ESP8266 Community Forum

How fast is I2C bus implementation for ESP8266?
Is it 400kHz or rather 100kHz? I ask, because I have some problem with PCF8574 port expanders, although DS3231 RTC is working fine. RTC is designed for 400kHz, expanders support only 100kHz.
I see I2C master library is a software implementation of this bus, utilizing bit banging.
Is it possible to set frequency manually?

Looking at the code, it seems to be just as you said ... it sets some bits, waits 2-5 micro seconds and then sets the next bits. There doesn't appear to be any "real-time" clock synchronization going on at all. Looking at the function "writeByte", I seem to see that there is an explicit 7 microseconds ( + data shifts + GPIO outputs) per bit ... which gives me about 100Khz - 140KHz ... it certainly won't be higher than 140KHz with explicit 7 microsecond "time to write a bit".

I had no problems with the PCF8574 ... see viewtopic.php?f=15&t=4447