VT82885 データシートの表示（PDF） - Unspecified

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VT82885

Real Time Clock

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VIA Technologies, Inc.

VT82885

Real Time Clock

should be written to a logic one to prevent

updates from occurring while access is be-

ing attempted. In addition to writing the ten

time, calendar and alarm registers in a se-

lected format (binary or BCD), the data

mode bit (DM) of Register B must be set to

the appropriate logic level. All ten time, cal-

endar and alarm bytes must use the same

data mode. The set bit in Register B should

be cleared after the data mode bit has been

written to allow the real time clock to update

the time and calendar bytes. Once initial-

ized, the real time clock makes all updates in

the selected mode. The data mode can-not

be changed without reinitializing the ten data

bytes. Table 2 shows the binary and BCD

formats of the ten time, calendar and alarm

locations. The 24-12 bit cannot be changed

without reinitializing the hour loca-tions.

When the 12-hour format is selected, the

high order bit of the hours byte repre-sents

PM when it is a logic one. 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 oc-curs during

an update, a problem exists where seconds,

minutes, hours, etc. may not correlate. The

probability of reading in-correct time and

calendar data is low. Sev-eral methods of

avoiding any possible incor-rect time and

calendar reads are covered later in this text.

The three alarm bytes can be used in two

ways. First, when the alarm time is written in

the appropriate hours, minutes and seconds

alarm locations, the alarm interrupt is initi-

ated at the specified time each day if the

alarm enable bit is high. The second use

condition is to insert a “don’t care” state in

one or more of the three alarm bytes. The

“don’t care” code is any hexadecimal value

from C0 to FF. The two most significant bits

of each byte set the “don’t care” condition

when at logic 1. An alarm will be generated

each hour when the “don’t care” bits are set

in the hours byte. Similarily, an alarm is

generated every minute with “don’t care”

codes in the hours and minute alarm bytes.

The “don’t care” codes in all three alarm

bytes create an interrupt every second.

TABLE 2: TIME. CALENDAR AND ALARM DATA MODES

ADDRESS

DECIMAL

RANGE

LOCATION

FUNCTION

RANGE BINARY DATA MODE BCD DATA MODE

0

Seconds

0-59

00-3B

00-59

1

Seconds Alarm

0-59

00-3B

00-59

2

Minutes

0-59

00-3B

00-59

3

Minutes Alarm

0-59

00-3B

00-59

4

Hours-12-hr Mode

1.12

01-0C AM, 81-8C PM 01-12AM,81-92PM

Hours-24-hr Mode

0-23

00-17

00-23

5

Hours Alarm-12-hr

1-12

01-0C AM, 81-8C PM 01-12AM,81-92PM

Hours Alarm-24-hr

0-23

00-17

00-23

6

Day of the Week

1-7

Sunday = 1

01-07

01-07

7

Date of the Month

1-31

01-1F

01-31

8

Month

1-12

01-0C

01-12

9

Year

0-99

00-63

00-99

NONVOLATILE RAM

6

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