Keypad Routine

[emufambirwi]

Mr Goodwin, I was checking your keypad routine in your tutorials. Could you explain the following subroutine.

call Scan_Keys
movlw 0x10 ;check for no key pressed
subwf key, w
btfss STATUS, Z
goto Key_Found
call Delay20
goto Keys

 

[Nigel Goodwin]

Pretty self explanitory really?, the first line scans the keypad, and the next two check if any pressed key was found. If one was, then it jumps to Key_Found to process the key, otherwise it loops back round.

 

[emufambirwi]

movlw 0x10 ;check for no key pressed
goto Keys

Why in particular movlw 0x10 and not movlw 0x00 because I thought that this would ensure we test all the keys

 

[Nigel Goodwin]

Why in particular movlw 0x10 and not movlw 0x00 because I thought that this would ensure we test all the keys

Because the routine 'walks' a bit along the rows (or columns), and it only gets to that point if no key was pressed.

 

[emufambirwi]

Because the routine 'walks' a bit along the rows (or columns), and it only gets to that point if no key was pressed.

Keys    call       Scan_Keys 
              movlw   0x10         	;check for no key pressed 
              subwf   key, w 
              btfss    STATUS, Z 
              goto     Key_Found 
              call   	Delay20 
              goto   	Keys

I understand that we are comparing the contents of 'key' with binary '00010000'.
Does this then mean that for 'no key pressed', in 'key' there should be 0x10 or 00010000

 

[Nigel Goodwin]

Keys    call       Scan_Keys 
              movlw   0x10         	;check for no key pressed 
              subwf   key, w 
              btfss    STATUS, Z 
              goto     Key_Found 
              call   	Delay20 
              goto   	Keys

I understand that we are comparing the contents of 'key' with binary '00010000'.
Does this then mean that for 'no key pressed', in 'key' there should be 0x10 or 00010000

Yes, that's right - any other result means there's a key pressed.

 

[emufambirwi]

Would you explain what happens in terms of voltage levels when I press say key '1' and scan that row? In my case, which I suppose is similar to yours, the columns are tied to Vcc through resistors, and I scan the rows. Initially, the rows are all high