GPIO0 behavior on bootup

[rith87]

Hello folks,

I would like to use GPIO0 as an output. After looking around, I came across this image (from https://zoetrope.io/tech-blog/esp8266-b ... te-startup):



It seems like there is some ringing at 350Hz during startup and I managed to reproduce this on my ESP-12F. Is there any way to avoid this 350 Hz ringing? Also, can the folks at Espressif explain why this ringing occurs? Some kind of RF calibration?

[MarsTechH]

1.You can try to use a low-pass filter, cutoff freq set around 1Hz or lower. If you dont think that's a good idea, use a simple RC circuit with a thansister to cut off those oscilitor when starting up(work likes a time-delay relay). Also... if you have extra GPIOs, use other GPIO to help : )

2. No idea, GPIO0 as a BOOT Mode selector, may trying to detecte? or... RF Calib, signaling something releated to crystal decting?

BTW, I'm not esp's folks : )

[Espressif_Kelly]

Hi,

1.Wifi chip will have an internal reset in 30ms ~50ms after CHIP_EN has been pulled up.
2.And then GPIO0 will output the clock frequency (e.g 26MHz) of the external cystal by default.
3.After that, voltage of GPIO15, GPIO0 and GPIO2 will be sampled and ESP8266EX will choose the boot mode.
4.After flash boot, GPIO0 can be controlled by software.

More questions, please feel free to let us know.

Thanks.