This section gives guidance on using this part to drive LEDs.
Figure 19 shows the suggested configuration when driving the LED and the pcf8574 with the same supply AND using the part as an input.
Given that this section specifically says "Input", I assumed that if I was writing the register, I didn't need to have the "parallel" resistor (with the resulting circuit looking like Figure 20).
I use two pcf8575s to drive multiple LEDs. I write a '0' to the port to turn the LED on. I write '1' to the port to turn the LED off.
All seem to work fine on one part. On the other, 4 LEDs work as expected. Another doesn't light, and when I write '0' to it, actually turns the 4 that are working off!
I have checked the schematic and layout and they all appear to be correct. I have checked this on multiple PCBs.
Is the "parallel" resistor required when "writing" to the ports?
Thanks!