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Chapter 10
Timer
Preliminary User’s Manual U15839EE1V0UM00
10.1.6 Sub Oscillator Calibration Function
For automotive dashboard application, customer need to achieve a watch timer accuracy of about
1 sec/week. This target is difficult to manage using a 32 KHz crystal for sub oscillator because of the
temperature dependency of these crystal types. The crystal type used for main oscillator has a
temperature deviation which compensates themselves over one year (summer/winter) while the
temperature deviation of sub oscillator accumulates over one year.
A sub-clock oscillator calibration function is available by measuring the sub-clock deviation with the
main clock oscillator. To perform this measurement in a power efficient way, a special clock path for
Timer C0 is supported. This clock supply switches between the peripheral macro clock prescaler output
and direct clock from the main oscillator input. In the Jupiter device the Timer C0 will be used to meas-
ure the sub-clock frequency by capture operation of the Watch timer interrupts. The device can be
switched to Watch mode during measurement operation.
To enable usage of the sub watch mode for real watch timer applications, a sub oscillator calibration
mode is implemented into Jupiter. In this mode the actual sub clock frequency can be measured taking
the main oscillator frequency as reference and the calculated watch time can be corrected according to
the actual sub clock deviation. The sub oscillator calibration mode is implemented in the following way:
•
Capture input for CCC01 input channel of Timer C can be switched to watch timer interrupt output
•
f
PCLK
input of Timer C can be clocked directly by the main oscillator with f
X
.
For the sub oscillator calibration, the Timer C peripheral clock and the CCC01 capture input need to be
multiplexed.
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