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Figure 20.1. RTC Overview
Peripheral bus
LFACLK RTC
Counter (CNT)
Com pare 0
(COMP0)
Com pare 1
(COMP1)
RTC Control and
Status
Clear
=
=
Com pare m atch 0
Com pare m atch 1
20.3.1 Counter
The RTC is enabled by setting the EN bit in the RTC_CTRL register. It counts up as long as it is
enabled, and will on an overflow simply wrap around and continue counting. The RTC is cleared when it
is disabled. The timer value is read-only and the RTC always starts counting from 0 when enabled. The
value of the counter can be read using the RTC_CNT register.
20.3.1.1 Clock Source
The RTC clock source and its prescaler value are defined in the Register Description section of the Clock
Management Unit (CMU). The clock used by the RTC has a frequency given by Equation 20.1 (p. 279) .
RTC Frequency Equation
f RTC = f LFACLK /2
RTC_PRESC
(20.1)
where f LFACLK is the LFACLK frequency (32.768 kHz) and RTC_PRESC is a 4 bit value. Table 20.1 (p.
280) shows the time of overflow and resolution of the RTC at the available prescaler values.
To use this module, the LE interface clock must be enabled in CMU_HFCORECLKEN0 in addition to
the module clock
2011-04-12 - d0001_Rev1.10
279
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