Dear Jesmond,

From your problem it is clear that u r having 4x4 keypad. please follow the steps.

1. as you r using P0 ensure u have proper PULLUPs.
2. considor 1 nibble of P0 is row & another as colums. (for e.g. P0.0 to P0.3 are rows & P0.4 to P0.7 are columns).
3. In background keep scanning. each row by writing 0 & reading columns (this can be done best using timer 0/1 intterupt.)
4. For scanning rows; write 1's on all columns & rows (P0 = 0xFF). now make row0 = 0 (i.e.P0.0 = 0 => P0 = 0xFE).
5. Now give some 'nops'(depending on pullups & osc. freq) & again read port. If all colomns are 1's then no key is pressed. come out of ISR. if any of column is zero then keep that in memory. come out of ISR.
6. In next ISR scan row1 instead (i.e. P0.1 = 0 or P0 = 0xFD). similarly scan row2 & row3 in consugative ISR.
7. when u come back to same row where u find key pressed last time, compare read no. with previous 1, stored in memory.(remember P0 when read will have maximum of 2 zeros)
8. if no. matches for minimum of 20 to 50 mSecs it is a valid key.


I hope this will help you.