M41ST87YMX6 datasheet(31/52 Pages) STMICROELECTRONICS

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M41ST87Y, M41ST87W

Clock operation

Doc ID 9497 Rev 8

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3.2

Calibrating the clock

The M41ST87Y/W is driven by a quartz controlled oscillator with a nominal frequency of

32,768 Hz. The devices are tested to not exceed ±35 ppm (parts per million) oscillator

frequency error at 25 °C, with ±20 ppm crystals, which translates to about ±1.53 minutes per

month. Even better accuracy can be achieved with higher accuracy crystals. When the

calibration circuit is properly employed, accuracy can be improved to better than ±2 ppm at

25 °C.

The oscillation rate of crystals changes with temperature (see Figure 22 on page 33).

Therefore, the M41ST87Y/W design employs periodic counter correction. The calibration

circuit adds or subtracts counts from the oscillator divider circuit at the divide by 256 stage,

as shown in Figure 23: Calibration waveform on page 33. 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 (08h).

These bits can be set to represent any value between 0 and 31 in binary form. Bit D5 is a

sign bit; '1' indicates positive calibration, '0' indicates negative 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.

Keys:

0 = Must be set to zero

RB0-RB1 = Watchdog resolution bits

32kE = 32 kHz output enable bit

RPT1-RPT5 = Alarm repeat mode bits

ABE = Alarm in battery backup mode enable bit RS0-RS3 = SQW frequency

AF = Alarm flag (read only)

S = Sign bit

AFE = Alarm flag enable bit

SQWE = Square wave enable

BL = Battery low flag (read only)

SQWOD = Square wave open drain bit

BMB0-BMB4 = Watchdog multiplier bits

ST = Stop bit

CB0-CB1 = Century bits

TB (1 and 2) = Tamper bits (read only)

CLR (1 and 2) = RAM clear bits

TCHI/TCLO (1 and 2) = Tamper current hi/tamper current low bits

CLR (1 and 2)EXT = RAM clear external bits

TCM (1 and 2) = Tamper connect mode bits

CLRPW0 = RAM clear pulse width 0 bit

TDS (1 and 2) = Tamper detect sampling bits

CLRPW1 = RAM clear pulse width 1 bit

TEB (1 and 2) = Tamper enable bits

FT = Frequency test bit

THS = Threshold bit

HT = Halt update bit

TIE (1 and 2) = Tamper interrupt enable bits

OF = Oscillator fail bit

TPM (1 and 2) = Tamper polarity mode bits

OFIE = Oscillator fail interrupt enable bit

TR = trec bit

OUT = Output level

WDS = Watchdog steering bit

PFOD = Power-fail output open drain bit

WDF = Watchdog flag (read only)

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