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Chapter 50 Subclock Calibration Unit
6.Application Note
■ Accuracy:
The accuracy of the calibration is dependent on the clock frequency used by the 4MHz Timer and
duration of the calibration. The maximum error of the 4MHz timer is +/- 1 digit. If the clock
frequency is 4MHz and duration of the calibration is 1 second, the achieved accuracy is
calculated in the following way:
0.25us (Clock cycle time) / 1 second (duration)=0.25 ppm.
In general:
Accuracy = (Clock cycle time of 4MHz Timer) / (Duration of calibration)
■ Power dissipation:
Suppose the current consumption I
RUN
in run mode is 20 times the consumption I
RTC
in RTC
mode ( I
RUN
= 20 x I
RTC
).
If the MCU is woken up from RTC mode by software to trigger the calibration measurement every
minute and the duration of the calibration is set to 1 second, the increase in the power dissipation
can be 20xI
RTC
/60= 1/3 * I
RTC
.
Therefore the software has to make sure that the increase should not affect the hardware
limitations coming from the system requirements. For example, the software has to be designed
to trigger the calibration least frequently.
It is generally recommended that the theoretical increase in power dissipation is not more than
5% particularly in the RTC mode of the MCU.
■ Measurement limits:
The limit to the duration of the calibration is roughly 2 seconds if the 32kHz/100kHz Timer is
operating with 32kHz clock (0.5 seconds if operating with 100kHz). On the other hand, the 4MHz
Timer can measure time up to 4 seconds if it is working with a 4MHz clock.