How to read switches.

Tutorial 2.1 - requires Main Board and Switch Board.

This simple program turns the corresponding LED on, when the button opposite it is pressed, extinguishing all other LED's.

;Tutorial 2.1 - Nigel Goodwin 2002
LIST p=16F628 ;tell assembler what chip we are using
include "P16F628.inc" ;include the defaults for the chip
__config 0x3D18 ;sets the configuration settings (oscillator type etc.)


LEDPORT Equ PORTA ;set constant LEDPORT = 'PORTA'
SWPORT Equ PORTA ;set constant SWPORT = 'PORTA'
LEDTRIS Equ TRISA ;set constant for TRIS register
SW1 Equ 7 ;set constants for the switches
SW2 Equ 6
SW3 Equ 5
SW4 Equ 4
LED1 Equ 3 ;and for the LED's
LED2 Equ 2
LED3 Equ 1
LED4 Equ 0

;end of defines

org 0x0000 ;org sets the origin, 0x0000 for the 16F628,
;this is where the program starts running 
movlw 0x07
movwf CMCON ;turn comparators off (make it like a 16F84)

bsf STATUS, RP0 ;select bank 1
movlw b'11110000' ;set PortA 4 inputs, 4 outputs
movwf LEDTRIS
bcf STATUS, RP0 ;select bank 0
clrf LEDPORT ;set all outputs low


Loop btfss SWPORT, SW1
call Switch1
btfss SWPORT, SW2
call Switch2
btfss SWPORT, SW3
call Switch3
btfss SWPORT, SW4
call Switch4
goto Loop

Switch1 clrf LEDPORT ;turn all LED's off
bsf SWPORT, LED1 ;turn LED1 on
retlw 0x00

Switch2 clrf LEDPORT ;turn all LED's off
bsf SWPORT, LED2 ;turn LED2 on
retlw 0x00

Switch3 clrf LEDPORT ;turn all LED's off
bsf SWPORT, LED3 ;turn LED3 on
retlw 0x00

Switch4 clrf LEDPORT ;turn all LED's off
bsf SWPORT, LED4 ;turn LED4 on
retlw 0x00


end 

As with the previous tutorials we first set things up, then the main program runs in a loop, the first thing the loop does is check switch SW1 with the 'btfss SWPORT, SW1' line, if the switch isn't pressed the input line is held high by the 10K pull-up resistor and it skips the next line. This takes it to the 'btfss SWPORT, SW2' line, where SW2 is similarly checked - this continues down checking all the switches and then loops back and checks them again. If a key is pressed, the relevant 'btfss' doesn't skip the next line, but instead calls a sub-routine to process the key press, each switch has it's own sub-routine. These sub-routines are very simple, they first 'clrf' the output port, turning all LED's off, and then use 'bsf' to turn on the corresponding LED, next the sub-routine exits via the 'retlw' instruction. As the switch is likely to be still held down, the same routine will be run again (and again, and again!) until you release the key, however for this simple application that isn't a problem and you can't even tell it's happening.

Tutorial 2.2 - requires Main Board and Switch Board.

This program toggles the corresponding LED on and off, when the button opposite it is pressed. It introduces the concept of 'de-bouncing' - a switch doesn't close immediately, it 'bounces' a little before it settles, this causes a series of fast keypresses which can cause chaos in operating a device.

;Tutorial 2.2 - Nigel Goodwin 2002
LIST p=16F628 ;tell assembler what chip we are using
include "P16F628.inc" ;include the defaults for the chip
__config 0x3D18 ;sets the configuration settings (oscillator type etc.)

cblock 0x20 ;start of general purpose registers
count1 ;used in delay routine
counta ;used in delay routine 
countb ;used in delay routine
endc

LEDPORT Equ PORTA ;set constant LEDPORT = 'PORTA'
SWPORT Equ PORTA ;set constant SWPORT = 'PORTA'
LEDTRIS Equ TRISA ;set constant for TRIS register
SW1 Equ 7 ;set constants for the switches
SW2 Equ 6
SW3 Equ 5
SW4 Equ 4
LED1 Equ 3 ;and for the LED's
LED2 Equ 2
LED3 Equ 1
LED4 Equ 0

SWDel Set Del50 ;set the de-bounce delay (has to use 'Set' and not 'Equ')

;end of defines

org 0x0000 ;org sets the origin, 0x0000 for the 16F628,
;this is where the program starts running 
movlw 0x07
movwf CMCON ;turn comparators off (make it like a 16F84)

bsf STATUS, RP0 ;select bank 1
movlw b'11110000' ;set PortA 4 inputs, 4 outputs
movwf LEDTRIS
bcf STATUS, RP0 ;select bank 0
clrf LEDPORT ;set all outputs low


Loop btfss SWPORT, SW1
call Switch1
btfss SWPORT, SW2
call Switch2
btfss SWPORT, SW3
call Switch3
btfss SWPORT, SW4
call Switch4
goto Loop

Switch1 call SWDel ;give switch time to stop bouncing
btfsc SWPORT, SW1 ;check it's still pressed
retlw 0x00 ;return is not
btfss SWPORT, LED1 ;see if LED1 is already lit
goto LED1ON
goto LED1OFF

LED1ON bsf LEDPORT, LED1 ;turn LED1 on
call SWDel
btfsc SWPORT, SW1 ;wait until button is released
retlw 0x00
goto LED1ON 

LED1OFF bcf LEDPORT, LED1 ;turn LED1 on
call SWDel
btfsc SWPORT, SW1 ;wait until button is released
retlw 0x00
goto LED1OFF 

Switch2 call SWDel ;give switch time to stop bouncing
btfsc SWPORT, SW2 ;check it's still pressed
retlw 0x00 ;return is not
btfss SWPORT, LED2 ;see if LED2 is already lit
goto LED2ON
goto LED2OFF

LED2ON bsf LEDPORT, LED2 ;turn LED2 on
call SWDel
btfsc SWPORT, SW2 ;wait until button is released
retlw 0x00
goto LED2ON 

LED2OFF bcf LEDPORT, LED2 ;turn LED2 on
call SWDel
btfsc SWPORT, SW2 ;wait until button is released
retlw 0x00
goto LED2OFF

Switch3 call SWDel ;give switch time to stop bouncing
btfsc SWPORT, SW3 ;check it's still pressed
retlw 0x00 ;return is not
btfss SWPORT, LED3 ;see if LED3 is already lit
goto LED3ON
goto LED3OFF

LED3ON bsf LEDPORT, LED3 ;turn LED3 on
call SWDel
btfsc SWPORT, SW3 ;wait until button is released
retlw 0x00
goto LED3ON 

LED3OFF bcf LEDPORT, LED3 ;turn LED3 on
call SWDel
btfsc SWPORT, SW3 ;wait until button is released
retlw 0x00
goto LED3OFF

Switch4 call SWDel ;give switch time to stop bouncing
btfsc SWPORT, SW4 ;check it's still pressed
retlw 0x00 ;return is not
btfss SWPORT, LED4 ;see if LED4 is already lit
goto LED4ON
goto LED4OFF

LED4ON bsf LEDPORT, LED4 ;turn LED4 on
call SWDel
btfsc SWPORT, SW4 ;wait until button is released
retlw 0x00
goto LED4ON 

LED4OFF bcf LEDPORT, LED4 ;turn LED4 on
call SWDel
btfsc SWPORT, SW4 ;wait until button is released
retlw 0x00
goto LED4OFF

;modified Delay routine, direct calls for specified times
;or load W and call Delay for a custom time.

Del0 retlw 0x00 ;delay 0mS - return immediately
Del1 movlw d'1' ;delay 1mS
goto Delay
Del5 movlw d'5' ;delay 5mS
goto Delay
Del10 movlw d'10' ;delay 10mS
goto Delay
Del20 movlw d'20' ;delay 20mS
goto Delay
Del50 movlw d'50' ;delay 50mS
goto Delay
Del100 movlw d'100' ;delay 100mS
goto Delay
Del250 movlw d'250' ;delay 250 ms
Delay movwf count1
d1 movlw 0xC7 ;delay 1mS
movwf counta
movlw 0x01
movwf countb
Delay_0
decfsz counta, f
goto $+2
decfsz countb, f
goto Delay_0

decfsz count1 ,f
goto d1
retlw 0x00
end 

In order to de-bounce the keypresses we delay for a short time, then check that the button is still pressed, the time delay is set by the variable SWDel, which is defined as Del50 in the defines section at the start of the program. I've extended the Delay routine to provide a selection of different delays (from 0mS to 250mS), called by simple 'call' instructions, the Delay routine itself can also be called directly - simply load the required delay into the W register and 'call Delay'. We then check to see if the corresponding LED is lit, with 'btfss SWPORT, LEDx', and jump to either 'LEDxON' or 'LEDxOFF', these routines are almost identical, the only difference being that the first turns the LED on, and the second turns it off. They first switch the LED, on or off, depending on which routine it is, and then delay again (calling SWDel as before), next they check to see if the button is still pressed, looping back around if it is. Once the key has been released the routine exits via the usual 'retlw' and returns to waiting for a keypress. I've used the variable SWDel (and provided the various delay times) so that you can easily try the effect of different delay times - in particular try setting SWDel to Del0, and see how the button pressing isn't reliable, you will probably find one of the buttons is worse than the others - particularly if you use old switches, wear makes them bounce more.

Tutorial 2.3 - requires Main Board, Switch Board, and LED Board.

Now for a more realistic example - this combines Tutorial 2.1 with Tutorial 1.9, the result is an LED sequencing program with four different patterns, selected by the four keys, with the key selected indicated by the corresponding LED.

;Tutorial 2.3 - Nigel Goodwin 2002
LIST p=16F628 ;tell assembler what chip we are using
include "P16F628.inc" ;include the defaults for the chip
__config 0x3D18 ;sets the configuration settings (oscillator type etc.)

cblock 0x20 ;start of general purpose registers
count ;used in table read routine
count1 ;used in delay routine
counta ;used in delay routine 
countb ;used in delay routine
endc

LEDPORT Equ PORTB ;set constant LEDPORT = 'PORTB'
LEDTRIS Equ TRISB ;set constant for TRIS register
SWPORT Equ PORTA
SWTRIS Equ TRISA
SW1 Equ 7 ;set constants for the switches
SW2 Equ 6
SW3 Equ 5
SW4 Equ 4
LED1 Equ 3 ;and for the LED's
LED2 Equ 2
LED3 Equ 1
LED4 Equ 0

org 0x0000 ;org sets the origin, 0x0000 for the 16F628,
;this is where the program starts running 
movlw 0x07
movwf CMCON ;turn comparators off (make it like a 16F84)

bsf STATUS, RP0 ;select bank 1
movlw b'00000000' ;set PortB all outputs
movwf LEDTRIS
movlw b'11110000' ;set PortA 4 inputs, 4 outputs
movwf SWTRIS
bcf STATUS, RP0 ;select bank 0
clrf LEDPORT ;set all outputs low
clrf SWPORT
bsf SWPORT, LED1 ;set initial pattern


Start clrf count ;set counter register to zero
Read movf count, w ;put counter value in W
btfsc SWPORT, LED1 ;check which LED is lit
call Table1 ;and read the associated table
btfsc SWPORT, LED2
call Table2
btfsc SWPORT, LED3
call Table3
btfsc SWPORT, LED4
call Table4
movwf LEDPORT
call Delay
incf count, w
xorlw d'14' ;check for last (14th) entry
btfsc STATUS, Z
goto Start ;if start from beginning
incf count, f ;else do next
goto Read

Table1 ADDWF PCL, f ;data table for bit pattern
retlw b'10000000'
retlw b'01000000'
retlw b'00100000'
retlw b'00010000'
retlw b'00001000'
retlw b'00000100'
retlw b'00000010'
retlw b'00000001'
retlw b'00000010'
retlw b'00000100'
retlw b'00001000'
retlw b'00010000'
retlw b'00100000'
retlw b'01000000'

Table2 ADDWF PCL, f ;data table for bit pattern
retlw b'11000000'
retlw b'01100000'
retlw b'00110000'
retlw b'00011000'
retlw b'00001100'
retlw b'00000110'
retlw b'00000011'
retlw b'00000011'
retlw b'00000110'
retlw b'00001100'
retlw b'00011000'
retlw b'00110000'
retlw b'01100000'
retlw b'11000000'

Table3 ADDWF PCL, f ;data table for bit pattern
retlw b'01111111'
retlw b'10111111'
retlw b'11011111'
retlw b'11101111'
retlw b'11110111'
retlw b'11111011'
retlw b'11111101'
retlw b'11111110'
retlw b'11111101'
retlw b'11111011'
retlw b'11110111'
retlw b'11101111'
retlw b'11011111'
retlw b'10111111'

Table4 ADDWF PCL, f ;data table for bit pattern
retlw b'00111111'
retlw b'10011111'
retlw b'11001111'
retlw b'11100111'
retlw b'11110011'
retlw b'11111001'
retlw b'11111100'
retlw b'11111100'
retlw b'11111001'
retlw b'11110011'
retlw b'11100111'
retlw b'11001111'
retlw b'10011111'
retlw b'00111111'

ChkKeys btfss SWPORT, SW1
call Switch1
btfss SWPORT, SW2
call Switch2
btfss SWPORT, SW3
call Switch3
btfss SWPORT, SW4
call Switch4
retlw 0x00

Switch1 clrf SWPORT ;turn all LED's off
bsf SWPORT, LED1 ;turn LED1 on
retlw 0x00

Switch2 clrf SWPORT ;turn all LED's off
bsf SWPORT, LED2 ;turn LED2 on
retlw 0x00

Switch3 clrf SWPORT ;turn all LED's off
bsf SWPORT, LED3 ;turn LED3 on
retlw 0x00

Switch4 clrf SWPORT ;turn all LED's off
bsf SWPORT, LED4 ;turn LED4 on
retlw 0x00


Delay movlw d'250' ;delay 250 ms (4 MHz clock)
movwf count1
d1 call ChkKeys ;check the keys
movlw 0xC7 ;delay 1mS
movwf counta
movlw 0x01
movwf countb
Delay_0
decfsz counta, f
goto $+2
decfsz countb, f
goto Delay_0

decfsz count1 ,f
goto d1
retlw 0x00

end 

The main differences here are in the Delay routine, which now has a call to check the keys every milli-second, and the main loop, where it selects one of four tables to read, depending on the settings of flag bits which are set according to which key was last pressed.

Tutorial 2.4 - requires Main Board, Switch Board, and LED Board.

Very similar to the last tutorial, except this one combines Tutorials 2.2 and 2.3 with Tutorial 1.9, the result is an LED sequencing program with three different patterns, selected by three of the keys, with the key selected indicated by the corresponding LED - the difference comes with the fourth switch, this selects slow or fast speeds, with the fast speed being indicated by a toggled LED.

;Tutorial 2.4 - Nigel Goodwin 2002
LIST p=16F628 ;tell assembler what chip we are using
include "P16F628.inc" ;include the defaults for the chip
__config 0x3D18 ;sets the configuration settings (oscillator type etc.)

cblock 0x20 ;start of general purpose registers
count ;used in table read routine
count1 ;used in delay routine
count2 ;used in delay routine
counta ;used in delay routine 
countb
countc
countd
speed
endc

LEDPORT Equ PORTB ;set constant LEDPORT = 'PORTB'
LEDTRIS Equ TRISB ;set constant for TRIS register
SWPORT Equ PORTA
SWTRIS Equ TRISA
SW1 Equ 7 ;set constants for the switches
SW2 Equ 6
SW3 Equ 5
SW4 Equ 4
LED1 Equ 3 ;and for the LED's
LED2 Equ 2
LED3 Equ 1
LED4 Equ 0


SWDel Set Del50

org 0x0000 ;org sets the origin, 0x0000 for the 16F628,
;this is where the program starts running 
movlw 0x07
movwf CMCON ;turn comparators off (make it like a 16F84)

bsf STATUS, RP0 ;select bank 1
movlw b'00000000' ;set PortB all outputs
movwf LEDTRIS
movlw b'11110000' ;set PortA 4 inputs, 4 outputs
movwf SWTRIS
bcf STATUS, RP0 ;select bank 0
clrf LEDPORT ;set all outputs low
clrf SWPORT ;make sure all LED's are off
bsf SWPORT, LED1 ;and turn initial LED on
movlw d'250'
movwf speed ;set initial speed



Start clrf count ;set counter register to zero
Read movf count, w ;put counter value in W
btfsc SWPORT, LED1 ;check which LED is on
call Table1 ;and call the associated table
btfsc SWPORT, LED2
call Table2
btfsc SWPORT, LED3
call Table3
movwf LEDPORT
call DelVar
incf count, w
xorlw d'14' ;check for last (14th) entry
btfsc STATUS, Z
goto Start ;if start from beginning
incf count, f ;else do next
goto Read

Table1 ADDWF PCL, f ;data table for bit pattern
retlw b'10000000'
retlw b'01000000'
retlw b'00100000'
retlw b'00010000'
retlw b'00001000'
retlw b'00000100'
retlw b'00000010'
retlw b'00000001'
retlw b'00000010'
retlw b'00000100'
retlw b'00001000'
retlw b'00010000'
retlw b'00100000'
retlw b'01000000'

Table2 ADDWF PCL, f ;data table for bit pattern
retlw b'11000000'
retlw b'01100000'
retlw b'00110000'
retlw b'00011000'
retlw b'00001100'
retlw b'00000110'
retlw b'00000011'
retlw b'00000011'
retlw b'00000110'
retlw b'00001100'
retlw b'00011000'
retlw b'00110000'
retlw b'01100000'
retlw b'11000000'

Table3 ADDWF PCL, f ;data table for bit pattern
retlw b'01111111'
retlw b'10111111'
retlw b'11011111'
retlw b'11101111'
retlw b'11110111'
retlw b'11111011'
retlw b'11111101'
retlw b'11111110'
retlw b'11111101'
retlw b'11111011'
retlw b'11110111'
retlw b'11101111'
retlw b'11011111'
retlw b'10111111'

ChkKeys btfss SWPORT, SW1
call Switch1
btfss SWPORT, SW2
call Switch2
btfss SWPORT, SW3
call Switch3
btfss SWPORT, SW4
call Switch4
retlw 0x00

Switch1 bcf SWPORT, LED2 ;turn unselected LED's off
bcf SWPORT, LED3 ;turn unselected LED's off
bsf SWPORT, LED1 ;turn LED1 on
retlw 0x00

Switch2 bcf SWPORT, LED1 ;turn unselected LED's off
bcf SWPORT, LED3 ;turn unselected LED's off
bsf SWPORT, LED2 ;turn LED2 on
retlw 0x00

Switch3 bcf SWPORT, LED1 ;turn unselected LED's off
bcf SWPORT, LED2 ;turn unselected LED's off
bsf SWPORT, LED3 ;turn LED3 on
retlw 0x00

Switch4 call SWDel ;give switch time to stop bouncing
btfsc SWPORT, SW4 ;check it's still pressed
retlw 0x00 ;return is not
btfss SWPORT, LED4 ;see if LED4 is already lit
goto FASTON
goto FASTOFF

FASTON bsf SWPORT, LED4 ;turn LED4 on
movlw d'80'
movwf speed ;set fast speed
call SWDel
btfsc SWPORT, SW4 ;wait until button is released
retlw 0x00
goto FASTON 

FASTOFF bcf SWPORT, LED4 ;turn LED4 on
movlw d'250'
movwf speed ;set slow speed
call SWDel
btfsc SWPORT, SW4 ;wait until button is released
retlw 0x00
goto FASTOFF

DelVar movfw speed ;delay set by Speed
movwf count1
d1 call ChkKeys ;check the keys
movlw 0xC7 ;delay 1mS
movwf counta
movlw 0x01
movwf countb
Delay_0
decfsz counta, f
goto $+2
decfsz countb, f
goto Delay_0

decfsz count1 ,f
goto d1
retlw 0x00

;use separate delay routines, as Del50 is called from ChkKeys
;which is called from within DelVar

Del50 movlw d'50' ;delay 50mS
movwf count2
d3 movlw 0xC7 ;delay 1mS
movwf countc
movlw 0x01
movwf countd
Delay_1
decfsz countc, f
goto $+2
decfsz countd, f
goto Delay_1

decfsz count2 ,f
goto d3
retlw 0x00


end