I2C weird behavior, SDA come high before SCL

[NoNamed]

 I was implementing i2c on the stm32f4 to read the as5600 magnetic encoder. Everything ran perfectly for a while, then the I2C got stuck, I check and see that after the start generate, sda transition when the scl still low. I checked many times and the same thing happened every time.

[AScha.3]

Where you see "wrong" signal?

For me it looks ok.

[NoNamed]

ah yes my bad, 

right after generated start, the sda somehow changed before the scl.

[AScha.3]

Ok, this looks ... like an error.  One clock pulse, then nothing...

1. What you doing there ? (command call ?)

2. Did you check for errors ? (state of I2S registers ?)

[Richard Li]

This is "start" only signal, is your code make I2C start without send package?

Maybe the delay too long, send package out your screen.

[QSHAO.1]

It seems your code generated a Stop and Re-start somehow.  It could be helpful to check status register to understand if something wrong, or read STOP/RESTART related statement in STM32F4 RM.