M41T93_08 Datasheet, PDF (26/51 Pages) STMicroelectronics – Serial SPI bus RTC with battery switchover

Clock operation
M41T93
Note:
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 512 Hz when FT and
OUT bits = '1' and ST = '0.' 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.
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
512Hz Output
Frequency Test
32kHz
÷2
÷2
÷2
÷2
÷2
CXI
Low Current
Oscillator
Remainder of
Divider Circuit
Square Wave
Watchdog Timer
8-Bit Timer
CXO
÷8
Digital Calibration Circuitry
(divide by 511/512/513)
Clock
Registers
1Hz Signal
Analog Calibration
Circuitry
AI11806a
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