Dear Chaitanya,

I have one suggestion that DONOT wait for a key press(while(col==0xff); use a timer isr instead(if your application permits).

Do it as follows,

1. for each instance of isr scan only one column check if you read any 0 bit on rows. If yes store it in one variable
2. ofcourse next instance will scan next column.
3. in this fashion 8 instances will cover 8 columns(lets call this as on set of scans).
4. if you find same key pressed for atleast (M) sets of scans, treat it as valid key(decide M by time of debounce required & what is configured in THx & TLx)
5. if you find 2 of the columns (N)th & (N + i)th has a key press; then depending on your need can ignore both or ignore one & store only another in variable.

Regards,
Mahesh