The delay routine required in this case is coded as follows:

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

;delay procedure ;=======================

;For a 10Mhz clock the Fosc32 option produces a conversion

;speed of 1/(10/32) = 3.2 microseconds. At 3.2 ms per bit

;13 bits require approximately 41 ms. The instruction time

;at 10Mhz is 10 ms. 4/10 = 0.4 ms per instruction. To delay

;41 ms a 10Mhz PIC must execute 11 instructions. Add one

;more for safety.

16.3 Realtime Clocks

In the context of microcontrollers and embedded systems, realtime clocks (also called RTCs) are integrated circuits designed to keep track of time in conventional hours, that is, in years, days, hours, minutes, and seconds. Many realtime clock ICs are available with various characteristics, data formats, modes of operation, and interfaces. Most of the ones used in PIC circuits have a serial interface in order to save access ports. Most RTC chips provide a battery connection so that time can be kept when the system is turned off.

In the sections that follow, we discuss one popular RTC chip: the NJU6355, but this is by no means the only option for embedded systems.

16.3.1 The NJU6355 Realtime Clock

The NJU6355 series is a serial I/O realtime clock used in microcontroller-based embedded systems. The IC includes its own quartz crystal oscillator, counter, shift register, voltage regulator, and interface controller. The PIC interface requires four lines. Operating voltage is TTL level so it can be wired directly on the typical PIC circuit. The output data includes year, month, day-of-week, hour, minutes, and seconds. Figure 16-9 is the pin diagram for the NJU6355.

NJU6355 output is in packed BCD format, that is, each decimal digit is represented by a 4-bit binary number. The chip’s logic correctly calculates the number of days in each month as well as the leap years. All unused bits are reported as binary 0. Figure 16-10 is a bitmap of the formatted timer data.

The NJU6355 does not report valid time data until after it has been initialized, even if there are power and clock signals into the chip. Initialization requires writing data into the 6355 registers. In order to write to the IC, code must set the I/O and the

CE lines high. At this moment all clock updates stop and the RTC goes into the write mode. Input data is latched in LSB first, starting with the year and concluding with the minutes. There is no provision for writing seconds into the RTC, so the total number of bits written is 44.

The 6355 contains a mechanism for detecting conditions that could compromise the clock’s operation, such as low power. In this case, the special value 0xee is written into each digit of the internal registers to inform processing routines that the timer has been compromised.

The NJU6355 requires the installation of an external crystal oscillator. The crystal must have a frequency of 32.768 kHz. The time-keeping accuracy of the RTC is determined by the quartz oscillator. The capacity of the oscillator must match that of the RTC and of the circuit. A standard crystal with a capacitance of 12.5pF works well for applications that do not demand high clock accuracy. For more exacting applications the 6355 can be programmed to check the clock frequency and determine its error. The chip’s frequency-checking mode is described in an NJU6355 Application Note available from New Japan Radio Co., Ltd.

16.3.2 RTC Demonstration Circuit and Program

The circuit shown in Figure 16-11 is a simple application of the 6355 RTC. The circuit uses a NJU6355 in conjunction with a 16F86 PIC and an LCD. The demonstration program, named RTC2LCD, sets up RTC and reads clock data in an endless loop. The hours, minutes, and seconds are displayed at the top line of the LCD as follows:

H:xx M:xx S:xx

where xx represents the two BCD digits read from the clock and converted to ASCII decimal for display. The program initializes the 6355 to some arbitrary values contained in the corresponding #define statements. These values are copied into program variables by a local procedure and then used to initialize the RTC registers. Two procedures relate to RTC operation: one to initialize the clock hardware and the other one to read the current time. In addition, two auxiliary procedures are implemented: one to read clock data and one to write clock data. Since clock data can be in 8- or 4-bit formats each procedure contains a separate entry point to handle the 4-bit option. The procedure to initialize and the one to write clock data are coded as follows:

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

;init RTC ;============================

;Procedure to initialize the real time clock chip. If chip

;is not initialized it will not operate and the values

;read will be invalid.

;Since the 6355 operates in BCD format the stored values must

;be converted to packed BCD.

;According to wiring diagram

;Seconds cannot be written to RTC. RTC seconds register

;is automatically initialized to zero

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

;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

The following procedures are used by the RTC2LCD program to read the data in the RTC registers:

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

;read RTC data ;============================

;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:

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: