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M41T00

CLOCK CALIBRATION

The M41T00 is driven by a quartz controlled oscil-

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

devices are tested not to exceed 35ppm (parts per

million) oscillator frequency error at 25°C, which

equates to about ±1.53 minutes per month. With

the calibration bits properly set, the accuracy of

each M41T00 improves to better than +2/–1 ppm

at 25°C.

The oscillation rate of any crystal changes with

temperature (see Figure 14). Most clock chips

compensate for crystal frequency and tempera-

ture shift error with cumbersome trim capacitors.

The M41T00 design, however, 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 13.

The number of times pulses are blanked (subtract-

ed, negative calibration) or split (added, positive

calibration) depends upon the value loaded into

the five bit Calibration byte found in the Control

Register. Adding counts speeds the clock up, sub-

tracting counts slows the clock down.

The Calibration byte occupies the five lower order

bits (D4-D0) in the Control register (Addr 7). This

byte can be set to represent any value between 0

and 31 in binary form. Bit D5 is a Sign bit; '1' indi-

cates positive calibration, '0' indicates negative

calibration. Calibration occurs within a 64minute

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 bi-

nary '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 in fact 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.

Figure 8. Bus Timing Requirements Sequence

Note: P = STOP and S = START

AI00589

SDA

P

tSU:STO

tSU:STA

tHD:STA

SR

SCL

tSU:DAT

tF

tHD:DAT

tR

tHIGH

tLOW

tHD:STA

tBUF

S

P

Figure 9. Slave Address Location

AI00602

R/W

SLAVE ADDRESS

START

A

0

100

0

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