M4T32-BR12SHX STMICROELECTRONICS [STMicroelectronics], M4T32-BR12SHX Datasheet - Page 25

• Manufacturer Part Number: M4T32-BR12SHX
• Description: Serial real-time clock with 44 bytes NVRAM and reset
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

M41T94
Clock operations
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 month 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 M41T94 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 in Application
Note AN934: TIMEKEEPER CALIBRATION. 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 the Calibration Byte.
The second approach is better suited to a manufacturing environment, and involves the use
of the IRQ/FT/OUT pin. The pin will toggle at 512Hz, when the Stop bit (ST, D7 of 1h) is '0,'
the Frequency Test bit (FT, D6 of 8h) is '1,' the Alarm Flag Enable bit (AFE, D7 of Ah) is '0,'
and the Watchdog Steering bit (WDS, D7 of 9h) is '1' or the Watchdog register (9h = 0) is
reset.
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 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 frequency.
The IRQ/FT/OUT pin is an open drain output which requires a pull-up resistor for proper
operation. A 500 to 10k
resistor is recommended in order to control the rise time. The FT
bit is cleared on power-down.
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