RTC Interfacing and Programming

1
The 8051 Microcontroller and
Embedded Systems
CHAPTER 15
RTC Interfacing and
Programming

Introduction of Real Time Clock
 The real-time clock (RTC) is a widely used
device that provides accurate time and date
for many applications
 Many systems such as the x86 IBM PC come
with such a chip on the motherboard
 The RTC chip in the IBM PC provides time
components of hour, minute, and second, in
addition to the date/calendar components of
year, month, and day
2

 The RTC chip uses an internal battery, which
keeps the time and date even when the
power is off
 One of the most widely used RTC chips is
the DS 12887 from Dallas Semiconductor
/Maxim Corp
 It uses an internal lithium battery to keep
operating for over 10 years in the absence of
external power
3

 According to the DS 12887 data sheet from
Maxim, it keeps track of “seconds, minutes,
hours, days, day of week, date, month, and
year with leap-year compensation valid up to
year 2100″
 The above information is provided in both
binary (hex) and BCD formats. The DS
12887 supports both 12-hour and 24-hour
clock modes with AM and PM in the 12-hour
4 mode

 It also supports the Daylight Savings Time
option
 The DS 12887 uses CMOS technology to
keep the power consumption low and it has
the designation DS12C887, where C is for
CMOS
 The DS12887 has a total of 128 bytes of
nonvolatile RAM
5

 It uses 14 bytes of RAM for clock/calendar
and control registers, and the other 114
bytes of RAM are for general-purpose data
storage
 In the x86 IBM PC, these 114 bytes of NV-RAM
are used for the CMOS configuration,
where the system setups are kept before the
operating system takes over
6

 The DS12887 has a total of 128 bytes of
RAM space with addresses 00 -7FH
 The first ten locations, 00 – 09, are set aside
for RTC values of time, calendar, and alarm
data
 The next four bytes are used for the control
and status registers
9
Address map of the DS12887

 They are registers A, B, C, and D and are
located at addresses 10-13 (OA – OD in hex)
 The next 114 bytes from addresses OEH to
7FH are available for data storage
 The entire 128 bytes of RAM are accessible
directly for read or write except the following:
– Registers C and D are read-only
– D7 bit of register A is read-only
– The high-order bit of the seconds byte is read-only
10
Address map of the DS12887

DS12887 Address Location for
Time, Calendar, and Alarm
11

Interfacing of DS12887 with 8051
12

Turning on the oscillator for the
first time
 The DS12887 is equipped with the internal
oscillator which is turned off in order to save
the lithium battery.
 We need to turn on the oscillator before we
use the time keeping features of the DS
12887
 To do that, bits D6 – D4 of register A must be
set to value 010
13

Register A Bits for Turning on the
DS12887′s Oscillator
14

Code for turning ON DS12887
 ACALL DELAY_200MS ;Giving settling time
 MOV R0,#0AH ; Reg A address of DS12887
 MOV A,#20H ; 010 DATA FOR D6-D4
 MOVX @R0,A
15

Register C
17
– IRQF =1: if PF = PIE= 1 orAF=AIE= 1 orUF =
UIE= 1
– PF Periodic interrupt flag. Periodic interrupts can
be generated at a rate of once every 500 ms to
once every 122 us. The rate is set by bits RS3 –
RSO of register A

Register C
 AF Alarm interrupt flag. The AF becomes 1
when the current real time matches the alarm
time. AF and AIE of register B together (if
both are 1) will allow the IRQ to be
asserted low when all the three bytes of the
real time (yy:mm:dd) are the same as the
bytes in the alarm time
 The AF also becomes 1 for cases of once
per second, once per minute, and once per
18 hour alarm

ALARM AND SQW FEATURES OF
THE DS12887 CHIP
19
The SQW pin
provides us a
square wave
output of various
frequencies
The frequency is
chosen by bits
RSO – RS3 of
register A

 Also enable SQWE bit in Register B
20
ALARM AND SQW FEATURES OF
THE DS12887 CHIP

ALARM FEATURE OF DS12887
CHIP
 The alarm interrupt can be programmed to
occur at rates of
– (a) once per day
– (b) once per hour
– (c) once per minute
– (d) once per second
21

Once-per-day alarm
 To program the alarm for once per day, we
write the desired time for the alarm into the
hour, minute, and second RAM locations 1,
3, and 5
 As the clock keeps the time, when all three
bytes of hour, minute, and second for the
real time clock match the values in the alarm
hour, minute, and second, the AF (alarm
flag) bit in register C of the DS 12887 will go
22 high

Once-per-hour, minute and second
alarms
 To program the alarm for once per hour, we
write value FFH into the alarm hour location
of 5 only
 To program the alarm for once per minute,
we write value FFH into both the alarm hour
and alarm minute locations of 5 and 3
 To program the alarm for once per second,
we write value FFH into all three locations of
alarm hour, alarm minute, and alarm second
23

 We can poll the AF bit in register C, which is
a waste of microcontroller resources, or allow
the IRQ pin to be activated upon matching
the alarm time with the real time
 It must be noted that in order to use the IRQ
pin of the DS 12887 for an alarm, the
interrupt-enable bit for alarm in register B
(AIE) must be set high
24
IRQ FEATURE OF THE DS12887
CHIP

CHIP
25

 Interrupt request (IRQ) is an output pin for
the DS12887 RTC chip
 There are three possible sources that can
activate the IRQ pin
– (a) alarm interrupt
– (b) periodic pulse interrupt
– (c) update interrupt
 We can choose which source to activate the
IRQ pin using the interrupt-enable bit in
register B of the DS 12887
26
CHIP

PERIODIC INTERRUPT
 The second source of interrupt is the periodic
interrupt flag (PF)
 The periodic interrupt flag is part of register C
 It will go high at a rate set by the RS3 -RSO
bits of register A
 This rate can be from once every 500 ms to
once every 122 us as shown
 The PF becomes 1 when an edge is detected
27 for the period

 Just like alarm interrupt, the periodic interrupt
can also be directed to the IRQ pin
 To use IRQ, the interrupt-enable bits of PIE
in register B must be set to 1
 In other words, we can poll the PF bit of
register C, which is a waste of the
microcontroller’s resources, or it can be
directed to the hardware IRQ pin
 If we set PIE = 1, the IRQ pin is asserted low
when PF goes high
28
PERIODIC INTERRUPT

 While the alarm interrupt gave us the options
from once per day to once per second, the
periodic interrupt gives us the option of
subsecond interrupts
 For example, we can write a program to send
a message to the screen twice per second (2
Hz)
29
PERIODIC INTERRUPT

Internal
Architecture
of DS12887
30

RTC Interfacing and Programming

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RTC Interfacing and Programming