In a previous post I signed off with questions about the functionality of the pfc8574. What is all this ‘quasi-bidirectional’ stuff in the data sheet about? It prevented me from implementing a getPin() method in my Android Things things-drivers library. I didn’t know what would happen when I set a pin value making it an output, then read the pin state. Would it be the value I wrote, or the pin state based on the electrical biasing of the pin.

Now that I have the pcf8574 test jig I can see what actually happens when I set pcf8574 pins high or low, and Raspberry Pi pins high or low. I can make a truth table of pin states to see if it would be OK to implement the readPin() method that’s missing from the driver.

The Hardware

The hardware for this experiment is the pcf8574 test jig setup that I used in the test jig experiment.

The PCF8574 Test Jig

Continue reading