Clock operations M48T08, M48T08Y, M48T18
16/31 Doc ID 2411 Rev 11
M48T08/18/08Y design, however, 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 9 on page 17. The number of times pulses are blanked (subtracted,
negative calibration) or split (added, positive calibration) depends upon the value loaded into
the five-bit calibration byte found in the control register. Adding counts speeds the clock up,
subtracting counts slows the clock down.
The calibration byte occupies the five lower order bits in the control register. This byte can
be set to represent any value between 0 and 31 in binary form. The sixth bit is the 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 in
fact running at exactly 32,768 Hz, 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 M48T08/18/08Y
may require. The first involves simply setting the clock, letting it run for a month and
comparing it to a known accurate reference (like WWV broadcasts). While that may seem
crude, it allows the designer to give the end user the ability to calibrate his clock as his
environment may require, even after the final product is packaged in a non-user serviceable
enclosure. All the designer has to do is provide a simple utility that accesses the calibration
byte.
The second approach is better suited to a manufacturing environment, and involves the use
of standard test equipment. When the frequency test (FT) bit, the seventh-most significant
bit in the day register, is set to a '1,' and the oscillator is running at 32,768 Hz, the LSB
(DQ0) of the seconds register will toggle at 512 Hz. Any deviation from 512 Hz indicates the
degree and direction of oscillator frequency shift at the test temperature. For example, a
reading of 512.01024 Hz would indicate a +20 ppm oscillator frequency error, requiring a
–10 (WR001010) 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 device must be selected and addresses must be stable at address 1FF9h when reading
the 512 Hz on DQ0.
The LSB of the seconds register is monitored by holding the M48T08/18/08Y in an extended
READ of the seconds register, but without having the READ bit set. The FT bit MUST be
reset to '0' for normal clock operations to resume.
For more information on calibration, see the application note AN934, “TIMEKEEPER
®
calibration.”
