M41T60

Clock operation

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These bits can be set to represent any value between 0 and 31 in binary format. Bit D5 is a

Sign Bit; '1' indicates positive calibration, '0' indicates negative calibration.

Calibration 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 loaded, the first 12 will be affected, and so on. Therefore, 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

calibration step in the calibration register. Assuming that the oscillator is running at exactly

32,768Hz, each of the 31 increments in the Calibration byte would represent +10.7 or –5.35

seconds per day which corresponds to a total range of +5.5 or –2.75 minutes per month.

Two methods are available for ascertaining how much calibration a given M41T60 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. Calibration

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

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.

●

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

use of the Frequency Test (FT) pin. The FT pin will toggle at 512Hz when the ST Bit is

set to '0,' and the OUT Bit and FT Bit are set to '1.' Any measured deviation from the

512Hz frequency 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 a –10 (XX001010) to be loaded into the Calibration

Byte for correction.

Note:

Setting or changing the Calibration Byte does not affect the Frequency Test output

operation. A 500-10k resistor is recommended in order to control the rise time.