A Virtual Peripheral Keyboard Scanner Introduction                  This   application   note   presents   programming   techniques   for   scanning   a   4x4   keyboard
usually  found  in  both  consumer  and  industrial  applications  for  simple  numeric  data  entry.  This
implementation uses the SX s internal interrupt feature to allow background operation of the code
as  a  virtual  peripheral,  uses  the  Parallax  demo  board  and  the  internal  pull-up  capability  of  SX
ports to eliminate the need of any external components.    RB0    RB1    RB2    RB3    RB4      1   2   3    RB5      4   5   6    RB6      7   8   9    RB7           0 Figure 1 - Keyboard scanner circuit diagram How the circuit and program work                  The circuit is just a simple connection to a keyboard matrix as indicated. Note that only 10
out of 16 positions are used. All 16 positions can be used by simply altering the virtual keyboard
matrix, which assigns value to the key pressed according to their row and column positions. No
external resistors are used due to the capabilities of SX to have internal weak pull up in the range of  20K.  (Note:  for  Parallax  demo  board  users,  the  connection  to  R13  for  LED  drive  must  be clipped off or disconnected since it will create a pull down effect.)                  The interrupt code segment uses a state machine approach to scan the keyboard so as not
to tie up the CPU for a long time during keyboard scan.