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M41T60 Datasheet(PDF) 10 Page - STMicroelectronics

Part No. M41T60
Description  Serial access real-time clock
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

M41T60 Datasheet(HTML) 10 Page - STMicroelectronics

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Clock operation
M41T60
10/24
3
Clock operation
The M41T60 is driven by a quartz-controlled oscillator with a nominal frequency of
32.768KHz. The accuracy of the Real-Time Clock depends on the frequency of the quartz
crystal that is used as the time-base for the RTC. The eight-byte Clock Register (see Table 2
on page 12) is used to both set the clock and to read the date and time from the clock, in a
binary coded decimal format. Seconds, Minutes, and Hours are contained within the first
three registers.
Bits D6 and D7 of Clock Register 05h (Century/Month Register) contain the CENTURY Bit 0
(CB0) and the CENTURY Bit 1 (CB1). See Table 3 on page 14 for additional explanation.
Bits D0 through D2 of Register 03h contain the Day (day of the week). Registers 04h, 05h,
and 06h contain the Date (day of the month), Century/Month, and Years. the eighth clock
register is the Calibration Register (this is described in the Clock Calibration section). Bit D7
of Register 00h contains the STOP Bit (ST). Setting this bit to a '1' will cause the oscillator to
stop. When reset to a '0,' the oscillator restarts within one second (typical).
Note:
Upon initial power-up, the user should set the ST Bit to a '1,' then immediately reset the ST
Bit to '0.' This provides an additional “kick-start” to the oscillator circuit.
Bit D7 of Register 01h contains the Oscillator Fail Interrupt Enable Bit (OFIE - see the
description in the Oscillator Fail Detection section).
Note:
A WRITE to ANY location within the first seven bytes of the clock register (0h-6h), including
the OFIE and ST Bit, will result in an update of the system clock and a reset of the divider
chain. This could result in an inadvertent change of the current time. These non-clock
related bits should be written prior to setting the clock, and remain unchanged until such
time as a new clock time is also written.
The seven Clock Registers may be read one byte at a time, or in a sequential block. The
Calibration Register (Address location 7h) may be accessed independently. Provision has
been made to ensure that a clock update does not occur while any of the clock addresses
are being read. If a clock address is being read, an update of the clock registers will be
halted. this will prevent a transition of data during the READ.
3.1
Calibrating the clock
The M41T60 is driven by a quartz-controlled oscillator with a nominal frequency of
32,768Hz. The accuracy of the clock is dependent upon the 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 M41T60 oscillator is designed for use with a 6pF 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 11 on page 12). The
M41T60 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 12 on page 13. The number of times pulses 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 07h.


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