Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

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

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

movwf dayOfWeek call delay_125

call readRTC movwf hour

call delay_125

call readRTC movwf minutes

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

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

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

;Value returned in w register

;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:

;Operation:

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

;otherwise bit read is a 0-bit

BCD Conversion Procedures

In addition to the RTC procedures to initialize the clock registers and to read clock data, the application requires auxiliary procedures to manipulate and display data in BCD format. BCD encodings, covered in Section 3.4, are a way of representing decimal digits in binary form. Two common BCD formats are used: packed and unpacked. In

the unpacked format each byte encodes a single BCD value. In packed form two BCD digits are encoded per byte. The 6355 uses the packed BCD format.

Since program data is usually in binary form, it is useful to have a routine to convert binary data into BCD form. A simple algorithm for converting binary to BCD is as follows:

1.The value 10 is subtracted from the source operand until the remainder is less than 0 (carry cleared). The number of subtractions is the high-order BCD digit.

2.The value 10 is then added back to the subtrahend to compensate for the last subtraction.

3.The final remainder is the low-order BCD digit.

The binary to BCD conversion procedure is coded as follows:

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

;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

;unpacked BCD digits

;w contains two packed BCD digits

;Routine logic:

;The value 10 is subtracted from the source operand

;until the remainder 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 remainder is the

;low-order BCD digit

;Variables:

Since the program requires displaying values encoded in BCD format, a routine is necessary to convert two packed BCD digits into two ASCII decimal digits. The conversion logic is quite simple since the BCD digit is converted to ASCII by adding 0x30 to its value. All that is necessary is to shift bits in the packed BCD operand so as to isolate each digit and then add 0x30 to each one. The routine’s code is as follows:

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

;BCD to ASCII decimal

;conversion ;==============================

;ON ENTRY:

;w register has two packed BCD digits

;ON EXIT:

;output variables asc10, and asc1 have

;two ASCII decimal digits

;Routine logic:

;The low order nibble is isolated and the value 30H

;added to convert to ASCII. The result is stored in

;the variable asc1. Then the same is done to the

;high-order nibble and the result is stored in the

;variable asc10

Bcd2asc: