Datasheet

3.2 Calibrating the clock
The M41T6x real-time clock is driven by a quartz controlled oscillator with a nominal frequency of 32.768 kHz.
This provides the time-base for the RTC. The accuracy of the clock depends on the frequency 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 M41T6x oscillator is designed for use with a 6 - 7 pF crystal load capacitance. 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 (see Figure 18. Crystal accuracy across temperature).
Therefore, the M41T6x 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 19. Calibration waveform.
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, '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 32.768 kHz, 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 (see Figure 19. Calibration waveform).
Two methods are available for ascertaining how much calibration a given M41T6x 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. 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 either the
SQW pin (M41T62/64) or the IRQ /FT/OUT pin (M41T65). The SQW pin will toggle at 512 Hz when RS3 =
'0,' RS2 = '1,' RS1 = '1,' RS0 = '0,' SQWE = '1,' and ST = '0.' Alternatively, for the M41T65, the IRQ /FT/OUT
pin will toggle at 512 Hz when FT and OUT bits = '1' and ST = '0.'
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
(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.
M41T62, M41T64, M41T65
Calibrating the clock
DS3840 - Rev 24
page 16/42