Keypad Routine - Electronic Circuits Projects Diagrams Free

emufambirwi · 26th July 2007, 07:50 AM

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** · 26th July 2007, 05:40 PM

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 · 27th July 2007, 06:36 AM

Quote:

Originally Posted by 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** · 27th July 2007, 06:42 AM

Quote:

Originally Posted by emufambirwi

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 · 27th July 2007, 08:16 AM

Quote:

Originally Posted by Nigel Goodwin

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

Code:

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** · 27th July 2007, 08:45 AM

Quote:

Originally Posted by emufambirwi

Code:

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 · 27th July 2007, 12:53 PM

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

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26th July 2007, 07:50 AM	(permalink)
emufambirwi	Keypad Routine 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 __________________ People perish because of lack of knowledge

26th July 2007, 05:40 PM	(permalink)
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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

27th July 2007, 12:53 PM	(permalink)
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 __________________ People perish because of lack of knowledge