M41T82RM6F datasheet(34/58 Pages) STMICROELECTRONICS

34 / 58 page

Clock operation

M41T82 M41T83

34/58

Two methods are available for ascertaining how much calibration a given M41T8x 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 IRQ1/FT/OUT pin. The IRQ1/FT/ OUT pin will toggle at 512Hz when FT and

OUT bits = '1' (M41T83 only) 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 19).

Figure 19.

Clock divider chain and calibration circuits

AI11806c

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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