In the main program, bit 4-7 of port B are initialized as inputs with Schmitt-trigger
and pull-ups while bit 3-0 are configure as outputs. Scanning is accomplished by outputting a  zero  on  a  column.  If  any  key  on  that  column  is  pressed,  a  zero  will  be  read  in  the  row where that key is located. Due to the internal pull-ups, all inactive inputs will be read as  1 s.                 The RTCC is initialized to be incremented on internal clock cycles. With a prescaler
ratio of 8, coupled with a value of 250 in the RETIW interrupt routine which will be loaded
into RTCC, it will give us approximately 1 mS per interrupt.                 The  state  machine  is  initialized  to  start  with  fast  scan  which  output  zeroes  to  all
columns and detect all keys at the same time. Once any key is detected, debouncing will be started.  Normally  this  is  dependent  on  the  mechanical  characteristics  of  the  keys.  In  this case, we are using a 20 mS debouncing time.                 After debouncing is done, a detailed scanning will be done. Each column will have a
zero output at a time to detect if any key in that certain column is pressed. If that is the case,
a  value  other  than    11110000  (binary)  will  be  read  from  the  inputs  (RB4-7)  and  we  will
know that a key is pressed. Otherwise, the detection that we made from the fast scan stage
may just have been noise and we transition back to fast scan state.                 When a value other than 11110000 (binary) is  read,  then  we  use  that  value  to  find
out  on  which  row  the  key  is.  That  row  count  and  the  column  count  that  we  keep  during
column scan will give us the exact location of the key hit by the formula: index to virtual matrix = 4*row count + column count Column count (cnt1)              3          2          1         0                                                                 RB3      RB2    RB1     RB0
scan pattern (RB3-0)          0111    1011    1101    1110 Row count (cnt2) input value (RB7-4) 3 F E D C 1110 RB4 B A 9 8 1101    RB5 2 1 7 6 5 4 1011    RB6 0 3 2 1 0 0111    RB7 Note: Index to virtual matrix is indicated at each intersection.