LCD (with PCF8574 I2C module ) doesn't work when using HSI in 0.5MHZ .

[HAbay.1]

I am working on a low power project using the stm32f103c8 (bluepill) and for some reason, the lcd refuses to work in low frequencies (500KHZ) , it works just fine in high frequencies (72MHZ) mind you. is it something related to the I2C clock? I've tried decreasing the value from 100khz to 10KHZ but that still didn't make a difference.

[Tesla DeLorean]

HSI can only be 8 MHz, you can divide down via the AHB / APB dividers.

Not sure what the minimums are for the I2C, but apt to be pretty low.

No idea if it's the IO Expander or the display. Expect that you'll have to do that analysis/work yourself.

Work with the vendor of the display, or if they provide contract support services.

Get a scope, or logic analyzer, on the signals and review.

Perhaps evaluate with bit-bang I2C