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M41T256YMT7F

256 Kbit (32K x8) Serial RTC

STMicroelectronics 

M41T256Y

Power-on Reset

The M41T256Y continuously monitors VCC. When

VCC falls to the power fail detect trip point, the RST

pulls low (open drain) and remains low on power-

up for tREC after VCC passes VPFD (max). The RST

pin is an open drain output and an appropriate

pull-up resistor should be chosen to control rise

time.

Tamper Indication Circuit

The M41T256Y provides an independent input

pin, the Tamper Pin (TP) which can be used to

monitor a signal which can result in the setting of

the Tamper Bit (TB) if the Tamper Enable Bit

(TEB) is set to a '1.'

The Tamper Pin is triggered by being connected to

VCC/VBAT through an external switch. This switch

is normally open in the application, allowing the pin

to be “floating” (internally latched to VSS when TEB

is set). When this switch is closed (connecting the

pin to VCC/VBAT), the Tamper Bit will be immedi-

ately set. This allows the user to determine if the

device has been physically moved or tampered

with. The Tamper Bit is a “read only” bit and is re-

set only by taking the Tamper Pin to ground and

resetting the Tamper Enable Bit to '0.'

This function operates both under normal power,

and in battery back-up. If the switch closes during

a power-down condition, the bit will still be set cor-

rectly.

Note: Upon initial battery attach or initial power

application without the battery, the state of TEB

(and TB) will be undetermined. Therefore TEB

must be initialized to a '0.'

Tamper Event Time-Stamp

If a tamper occurs, not only will the Tamper Bit be

set, but the event will also automatically be time-

stamped. This is accomplished by freezing the

normal update of the clock registers (7FF7h

through 7FFFh) immediately following a tamper

event. Thus, when tampering occurs, the user may

first read the time registers to determine exactly

when the tamper event occurred, then re-enable

the clock update to the current time (and reset the

Tamper Bit, TB) by resetting the Tamper Enable

Bit (TEB).

The time update will then resume, and after either

a Stop Condition or incrementing the address

pointer to a RAM address and back, the clock can

be read to determine the current time.

Note: The Tamper Bit (TB) must always be set to

'0' in order to read the current time.

Calibrating the Clock

The M41T256Y is driven by a quartz controlled os-

cillator with a nominal frequency of 32,768Hz. The

devices are tested not exceed ±35 ppm (parts per

million) oscillator frequency error at 25oC, which

equates to about ±1.53 minutes per month. When

the Calibration circuit is properly employed, accu-

racy improves to better than +1/–2 ppm at 25°C.

The oscillation rate of crystals changes with tem-

perature (see Figure 14., page 16). Therefore, the

M41T256Y design employs periodic counter cor-

rection. The calibration circuit adds or subtracts

counts from the oscillator divider circuit at the di-

vide by 256 stage, as shown in Figure

15., page 16. The number of times pulses which

are blanked (subtracted, negative calibration) or

split (added, positive calibration) depends upon

the value loaded into the five Calibration bits found

in the Control Register. Adding counts speeds the

clock up, subtracting counts slows the clock down.

The Calibration bits occupy the five lower order

bits (D4-D0) in the Control Register (7FF8h).

These bits can be set to represent any value be-

tween 0 and 31 in binary form. Bit D5 is a Sign Bit;

'1' indicates positive calibration, '0' indicates nega-

tive calibration. Calibration occurs within a 64

minute cycle. The first 62 minutes in the cycle

may, once per minute, have one second either

shortened by 128 or lengthened by 256 oscillator

cycles. If a binary '1' is loaded into the register,

only the first 2 minutes in the 64 minute cycle will

be modified; if a binary 6 is loaded, the first 12 will

be affected, and so on.

Therefore, each calibration step has the effect of

adding 512 or subtracting 256 oscillator cycles for

every 125,829,120 actual oscillator cycles, that is

+4.068 or –2.034 ppm of adjustment per calibra-

tion step in the calibration register. Assuming that

the oscillator is running at exactly 32,768Hz, each

of the 31 increments in the Calibration byte would

represent +10.7 or –5.35 seconds per month

which corresponds to a total range of +5.5 or –2.75

minutes per month.

15/27

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