M48T86 Просмотр технического описания (PDF) - Unspecified

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M48T86

5.0V PC Real-Time Clock

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CLOCK OPERATIONS

Address Map

The address map of the M48T86 is shown in Fig-

ure 10. It consists of 114 bytes of user RAM, 10

bytes of RAM that contain the RTC time, calendar

and alarm data, and 4 bytes which are used for

control and status. All bytes can be read or written

to except for the following:

1. Registers C & D are “Read only.”

2. Bit 7 of Register A is “Read only.”

The contents of the four Registers A, B, C, and D

are described in the “Registers” section.

Time, Calendar, and Alarm Locations

The time and calendar information is obtained by

reading the appropriate memory bytes. The time,

calendar, and alarm registers are set or initialized

by writing the appropriate RAM bytes. The con-

tents of the time, calendar, and alarm bytes can be

either Binary or Binary-Coded Decimal (BCD) for-

mat. Before writing the internal time, calendar, and

alarm register, the SET Bit (Register B; Bit 7)

should be written to a logic '1.' This will prevent up-

dates from occurring while access is being at-

tempted. In addition to writing the time, calendar,

and alarm registers in a selected format (binary or

BCD), the Data Mode (DM) Bit (Register B; Bit 2),

must be set to the appropriate logic level ('1' signi-

Figure 10. Address Map

M48T86

fies binary data; '0' signifies Binary Coded Decimal

(BCD data). All time, calendar, and alarm bytes

must use the same data mode. The SET Bit

should be cleared after the Data Mode Bit has

been written to allow the Real Time Clock to up-

date the time and calendar bytes. Once initialized,

the Real Time Clock makes all updates in the se-

lected mode. The data mode cannot be changed

without reinitializing the ten data bytes. Table

3., page 12 shows the binary and BCD formats of

the time, calendar, and alarm locations. The 24/12

Bit (Register B; Bit 1) cannot be changed without

reinitializing the hour locations. When the 12-hour

format is selected, a logic '1' in the high order bit of

the hours byte represents PM. The time, calendar,

and alarm bytes are always accessible because

they are double-buffered. Once per second the ten

bytes are advanced by one second and checked

for an alarm condition. If a READ of the time and

calendar data occurs during an update, a problem

exists where data such as seconds, minutes, or

hours may not correlate. However, the probability

of reading incorrect time and calendar data is low.

Methods of avoiding possible incorrect time and

calendar READs are reviewed later in this text.

0

00

14

BYTES

CLOCK AND CONTROL

STATUS REGISTERS

13

0D

14

0E

114

BYTES

STORAGE REGISTERS

0

SECONDS

1

SECONDS ALARM

2

MINUTES

3

MINUTES ALARM

4

HOURS

5

HOURS ALARM

6

DAY OF WEEK

7

DATE OF MONTH

8

MONTH

9

YEAR

10

REGISTER A

11

REGISTER B

12

REGISTER C

13

REGISTER D

BCD OR

BINARY

FORMAT

127

7F

AI01650

11/29

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