M4T32-BR12SHX STMICROELECTRONICS [STMicroelectronics], M4T32-BR12SHX Datasheet - Page 24

• Manufacturer Part Number: M4T32-BR12SHX
• Description: Serial real-time clock with 44 bytes NVRAM and reset
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

4.8
24/41
Figure 13. RSTIN1 and RSTIN2 timing waveforms
Table 7.
1. Valid for ambient operating temperature: T
2. Pulse width less than 50ns will result in no RESET (for noise immunity).
3. Pulse width less than 20ms will result in no RESET (for noise immunity).
4. Programmable (see
Calibrating the clock
The M41T94 is driven by a quartz-controlled oscillator with a nominal frequency of
32,768Hz. Uncalibrated clock accuracy will not exceed ±35 ppm (parts per million) oscillator
frequency error at 25°C, which equates to about ±1.53 minutes per month. 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
Therefore, the M41T94 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
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 (8h).
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
RSTIN1
RSTIN2
RST
t
t
t
t
Symbol
R1HRH
R2HRH
RLRH1
RLRH2
(1)
Figure 15 on page
(2)
(3)
(4)
(4)
Reset AC characteristics
RSTIN1 low to RSTIN1 high
RSTIN2 low to RSTIN2 high
RSTIN1 high to RST high
RSTIN2 high to RST high
Table on page
26. The number of times pulses are blanked (subtracted,
Parameter
tRLRH1
28).
tR1HRH
A
= –40 to 85°C; V
(1)
tRLRH2
CC
= 2.7 to 5.5V (except where noted).
Min
200
100
40
40
Figure 14 on page
tR2HRH
Max
200
200
26).
AI03665
Unit
ms
ms
ms
ns