Wire.setClock() keeps SCL low with 800kHz

[Koepel]

The default 100 kHz works, and the Wire.setClock(100000L) works. If the clock is set to 800 kHz or above, then the SCL signal stays low because of a missing stop condition. The I2C speed for normal signals should be from 100kHz to 400kHz, but someone might try 800kHz.

Tested with Arduino IDE 1.8.15 and Nano Every board. Nothing is connected to the I2C bus. Test sketch:

#include <Wire.h>

void setup()
{
  Wire.begin();
  Wire.setClock( 100000L);
}

void loop()
{
  Wire.beginTransmission( 0x2B);
  Wire.endTransmission();

  delay( 100);
}

Result with a sample rate of 24 MHz with LHT00SU1 and PulseView/sigrok). The measured frequency of SCL varies a few percentage. It is 99 kHz with no pullup resistors here:

With Wire.setClock(400000L); the measured frequency of SCL is 303 kHz with no pullup resistors:

With Wire.setClock(800000L); the SCL signal stays low. There is no stop condition. The SCL raises before the next start condition. The measured frequency of SCL is 525 kHz with no pullup resistors.

[Update] I have edited this Issue because I learned that not the clock speed of SCL is set, but that the resulting clock speed is relative to the rise time.

[softhack007]

Just looked into DxCore, and saw that two things are done differently:

• TWI baud rate calculation has an offset that depends on CPU speed
• setting of a register bit "FMPEN" for TWI speed above 600KHz

Would this also help in megaavr core?

https://github.com/SpenceKonde/DxCore/blob/f49e0d61e066b4c9d606a5f8297da90574c93012/megaavr/libraries/Wire/src/twi.c#L230-L234