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608	Chapter 16

;Procedure to read the current time from the RTC and store

;data (in packed BCD format) in local time registers.

;According to wiring diagram

;NJU6355 Interface for read operations:

; DAT	PORTB,0	Input
; CLK	PORTB,1	Output
; CE	PORTB,2	Output
; IO	PORTB,3	Output
Get_Time
; Clear Port-B
movlw	b’00000000’
movwf	PORTB

;Make data line input Bank1

movlw b’00000001’ movwf TRISB

Bank0

;Reading RTC data requires that the IO line be low and the

;CE line be high. CLK line is held low

	bcf	PORTB,CLK	; CLK low
	call	delay_125
	bcf	PORTB,IO ;	IO line low
	call	delay_125
	bsf	PORTB,CE ;	and CE line high
; Data is read from RTC as follows:
;	year	8	bits (0 to 99)
;	month	8	bits (1 to 12)
;	day	8	bits (1 to 31)
;	dayOfWeek	4	bits (1 to 7)
;	hour	8	bits (0 to 23)
;	minutes	8	bits (0 to 59)
;	seconds	8	bits (0 to 59)
;		======
;	Total	52 bits
	call	readRTC
	movwf	year
	call	delay_125
	call	readRTC
	movwf	month
	call	delay_125
	call	readRTC
	movwf	day
	call	delay_125

; dayOfWeek of week is a 4-bit value call read4RTC

Analog to Digital and Realtime Clocks

609

movwf dayOfWeek call delay_125

call readRTC movwf hour

call delay_125

call readRTC movwf minutes call delay_125

call readRTC movwf seconds

bcf	PORTB,CE ; CE line low to end output
return

;============================

;read 4/8 bits from RTC ;============================

;Procedure to read 4/8 bits stored in 6355 registers

;Value returned in w register

read4RTC
movlw	.4	; 4	bit read
goto	anyBits
readRTC
movlw	.8	; 8	bits read
anyBits:
movwf	counter

;Read 6355 read operation requires the IO line be set low

;and the CE line high. Data is read in the following order:

;year, month, day, day-of-week, hour, minutes, seconds readBits:

bsf	PORTB,CLK; Set CLK high to validate data
bsf	STATUS,C	; Set the carry flag (bit = 1)

;Operation:

;If data line is high, then bit read is a 1-bit

;otherwise bit read is a 0-bit

btfss	PORTB,DAT	; Is data line high?
	; Leave carry set (1 bit) if high
bcf	STATUS,C ; Clear the carry bit (make bit 0)
; At this point the carry bit matches the data line
bcf	PORTB,CLK	; Set CLK low to end read
; The carry bit is now rotated into the temp1 register
rrf	temp1,1
decfsz	counter,1	; Decrement the bit counter
goto	readBits ; Continue if not last bit

; At this point all bits have been read (8 or 4)

610		Chapter 16
movf	temp1,0	; Result to w
return

;============================

;write 4/8 bits to RTC ;============================

;Procedure to write 4 or 8 bits to the RTC registers

;ON ENTRY:

;temp1 register holds value to be written

;ON EXIT:

;nothing

write4RTC
movlw	.4	; Init for	4 bits
goto	allBits
writeRTC
movlw	.8	; Init for	8 bits
allBits:
movwf	counter	; Store in	bit counter
writeBits:
bcf	PORTB,CLK	; Clear the CLK line
call	delay_5	; Wait
bsf	PORTB,DAT	; Set the data line to RTC
btfss	temp1,0	; Send LSB
bcf	PORTB,DAT	; Clear data line
call	delay_5	; Wait for	operation to
		; complete
bsf	PORTB,CLK	; Bring CLK line high to
		; validate
rrf	temp1,f	; Rotate bits in storage
decfsz	counter,1	; Decrement bit counter
goto	writeBits	; Continue	if not last bit
return

;============================

;init time variables ;============================

;Procedure to initialize time variables for testing

;Constants used in initialization are located in

;#define statements.

initRTC:

movlw iYear

movwf year

movlw iMonth

movwf month

movlw iDay

movwf day

movlw iDoW

movwf dayOfWeek

Analog to Digital and Realtime Clocks

611

movlw iHour

movwf hour

movlw iMin

movwf minutes

movlw iSec

movwf seconds

return

;============================

;binary to BCD conversion ;============================

;Convert a binary number into two packed BCD digits

;ON ENTRY:

;w register has binary value in range 0 to 99

;ON EXIT:

;output variables bcdLow and bcdHigh contain two

;packed unpacked BCD digits

;w contains two packed BCD digits

;Routine logic:

;The value 10 is subtracted from the source operand

;until the reminder is < 0 (carry cleared). The number

;of subtractions is the high-order BCD digit. 10 is

;then added back to the subtrahend to compensate

;for the last subtraction. The final reminder is the

;low-order BCD digit

;Variables:

;	inNum	storage for source operand
;	bcdHigh	storage for high-order nibble
;	bcdLow	storage for low-order nibble
;	thisDig	Digit counter
bin2bcd:
	movwf	inNum	; Save copy of source value
	clrf	bcdHigh ; Clear storage
	clrf	bcdLow
	clrf	thisDig
min10:
	movlw	.10
	subwf	inNum,f	; Subtract 10
	btfsc	STATUS,C	; Did subtract overflow?
	goto	sum10	; No. Count subtraction
	goto	fin10
sum10:
	incf	thisDig,f	;increment digit counter
	goto	min10
; Store 10th digit
fin10:
	movlw	.10
	addwf	inNum,f	; Adjust for last subtract
	movf	thisDig,w	; get digit counter contents

612		Chapter 16
movwf	bcdHigh	; Store it
; Calculate and store low-order BCD digit
movf	inNum,w	; Store units value
movwf	bcdLow	; Store digit
; Combine both digits
swapf	bcdHigh,w	; High nibble to HOBs
iorwf	bcdLow,w ; ORin low nibble
return

;============================================================

end ; END OF PROGRAM

;============================================================

Appendix A

Resistor Color Codes

The resistor color coding system applies to carbon film, metal oxide film, fusible, precision metal film, and wirewound resistors of the axial lead type. This system is employed when the surface area is not sufficient to print the actual resistance value. Several color codes are used, the most common ones are the 4-band and 5-band codes. In the 4-band code the first two bands represent the magnitude of the resistance, the third band is a multiplier for this value, and the fourth one encodes the error tolerance. In the 5-band code the first three bands represent the magnitude, the fourth one serves as a multiplier, and the fifth one is the error tolerance.