I am reverse engineering a circuit so that I can interface to it and electronically control the button input. I have successfully accomplished this on an earlier version of the product, however, the next revision/generation is giving me issues. I wanted to bounce this off the group...
The keypad for both versions is 12 buttons with 3 commons (15 pins total). The keypad is an FPC which plugs into a ZIF socket. I interface at the point of the ZIF socket. There are no components on the FPC. It consists only of the 15 pins wired to membrane switches.
The current version (the one that I can interface to) is very simple in that the 3 common lines are tied together and are at ground potential. The remaining 12 pins are all routed back and individually pulled high. This makes interfacing very easy as I simply route the 12 pins to 12 N-Channel MOSFETs tying the drain to the pin and the source to any of the commons (ground).
The newest version has 3 commons. Each common is routed to a group of four buttons. So, common 1 is associated with buttons 1,2,3 and 4... and so on. The commons are at roughly 3VDC and the remaining 12 pins are at approximately .65VDC. Neither has a low impedance path back to the systems positive rail or ground. The target circuit is very complex (>6 layers) so I can't trace anything around the board.
I can physically short a pin to the appropriate common and the functionality associated with that button works. I attempted to control the functionality by wiring in both P and N channel FETs (separately obviously). They both work on some buttons and not on others. For example, in the set of buttons (4) associated with common 1, button 2 and 3 may work reliably all the time but buttons 1 and 4 will work sometimes yet trigger the functionality associated with buttons 2 and 3 at other times.
I can't seem to wrap my head around what they might be doing in the target circuit. The only reason I can see for them to change the interface is to minimize I/O count... but I don't see how this would achieve that goal.
Any suggestions?