M48T58, M48T58Y Clock operations
Doc ID 2412 Rev 8 17/33
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 M48T58/Y 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 some test equipment. When the frequency test (FT) bit (D6 in the day register) is set to a
'1,' and D7 of the seconds register is a '0' (oscillator running), The frequency test (pin 1) 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.
The frequency test pin is an open drain output which requires a pull-up resistor for proper
operation. A 500-10 kΩ resistor is recommended in order to control the rise time.
For more information on calibration, see application note AN934, “TIMEKEEPER
®
calibration.”
