M41T82 datasheet(32/61 Pages) STMICROELECTRONICS

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Clock operation

M41T82-M41T83

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Doc ID 12578 Rev 12

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 uses the

512 Hz frequency test output. This is the IRQ1/FT/OUT pin on the M41T83, and the

FT/RST pin on the M41T82 (see Section 3.14 and Section 3.15 for more information on

enabling the FT output). The 512 Hz frequency test signal can be measured using a

highly accurate timing device such as a frequency counter. The measured value is then

compared to 512 Hz and the oscillator error in ppm is then determined.

Any deviation from 512 Hz indicates the degree and direction of oscillator frequency

shift at the test temperature. For example, a reading of 512.010124 Hz would indicate a

+20 ppm oscillator frequency error, requiring either a –10 (xx001010) to be loaded into

the digital calibration byte, or +6 pF (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

÷64

÷2

Digital Calibration Circuitry

(divide by 511/512/513)

Clock

Counters

Square Wave

Watchdog Timer

8-bit Timer

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