M48T559Y datasheet(10/18 Pages) STMICROELECTRONICS

10 / 18 page

M48T559Y

10/18

Figure 9. Crystal Accuracy Across Temperature

AI02124

-80

-60

-100

-40

-20

∆F

= -0.038

(T - T

± 10%

ppm

0 = 25 °C

ppm

°C

The Calibration byte occupies the five lower order

bits (D4-D0) in the Control register (1FF8h). These

bits 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 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 modi-

fied; if a binary 6 is loaded, the first 12 will be af-

fected, 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 calibration

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.

Two methods are available for ascertaining how

much calibration a given M48T559Y may require.

The first involves simply setting the clock, letting it

run for a month and comparing it to a known accu-

rate reference (like WWV broadcasts). While that

may seem crude, it allows the designer to give the

end user the ability to calibrate his clock as his en-

vironment may require, even after the final product

is packaged in a non-user serviceable enclosure.

All the designer has to do is provide a simple utility

that accesses the Calibration byte.

The second approach is better suited to a manu-

facturing environment, and involves the use of the

IRQ/FT pin. The pin will toggle at 512Hz when the

Stop bit (D7 of 1FF9h) is ’0’, the FT bit (D6 of

1FFCh) is ’1’, the AFE bit (D7 of 1FF6h) is ’0’, and

the Watchdog Steering bit (D7 of 1FF7h) is ’1’ or

the Watchdog Register is reset (1FF7h = 0).

Any deviation from 512Hz indicates the degree

and direction of oscillator frequency shift at the test

temperature. For example, a reading of 512.01024

Hz would indicate a +20 ppm oscillator frequency

error, requiring a -10 (WR001010) to be loaded

into the Calibration Byte for correction. Note that

setting or changing the Calibration Byte does not

affect the Frequency test output frequency.

The FT bit is cleared on power-down.

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