AN2678 Datasheet, PDF (11/16 Pages) STMicroelectronics – Extremely accurate timekeeping over

AN2678
3
Digital calibration
Digital calibration
The digital calibration feature of the M41T82, M41T83 and M41T93 uses periodic counter
correction. The clock counters are adjusted by adding or subtracting pulses at the 512 Hz
divider stage. This approach provides compensation over an approximate range of –63 ppm
to +126 ppm.
This method employs the use of periodic counter correction by adjusting the ratio of the 100
Hz divider stage to the 512 Hz divider stage. Under normal operation, the 100 Hz divider
stage outputs precisely 100 pulses for every 512 pulses of the 512 Hz stage. This 100 Hz
signal is the input to the counter for the 10ths/100ths of seconds register in the RTC. By
adjusting the number of 512 Hz input pulses used to generate 100 output pulses, the clock
can be sped up or slowed down. To provide digital correction, the device will, depending on
the digital calibration value, periodically produce one or more long or short seconds.
When a non-zero value is loaded into the five Calibration Bits (DC4 - DC0) of the Digital
Calibration Register (0x08) and the sign bit is 1, (indicating positive calibration), the 100 Hz
stage will output 100 pulses for every 511 input pulses instead of the normal 512. Since the
100 pulses are now being output in a shorter window, this has the effect of speeding up the
clock by 1/512 seconds for each second the circuit is active.
Table 2. Digital calibration values
Calibration value, DC4-DC0
Decimal
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Binary
00000
00001
00010
00011
00100
00101
00110
00111
01000
01001
01010
01011
01100
01101
01110
01111
Calibration effect, in ppm,
rounded to the nearest integer
Slowing
Sign DCS = 0
negative calibration
– 0 ppm
– 2 ppm
– 4 ppm
– 6 ppm
– 8 ppm
– 10 ppm
– 12 ppm
– 14 ppm
– 16 ppm
– 18 ppm
– 20 ppm
– 22 ppm
– 24 ppm
– 26 ppm
– 28 ppm
– 31 ppm
Speeding
Sign DCS = 1
positive calibration
+ 0 ppm
+ 4 ppm
+ 8 ppm
+ 12 ppm
+ 16 ppm
+ 20 ppm
+ 24 ppm
+ 28 ppm
+ 33 ppm
+ 37 ppm
+ 41 ppm
+ 45 ppm
+ 49 ppm
+ 53 ppm
+ 57 ppm
+ 61 ppm
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