Cypress Board Analog Input Help Needed

[Tom Bottiglieri]

I saw the same behavior with last year's and this year's firmware. Do you have any signals running to the other inputs, or just the one pot? I ended up pulling the rest up manually last year because we were using some of the analog input lines for push button switches (ran out of IO) and the joystick moving to one side would pull the rest down a bit and trigger the switch. This year I just left them because we aren't using any other analog inputs either than 2 pots for our joysticks.

Is it common for a floating ADC input to swing with the other ports? It seems like this is caused by some high impedance connection between the input pins.

[mandrews281]

I have about 9 or 10 digital inputs, but nothing else analog, so those pins are unconnected. We are using the stock joysticks and they are connected via USB hub to one of the USB inputs on the classmate.

One thing I forgot to verify is if the other analog lines are following the AI1 voltage directly or are inverted.

[Mark McLeod]

It's normal for the unused lines to follow the previous analog input.
The last analog voltage sampled will linger until new input voltage replaces it.

[mandrews281]

Well that explains it. I just checked the wiring on our potentiometer. It's plugged into AI2!! (the J3 side not J2). So AI2 is tracking the pot and the other floating AI's are simply tracking AI2.

Thanks everyone.

[Joe Ross]

Originally Posted by Tom Bottiglieri

Is it common for a floating ADC input to swing with the other ports? It seems like this is caused by some high impedance connection between the input pins.

It's common for there to only be one ADC on a microcontroller, and use a mux to select each one to sample one at a time. If the input is floating, and the mux switches to it, the previous sample is retained. That's what's happening in this case.