M48T512V datasheet(12/21 Pages) STMICROELECTRONICS

12 / 21 page

M48T512Y, M48T512V*

12/21

Calibrating the Clock

The M48T512Y/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.

The

M48T512Y/V design employs periodic counter

correction. The calibration circuit adds or subtracts

counts from the oscillator divider circuit at the di-

vide by 256 stage (see Figure 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 7FFF8h. 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 Cali-

bration byte would represent +10.7 or –5.35 sec-

onds per month which corresponds to a total range

of +5.5 or –2.75 minutes per month.

One method for ascertaining how much calibration

a given M48T512Y/V may require involves setting

the clock, letting it run for a month and comparing

it to a known accurate reference and recording de-

viation over a fixed period of time.

Calibration values, including the number of sec-

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

in STMicroelectronics Application Note AN934,

TIMEKEEPER Calibration.” This allows the de-

signer to give the end user the ability to calibrate

the clock as the environment requires, even if the

final product is packaged in a non-user service-

able enclosure. The designer could provide a sim-

ple utility that accesses the Calibration bits. For

more information on calibration (see the Applica-

tion Note AN934, “TIMEKEEPER® Calibration” on

the ST Web Site).

Figure 8. Crystal Accuracy Across Temperature

AI00999

–160

10203040506070

Frequency (ppm)

Temperature

°C

–10

–20

–30

–40

–100

–120

–140

–40

–60

–80

–20

∆F

= -0.038

(T - T

± 10%

ppm

0 = 25 °C

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