M41T93 datasheet(25/49 Pages) STMICROELECTRONICS

25 / 49 page

M41T93

Clock operation

25/49

Two methods are available for ascertaining how much calibration a given M41T93 may

require:

●

The first 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. 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 either or both of the Calibration

bytes.

●

The second approach is better suited to a manufacturing environment, and involves the

use of the IRQ/FT/OUT pin. The IRQ/FT/ OUT pin will toggle at 512Hz when FT and

OUT Bits = '1' and ST = '0.' Any deviation from 512Hz indicates the degree and

direction of oscillator frequency shift at the test temperature. For example, a reading of

512.010124Hz would indicate a +20 ppm oscillator frequency error, requiring either a –

10 (xx001010) to be loaded into the Digital Calibration byte, or +6pF (00011000) into

the Analog Calibration byte for correction.

Note:

Setting or changing the Digital Calibration Byte does not affect the Frequency Test, Square

Wave, or Watchdog Timer frequency, but changing the Analog Calibration byte DOES affect

all functions derived from the low current oscillator (see Figure 12).

Figure 12.

Clock divider chain and calibration circuits

AI11806a

Analog Calibration

Circuitry

Remainder of

Divider Circuit

1Hz Signal

512Hz Output

Frequency Test

32kHz

Low Current

Oscillator

CXI

CXO

÷2

÷8

÷2

Digital Calibration Circuitry

(divide by 511/512/513)

Clock

Registers

Square Wave

Watchdog Timer

8-Bit Timer

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