
Analog Integrated Circuit Device Data
Freescale Semiconductor
104
PC34708
Functional Block Requirements and Behaviors
32.768 kHz Crystal Oscillator RTC Block Description and Application Information
RTCPORB. To inform the processor that the contents of the RTC are no longer valid due to the reset, a timer reset interrupt
function is implemented with the RTCRSTI bit.
RTC Timer Calibration
A clock calibration system is provided to adjust the 32,768 cycle counter that generates the 1.0 Hz timer for RTC timing registers.
The general implementation relies on the system processor to measure the 32.768 kHz crystal oscillator against a higher
frequency and more accurate system clock such as a TCXO. If the RTC timer needs a correction, a 5 bit 2’s complement
calibration word can be sent via the SPI to compensate the RTC for inaccuracy in its reference oscillator.
The available correction range should be sufficient to ensure drift accuracy in compliance with standards for DRM time keeping.
Note that the 32.768 kHz oscillator is not affected by RTCCAL settings; calibration is only applied to the RTC time base counter.
Therefore, the frequency at the clock output CLK32K is not affected.
The RTC system calibration is enabled by programming the RTCCALMODE[1:0] for desired behavior by operational mode.
The RTC Calibration circuitry can be automatically disabled when main battery contact is lost or if it is so deeply discharged that
RTC power draw is switched to the coin cell (configured with RTCCALMODE=01).
Because of the low RTC consumption, RTC accuracy can be maintained through long periods of the application being shut down,
even after the main battery has discharged. However, it is noted that the calibration can only be as good as the RTCCAL data
that has been provided, so occasional refreshing is recommended to ensure that any drift influencing environmental factors have
not skewed the clock beyond desired tolerances.
Table 107. RTC calibration Settings
Code in RTCCAL[4:0]
Correction in Counts per 32768 Relative correction in ppm
01111
+15
+458
00011
+3
+92
00001
+1
+31
00000
0
11111
-1
-31
11101
-3
-92
10001
-15
-458
10000
-16
-488
Table 108. RTC Calibration Enabling
RTCCALMODE
Function
00
RTC Calibration disabled (default)
01
RTC Calibration enabled in all modes except coin cell only
10
Reserved for future use. Do not use.
11
RTC Calibration enabled in all modes