M48T128YPM datasheet(12/22 Pages) STMICROELECTRONICS

12 / 22 page

M48T128Y, M48T128V*

12/22

Calibrating the Clock

The M48T128Y/V is driven by a quartz controlled

oscillator with a nominal frequency of 32,768Hz.

The devices are factory calibrated at 25°C and

tested for accuracy. Clock accuracy will not ex-

ceed 35 ppm (parts per million) oscillator frequen-

cy error at 25°C, which equates to about ±1.53

minutes per month. When the Calibration circuit is

properly employed, accuracy improves to better

than +1/–2 ppm at 25°C. The oscillation rate of

crystals changes with temperature (see Figure

8., page 13). The M48T128Y/V design employs

periodic counter correction. The calibration circuit

adds or subtracts counts from the oscillator divider

circuit at the divide by 128 stage, as shown in Fig-

ure 9., page 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 Calibration bits found in the Control Regis-

ter. Adding counts speeds the clock up, subtract-

ing counts slows the clock down. The Calibration

bits occupy the five lower order bits (D4-D0) in the

Control Register 1FFF8h. 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 cal-

ibration, '0' indicates negative calibration. Calibra-

tion 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 load-

ed, the first 12 will be affected, and so on. There-

fore, 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.

One method is available for ascertaining how

much calibration a given M48T128Y/V may re-

quire. This involves setting the clock, letting it run

for a month and comparing it to a known accurate

reference and recording deviation over a fixed pe-

riod of time.

Calibration values, including the number of sec-

onds lost or gained in a given period, can be found

the

STMicroelectronics

Application

Note,

“TIMEKEEPER CALIBRATION.”

This allows the designer to give the end user the

ability to calibrate the clock as the environment re-

quires, even if the final product is packaged in a

non-user serviceable enclosure. The designer

could provide a simple utility that accesses the

Calibration byte. For example, a deviation of 21

seconds slow over a period of 30 days would indi-

cate a –8 ppm oscillator frequency error, requiring

a +2(WR100010) to be loaded into the Calibration

Byte for correction.

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