Project 1 - A Clock for the Shack
Decimal Point

Problem:
No details are given on how to light the decimal point. Indeed, if the instructions are followed, all the decimal points on all of the digits will light.

Solution:
On my clock, I hard wired the decimal point of the second digit to zero volts via a 180 ohm resistor, which is the simplest solution. However, we missed this when we moved the display into a different section. To make the change is software, change the table so that the decimal point is normally out.
RETLW 0x81 ;Zero
RETLW 0xD7 ;One
RETLW 0xA2 ;Two
RETLW 0x92 ;Three
RETLW 0xD4 ;Four
RETLW 0x98 ;Five
RETLW 0x88 ;Six
RETLW 0xD3 ;Seven
RETLW 0x80 ;Eight
RETLW 0xD0 ;Nine
RETLW 0x8F ;10 = "U"
RETLW 0xAC ;11 = "T"
RETLW 0xAE ;12 = "C"
Then, change the display code by adding the code in bold so that the decimal point is lit on the second character: 
;Update Display

TICK	MOVLW	0xFF
	MOVWF	PORTB     ;Clear Display

	MOVF	DIGIT, W  ;Update PortA

	ADDLW	0x04

	ANDLW	0x1C

	MOVWF	DIGIT
	MOVWF	PORTA

	INCF	CHAR, W   ;Update character
	ANDLW	0x07
	MOVWF	CHAR
	ADDLW	0x10
	MOVWF	FSR
	MOVF	INDR,  W   ;Get char
	ANDLW	0x0F
	CALL	TABLE      ;Returns with display in W

	MOVWF	0x21	   ;STORE RESULTS IN TEMPORARY REGISTER
	MOVF	CHAR, W	   ;MOVE OFFSET TO W
	XORLW   0x01	   ;IS IT SECOND DIGIT
	BTFSS   STATUS,2   ;CHECK ZERO BIT
	GOTO	OK	   ;NOT THIS DIGIT
	MOVLW	0x80	   ;THIS DIGIT SO SWITCH ON DOT
	SUBWF	0x21, 1
OK	MOVF	0x21, 0	   ;GET CHARACTER TO DISPLAY

	MOVWF	PORTB

	MOVLW	0xC9
	MOVWF	DELAY
LOOP1	DECFSZ	DELAY, F
	GOTO	LOOP1
Note that this will take some additional time and therefore the delay loops will also need to be modified.

Other Projects:
This change does not affect the other projects.

Alternatives:
If you have Followed the alternative solution for wiring up the display board:
Change the routine called "TABLE" to the one given below:

ADDWF  	PCL, F

RETLW	0x09	;Zero
RETLW	0xED	;One
RETLW	0x43	;Two
RETLW	0xC1	;Three
RETLW	0xA5	;Four
RETLW	0x91	;Five
RETLW	0x31	;Six
RETLW	0xCD	;Seven
RETLW	0x01	;Eight
RETLW	0x85	;Nine
RETLW	0x0F	;Letter "U">
RETLW	0x2D	;Letter "T" 
RETLW	0x2F	;Letter "C"
RETLW	0xFF	;13
RETLW	0xFF	;14
RETLW	0xFF	;15
RETLW	0xFF	;16
Then change the additional code:

	MOVWF	0x21	   ;STORE RESULTS IN TEMPORARY REGISTER
	MOVF	CHAR, W	   ;MOVE OFFSET TO W
	XORLW   0x01	   ;IS IT SECOND DIGIT
	BTFSS   STATUS,2   ;CHECK ZERO BIT
	GOTO	OK	   ;NOT THIS DIGIT

	DECF	0x21, 1	   ;THIS DIGIT SO SWITCH ON DOT

OK	MOVF	0x21, 0	   ;GET CHARACTER TO DISPLAY


See also:
Pin connections missing from the book.


Further information:
I hope that this page helps you with your projects but if you have found any other problems or if you have any further questions please contact me though the email address of gares-AT-g4aym-DOT-org-DOT-uk.

Sorry about the email address but it seems to be the only way to avoid the spammers



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