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STM32F I2C continual AF when sending address to known good slave

Question asked by averill.edward.001 on Dec 1, 2015
Latest reply on Dec 2, 2015 by averill.edward.001
STM32F semi-newbie here, so forgive me for any cluelessness.

I have a nice daughtercard for our STM32F board that takes the I2C1 lines from the STM32F into a GPIO expander.  When I hook it up to our bouard, I get AF errors 100% of the time, but when I hook it to an older proto board it talks.

Scope shows timings on the I2C out from the STM32F are identical to our proto board, and the daughtercard timings appear the same as well.

I'm assuming that an AF error indicates that, for whatever reason, the address was clocked out but the slave device didn't ACK it.  Is this true?

I'm off to check 3.3V levels on the new board in case spin1 has "issues" on the 3.3v rail.

All info appreciated!