M41T81
Clock operation
3.3
Calibrating the clock
The M41T81 is driven by a quartz controlled oscillator with a nominal frequency of
32,768Hz. The devices are tested not exceed 35 ppm (parts per million) oscillator
o
frequency error at 25 C, which equates to about +1.9 to –1.1 minutes per month (see
Figure 10 on page 16). When the Calibration circuit is properly employed, accuracy
improves to better than 2 ppm at 25°C.
The oscillation rate of crystals changes with temperature. The M41T81 design employs
periodic counter correction. The calibration circuit adds or subtracts counts from the
oscillator divider circuit at the divide by 256 stage, as shown in Figure 11 on page 16. The
number of times pulses which are blanked (subtracted, negative calibration) or split (added,
positive calibration) depends upon the value loaded into the five Calibration Bits found in the
Control Register. Adding counts speeds the clock up, subtracting counts slows the clock
down.
The Calibration Bits occupy the five lower order bits (D4-D0) in the Control Register 08h.
These bits can be set to represent any value between 0 and 31 in binary form. 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 (see Figure 11 on page 16).
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 M41T81 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/SQW pin. The pin will toggle at 512Hz, when the Stop Bit (ST, D7 of
01h) is '0,' the Frequency Test Bit (FT, D6 of 08h) is '1,' the Alarm Flag Enable Bit (AFE, D7
of 0Ah) is '0,' and the Square Wave Enable Bit (SQWE, D6 of 0Ah) is '0' and the Watchdog
Register (09h = 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 that setting or changing the Calibration Byte does not affect the
Frequency Test output frequency.
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STMICROELECTRONICS [ ST ]
