m41t256y STMicroelectronics, m41t256y Datasheet - Page 20

• Manufacturer Part Number: m41t256y
• Description: 256 Kbit 32k X8 Serial Rtc
• Manufacturer: STMicroelectronics

Figure 11. Crystal accuracy across temperature
Figure 12. Clock calibration
20/30
Frequency (ppm)
NORMAL
POSITIVE
CALIBRATION
NEGATIVE
CALIBRATION
–100
–120
–140
–160
–20
–40
–60
–80
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 32,768Hz, 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 M41T256Y may
require.
20
0
–40
–30
–20
–10
0
10
Temperature C
T
K = –0.036 ppm/ C
F
O
F = K x (T –T
= 25 C ± 5 C
20
30
O
)
2
40
2
± 0.006 ppm/ C
50
60
2
70
80
AI00999b
AI00594B