M48T251Y datasheet(13/24 Pages) STMICROELECTRONICS

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M48T251Y, M48T251V

Clock Register Information

Clock information is contained in eight registers of

8 bits, each of which is sequentially accessed one

(1) bit at a time after the 64-bit pattern recognition

sequence has been completed. When updating

the clock registers, each must be handled in

groups of 8 bits. Writing and reading individual bits

within a register could produce erroneous results.

These READ/WRITE registers are defined in the

clock register map (see Table 5.).

Data contained in the clock registers is in Binary

Coded Decimal format (BCD). Reading and writing

the registers is always accomplished by stepping

through all eight registers, starting with Bit 0 of

Clock Accuracy

The RTC is guaranteed to keep time accuracy to

with ±1 minute per month at 25°C. The clock is fac-

tory-tuned with special calibration elements, and

does not require additional calibration. Moderate

temperature deviation will have a negligible effect

in most applications.

AM-PM/12/24 Mode

Bit 7 of the hours register is defined as the 12-hour

or 24-hour mode select bit. When it is high, the 12-

hour mode is selected. In the 12-hour mode, Bit 5

is the AM/PM bit with the logic high being “PM.” In

the 24-hour mode, Bit 5 is the second 10-hour bit

(20-23 hours).

Oscillator and Reset Bits

Bits 4 and 5 of the day register are used to control

the reset and oscillator functions. Bit 4 controls the

reset pin input. When the reset bit is set to logic '1,'

the Reset Input pin is ignored. When the reset bit

logic is set to '0,' a low input on the reset pin will

cause the device to abort data transfer without

changing data in the timekeeping registers. Reset

operates independently of all other inputs. Bit 5

controls the oscillator. When set to logic '0,' the os-

cillator turns on and the RTC/calendar begins to

increment.

Zero Bits

Registers 1, 2, 3, 4, 5, and 6 contain one (1) or

more bits that will always read logic '0.' When writ-

ing to these locations, either a logic '1' or '0' is ac-

ceptable.

Table 5. Phantom Clock Register Map

Keys: A/P = AM/PM Bit

12/24 = 12 or 24-hour mode Bit

OSC = Oscillator Bit

RST = Reset Bit

0 = Must be set to '0'

Function/Range

BCD Format

0.1 Seconds

0.01 Seconds

Seconds

00-99

10 Seconds

Seconds

00-59

10 Minutes

Minutes

00-59

312/24

10 /

A/P

Hrs

Hours (24 Hour Format)

Hours

01-12/

00-23

OSC

RST

Day of the Week

Day

01-7

10 date

Date: Day of the Month

Date

01-31

10M

Month

01-12

10 Years

Year

00-99

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