mc68hc908lj24 Freescale Semiconductor, Inc, mc68hc908lj24 Datasheet - Page 225

• Manufacturer Part Number: mc68hc908lj24
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

12.7 RTC Clock Calibration and Compensation
MC68HC908LJ24/LK24 — Rev. 2.1
Freescale Semiconductor
The RTC module is built with a calibration and compensation circuit for
the CGMXCLK. The circuit is used to compensate frequency errors of
the 32.768kHz crystal so that a more accurate 1-Hz RTC clock can be
achieved. Using this method, it is possible to use a less expensive
crystal. If the crystal temperature profile and the current temperature is
known, the calibration circuit can also be used to compensate crystal
frequency errors due to ambient temperature change and crystal aging.
The circuit provides a ±60ppm compensation range, which is suitable for
common 30ppm crystals, and can achieve a compensated RTC clock
accuracy between –3ppm and +2ppm.
The automatic calibration process works by comparing an external 1-Hz
(1ppm) signal driven into the CALIN pin, with the CGMXCLK signal
driven by the 32.768kHz crystal. The calibration is started by setting the
CAL bit followed by the AUTOCAL bit in the RTC calibration register
(RTCCOMR). During calibration, the tdiff block compares the time
difference between the two clock sources for 15 seconds. This time
difference is expressed in the number of CGMXCLK clock cycles and
stored in two’s complement format in the 6-bit E-register (the RTC
calibration data register, RTCCDAT).
CGMXCLK
CGMXCLK
(32.768-kHz)
CALIN
Figure 12-3. RTC Clock Calibration and Compensation
1-Hz REF
Real Time Clock (RTC)
÷ 983040
÷ 32766
÷ 32767
÷ 32768
÷ 32769
÷ 32770
÷ 30
1-Hz FROM
CGMXCLK ÷ 32768
COMPENSATED
1-Hz
RTC Clock Calibration and Compensation
tdiff
E-REGISTER
A
B
COMEN
1
S
EVOL
Real Time Clock (RTC)
1-Hz
RTC CLOCK
Data Sheet
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