STFPC320 STMicroelectronics, STFPC320 Datasheet - Page 25

• Manufacturer Part Number: STFPC320
• Description: Front Panel Controller/driver With Standby Power Management And Real-time Clock
• Manufacturer: STMicroelectronics

STFPC320
6.7
Calibrating the clock
The STFPC320 is driven by a quartz controlled oscillator with a nominal frequency of
32.768kHz. The accuracy of the RTC depends on the frequency of the quartz crystal that is
used as the time-base for the RTC. The accuracy of the clock is dependent upon the
accuracy of the crystal, and the match between the capacitive load of the oscillator circuit
and the capacitive load for which the crystal was trimmed. The STFPC320 crystal is
designed for use with a 6pF crystal load capacitance. When the calibration circuit is properly
employed, accuracy improves to better than ±2 ppm at 25 deg C.
The oscillation rate of crystals changes with temperature. Therefore, the STFPC320 design
employs periodic counter correction. The calibration circuit adds or subtracts counts from
the oscillator divider circuit at the divide by 256 stage. 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 Calibration 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 Calibration 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 and ‘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 running at exactly 32.768KHz, each of the 31 increments in
the Calibration byte would represent +10.7 or -5.35 seconds per day 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 STFPC320 may
require:
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Any deviation from 512Hz indicates the degree and direction of oscillator frequency shift at
the test temperature. For example, a reading of 512.010124 Hz would indicate a +20ppm
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 or Square Wave output frequency.
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 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 IRQ_N/SQW pin. This pin will toggle at 512Hz when RS3 = ‘0’, RS2 = ‘1’, RS1 =
‘1’, RS0 = ‘0’, SQWE = ‘1’ and ST = ‘0’. In normal mode, it is always advised to keep the
SQWE to a ‘0’.
Real Time Clock (RTC) operation
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