M48T201Y, M48T201V Clock operation
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The oscillation rate of crystals changes with temperature (see Figure 10 on page 24). The
M48T201Y/V 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 24.
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 7FFF8h.
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 (see Figure 11 on
page 24). 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
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 M48T201Y/V 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 the
STMicroelectronics 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 pin. The pin will toggle at 512 Hz, when the stop bit (ST, D7 of 7FFF9h) is '0,'
the frequency test bit (FT, D6 of 7FFFCh) is '1,' the alarm flag enable bit (AFE, D7 of
7FFF6h) is '0,' and the watchdog steering bit (WDS, D7 of 7FFF7h) is '1' or the watchdog
register (7FFF7h=0) is reset.
Note: A 4-second settling time must be allowed before reading the 512 Hz output.
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 a –10 (WR001010) 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.
The IRQ
/FT pin is an open drain output which requires a pull-up resistor to V
CC
for proper
operation. A 500-10 kΩ resistor is recommended in order to control the rise time. The FT bit
is cleared on power-down.
