M Heimbach

Driving Keypads with Microcontroller

Discussion created by M Heimbach on Mar 25, 2010
Latest reply on Mar 29, 2010 by Peter House

Hi, I'm using an HCS08LG32 part.  Starting from an older design, I'm looking at upgrading the design and perhaps removing some redundant or un-needed parts.  Currently, the uP is driving a buffer chip (74LS244) thru some series resistors to the keypad connector.  I am basically driving 4 inputs and, depending which key is pressed, an input is shorted to an output.  This in turn goes thru more series resistors to a CMOS NOR logic gate for eventual analysis to determine which key was pressed.  Please see the attached circuit.


My question is what are the implications of removing the buffer chip and/or all the series resistors ?  Since the NOR gate is CMOS, I will keep the pull-downs for the inputs (since they'll be floating most of the time when no key is pressed).  I'm pretty sure that the uP can drive the NOR chip OK, but am not sure of what transients/problems might develop when keys are pressed/released.  Could these damage the uP ?  Could I get away with just some series resistors ?