M48T128Y, M48T128V

Clock operations

Doc ID 5746 Rev 6

13/23

One method is available for ascertaining how much calibration a given M48T128Y/V may

require. 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 period of time.

Calibration values, including the number of seconds lost or gained in a given period, can be

found in the STMicroelectronics application note, “TIMEKEEPER calibration.”

This allows the designer 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 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 indicate a –8 ppm

oscillator frequency error, requiring a +2(WR100010) to be loaded into the calibration byte

for correction.

Figure 7.

Crystal accuracy across temperature

Figure 8.

Clock calibration

AI02124

-80

-60

-100

-40

-20

0

20

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

ΔF

= -0.038

(T - T

F

ppm

C

2

T

ppm

°C

AI00594B

NORMAL

POSITIVE

CALIBRATION

NEGATIVE

CALIBRATION