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Guys,
I know that I'm missing something basic here but I need some help. We've used the TMUX1108 in many different products with no issues. We're working on a new design and we're seeing something we can't explain.
We are multiplexing 8 switches into a single pin on a processor. Each of the inputs is pulled up to 5 volts through a 10k resistor. The switches pull these inputs to ground. Here is what we're seeing.
We have the EN enable pin pulled up to 5 volts on the board.
Regardless of the states of the address pins (A0, A1 & A2) turning on switch 8 pulls the D signal to ground. We've checked the artwork and the schematic and can see no issues.
As an example, we have the processor pins connected to A0, A1 and A2 all pulled low. In theory, the TMUX1108 should connect the D pin to S1. It doesn't, even though S1 should be enabled, we're getting S8. The same is true for any other combination of the address pins. We can confirm that we are getting the correct voltages from the processor onto the A0, A1 and A2 pins on the TMUX1108.
The layout on the board is pretty simple. Here is an image of that part of the board with the TMUX1108, the traces and the pull-up resistors.
We get the same result on different boards with the exact same hardware and code. We've swapped the TMUX1108 on these boards with new parts out of the tape and we see the same behavior.
We can only guess that we're doing something to damage the chip in our testing. We did see one board that exhibited this behavior that also had the other pins going to the TMUX1108 getting pulled down by about 600 mV when we grounded the switch connected to S8.
I know that we are missing something basic. I really appreciate any feedback or thoughts on how to fix this.
Thanks,
Jay
