mode has been determined and has no effect on it. It occurs even with an external pullup.

A static measurement of the input pullup can also be made by setting the GPIO to INPUT_PULLUP and this gives similar results.

If no wire is attached to these pins then it would take a huge amount of noise to disturb the boot mode with this impedance.

Now if one is attaching external equipment then its characteristics need to be taken into account, but the question was about floating pins.