MCP79520-ISLASHMS datasheet(16/58 Pages) MICROCHIP

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MCP7951X/MCP7952X

DS20002300D-page 16

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5.3

Timekeeping

The MCP795XX maintains the current time and date

using an external 32.768 kHz crystal or clock source.

Separate registers are used for tracking hundredths of

seconds, seconds, minutes, hours, day of week, date,

month and year. The MCP795XX automatically adjusts

for months with less than 31 days and compensates for

leap years from 2001 to 2399. The year is stored as a

two-digit value.

Both 12-hour and 24-hour time formats are supported

and are selected using the 12/24 bit.

The day of week value counts from 1 to 7, increments

at midnight and the representation is user-defined (i.e.,

the MCP795XX does not require 1 to equal Sunday,

etc.).

All time and date values are stored in the registers as

binary-coded decimal (BCD) values. The MCP795XX

will continue to maintain the time and date while

operating off the backup supply.

When reading from the timekeeping registers, the

registers are buffered to prevent errors due to rollover

of counters. The following events cause the buffers to

be updated:

• When a read is initiated from the RTCC registers

(addresses 0x00 to 0x1F)

• During an RTCC register read operation, when

the register address rolls over from 0x1F to 0x00

The timekeeping registers should be read in a single

operation to utilize the on-board buffers and avoid

rollover issues.

5.3.1

DIGIT CARRY RULES

The following list explains which timer values cause a

digit carry when there is a rollover:

• Time of day: from 11:59:59.99 PM to 12:00:00.00

AM (12-hour mode) or 23:59:59.99 to 00:00:00.00

(24-hour mode), with a carry to the Date and

Weekday fields

• Date: carries to the Month field according to

Table 5-3

• Weekday: from 7 to 1 with no carry

• Month: from 12/31 to 01/01 with a carry to the

Year field

• Year: from 99 to 00 with no carry

TABLE 5-2:

SUMMARY OF REGISTERS ASSOCIATED WITH OSCILLATOR

CONFIGURATION

Name

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

on Page

RTCSEC

SECTEN2 SECTEN1 SECTEN0

SECONE3

SECONE2 SECONE1 SECONE0

RTCWKDAY

—

OSCRUN

PWRFAIL

VBATEN

WKDAY2

WKDAY1

WKDAY0

CONTROL

—

SQWEN

ALM1EN

ALM0EN

EXTOSC

CRSTRIM

SQWFS1

SQWFS0

Legend:

— = unimplemented location, read as ‘0’. Shaded cells are not used by oscillator configuration.

Note 1: Loading invalid values into the time and

date registers will result in undefined

operation.

2: To avoid rollover issues when loading

new

time

and

date

values,

the

oscillator/clock input should be disabled

by clearing the ST bit for External Crystal

mode and the EXTOSC bit for External

Clock Input mode. After waiting for the

OSCRUN bit to clear, the new values can

be loaded and the ST or EXTOSC bit can

then be re-enabled.

TABLE 5-3:

DATE TO MONTH ROLLOVER

SCHEDULE

Month

Name

Maximum Date

January

February

28 or 29(1)

March

April

May

June

July

August

September

October

November

December

Note 1: 29 during leap years, otherwise 28.

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