Here's some stuff I tried that *didn't* work.
1. I tried just using wire ties to jumper the other inputs to the first GPIO. I got this error:
"The digital input of pin "Pin_1(0)" is occupied. This pin is connected to an AMuxHW component, which indicates its digital input is expected to be controlled by the only matching hardware control signal on the AMuxHW component."
2. I tried using a hardware analog switch with a digital constant attached so all 4 lines connect to the GPIO. I got these two errors for each of the 4 lines:
Signal "Net_###" is expected to connect to only one GPIO analog pin but instead it connects to "\ScanComp_1:ScanCompAMuxSingle\". Please fix the connection.
Signal "Net_###" is expected to connect to only one GPIO analog pin but instead it connects to "AMuxHw_1". Please fix the connection.
Of course after all this I find basically the same project for PSoC4 in the 100 days thing at element14:
Since PSoC4 doesn't have a VDAC they run the IDAC output to external resistors that generate the reference voltage for the Scan_Comp. I'm not sure why they used 4 different resistors and a mux instead of using just 1 and changing the IDAC value.
(Probably because it's cooler to do it all in hardware).
I'll probably change mine to use this method, since I was planning on modifying it for PSoC4 at a later time, anyway.
I'll only be saving 1 pin on the PSoC4 version, but I can add more steps to the reference voltage without adding any pins.
(The PSoC5LP will only need 1 external input!)