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参数资料
| 型号： | M48T212A-85MH1TR |
| 厂商： | STMICROELECTRONICS |
| 元件分类： | 时钟/数据恢复及定时提取 |
| 英文描述： | 0 TIMER(S), REAL TIME CLOCK, PDSO44 |
| 封装： | HATLESS, PLASTIC, SOIC-44 |
| 文件页数： | 13/30页 |
| 文件大小： | 155K |
| 代理商： | M48T212A-85MH1TR |
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M48T212A
20/30
Calibrating the Clock
The M48T212A is driven by a quartz-controlled os-
cillator with a nominal frequency of 32,768 Hz. The
devices are tested not to exceed
±35 PPM (parts
per million) oscillator frequency error at 25
°C,
which equates to about
±1.53 minutes per month
(see Figure 13, page 25). When the Calibration cir-
cuit is properly employed, accuracy improves to
better than +1/–2 ppm at 25
°C.
The oscillation rate of crystals changes with tem-
perature. The M48T212A 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 14,
page 25. 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 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 calibra-
tion. 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 modi-
fied; if a binary 6 is loaded, the first 12 will be af-
fected, 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 calibra-
tion step in the calibration register. Assuming that
the oscillator is running at exactly 32,768 Hz, 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 M48T212A may require.
The first involves setting the clock, letting it run for
a month and comparing it to a known accurate ref-
erence 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, “TIMEKEEP-
ER Calibration.”
This allows the designer to give the end user the
ability to calibrate the clock as the environment re-
quires, 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 manu-
facturing environment, and involves the use of the
IRQ/FT pin. The pin will toggle at 512Hz, when the
Stop Bit (ST, D7 of 9h) is ’0,’ the Frequency Test
Bit (FT, D6 of Ch) is ’1,’ the Alarm Flag Enable Bit
(AFE, D7 of 6h) is ’0,’ and the Watchdog Steering
BIt (WDS, D7 of 7h) is ’1’ or the Watchdog Register
(7h=0) is reset.
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 oscilla-
tor frequency error, requiring a –10 (WR001010)
to be loaded into the Calibration Byte for correc-
tion.
Note: Setting or changing the Calibration Byte
does not affect the Frequency Test output fre-
quency.
The IRQ/FT pin is an open drain output which re-
quires a pull-up resistor to VCC for proper opera-
tion. A 500-10k
resistor is recommended in order
to control the rise time. The FT Bit is cleared on
power-down.
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相关代理商/技术参数 |
参数描述 |
|---|---|
| M48T212A-85MH6 | 制造商:STMICROELECTRONICS 制造商全称:STMicroelectronics 功能描述:3.3V TIMEKEEPER CONTROLLER |
| M48T212A-85MH6TR | 制造商:STMICROELECTRONICS 制造商全称:STMicroelectronics 功能描述:3.3V TIMEKEEPER CONTROLLER |
| M48T212AMH | 制造商:STMICROELECTRONICS 制造商全称:STMicroelectronics 功能描述:3.3V TIMEKEEPER CONTROLLER |
| M48T212V | 制造商:STMICROELECTRONICS 制造商全称:STMicroelectronics 功能描述:3.3V TIMEKEEPER supervisor |
| M48T212V_07 | 制造商:STMICROELECTRONICS 制造商全称:STMicroelectronics 功能描述:3.3V TIMEKEEPER supervisor |
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