M41T256YMT7 查看數據表（PDF） - STMicroelectronics

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M41T256YMT7

256 Kbit (32K x8) Serial RTC

STMicroelectronics 

M41T256Y

Two methods are available for ascertaining how

much calibration a given M41T256Y may require.

The first involves setting the clock, letting it run for

a month and comparing it to a known accurate ref-

erence and recording deviation over a fixed period

of time. Calibration values, including the number of

seconds lost or gained in a given period, can be

found in Application Note AN934: TIMEKEEPER

CALIBRATION. This allows the designer to give

the end user the ability to calibrate the clock as the

environment requires, even if the final product is

packaged in a non-user serviceable enclosure.

The designer could provide a simple utility that ac-

cesses the Calibration byte.

The second approach is better suited to a manu-

facturing environment, and involves the use of the

FT pin. The pin will toggle at 512Hz, when the Stop

Bit (ST) is '0,' and the Frequency Test Bit (FT) is

'1.'

Any deviation from 512Hz indicates the degree

and direction of oscillator frequency shift at the test

temperature. For example, a reading of

512.010124Hz would indicate a +20 ppm oscillator

frequency error, requiring a –10 (XX001010) to be

loaded into the Calibration Byte for correction.

Note that setting or changing the Calibration Byte

does not affect the Frequency Test output fre-

quency.

The FT pin is an open drain output which requires

a pull-up resistor to VCC for proper operation. A

500 to 10k resistor is recommended in order to

control the rise time. The FT Bit is cleared on pow-

er-down.

Battery Low Warning

The M41T256Y automatically performs battery

voltage monitoring upon power-up. The Battery

Low (BL) Bit, Bit D7 of Day Register, will be assert-

ed if the battery voltage is found to be less than ap-

proximately 2.5V. The BL Bit will remain asserted

until completion of battery replacement and sub-

sequent battery low monitoring tests, during the

next power-up sequence.

If a battery low is generated during a power-up se-

quence, this indicates that the battery is below ap-

proximately 2.5 volts and may not be able to

maintain data integrity in the SRAM. Data should

be considered suspect and verified as correct. A

fresh battery should be installed. The battery may

be replaced while VCC is applied to the device.

The M41T256Y only monitors the battery when a

nominal VCC is applied to the device. Thus appli-

cations which require extensive durations in the

battery back-up mode should be powered-up peri-

odically (at least once every few months) in order

for this technique to be beneficial. Additionally, if a

battery low is indicated, data integrity should be

verified upon power-up via a checksum or other

technique.

Preferred Power-on/Battery Attach Defaults

See Table 4, below.

Table 4. Preferred Default Values

Condition

WC

TEB(1)

TB(1)

FT

ST(1)

SLP(1)

Battery Attach or Initial Power-up

0

X

X

0

X

X

Power-Cycling (with battery)

0

UC

UC

0

UC

UC

Note: 1. X = Undetermined; UC = Unchanged

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