M48T35AY datasheet(12/25 Pages) STMICROELECTRONICS

12 / 25 page

M48T35AY, M48T35AV

12/25

Calibrating the Clock

The M48T35AY/V is driven by a quartz-controlled

oscillator with a nominal frequency of 32,768 Hz.

The devices are tested not to exceed 35 ppm

(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 M48T35AY/V improves to better

than +1/–2 ppm at 25°C.

The oscillation rate of any crystal changes with

temperature (see Figure 11., page 13). Most clock

chips compensate for crystal frequency and tem-

perature shift error with cumbersome “trim” capac-

itors. The M48T35AY/V 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 Fig-

ure 12., page 13. The number of times pulses 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 Byte occupies the five lower order

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

bits can be set to represent any value between 0

and 31 in binary form. Bit D5 is the Sign Bit; '1' in-

dicates 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 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 32,768

Hz, 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 M48T35AY/V may re-

quire. The first involves simply setting the clock,

letting it run for a month and comparing it to a

known accurate 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 environment may require, even after the fi-

nal product is packaged in a non-user serviceable

enclosure.

The second approach is better suited to a manu-

facturing environment, and involves the use of

some test equipment. When the Frequency Test

(FT) Bit, the seventh-most significant bit in the Day

ister is a '0' (Oscillator Running), DQ0 will toggle at

512 Hz during a READ of the Seconds Register.

Any deviation from 512 Hz 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: Setting or changing the Calibration Byte

does not affect the Frequency Test output fre-

quency.

The FT Bit MUST be reset to '0' for normal clock

operations to resume. The FT Bit is automatically

Reset on power-down.

For more information on calibration, see Applica-

tion Note AN934, “TIMEKEEPER® Calibration.”

Century Bit

Bit D5 and D4 of Clock Register 7FFCh contain

the CENTURY ENABLE Bit (CEB) and the CEN-

TURY Bit (CB). Setting CEB to a '1' will cause CB

to toggle, either from a '0' to '1' or from '1' to '0' at

the turn of the century (depending upon its initial

state). If CEB is set to a '0,' CB will not toggle.

Note: The WRITE Bit must be set in order to write

to the CENTURY Bit.

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