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We have a circuit board with a Linux System on Module. One of the SOM's I2C ports is connected to two PCA9534A I/O expanders. One of these expanders has its address pins all tied low, so its I2C address is 20H. The other has its A0 pin connected to +3.3V, so its I2C address is 21H. Neither of these ICs is responding with an ACK bit when it receives its I2C address.
The power rail is 3.3V. SCL is running at 100 KHz. SCL and SDA are pulled up to 3.3V with 4.7K resistors. (We have also tried 1K without any noticeable effect on this issue.)
