EFM32WG Datasheet, PDF (566/834 Pages) List of Unclassifed Manufacturers – The EFM32WG Wonder Gecko is the ideal choice for demanding 8-, 16-, and 32-bit energy sensitive applications.

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22.3 Functional Description
The Backup RTC is a 32-bit counter with one compare channel. The Backup RTC resides in a power
domain which can be configured to always be on, in EM0 through EM4. This domain also has the
possibility to be powered by a backup battery. For further details on the backup power domain, refer to
Section 10.3.4 (p. 111) . Available in all energy modes, the Backup RTC is ideal for applications where
keeping track of time in combination with extremely low energy consumption is essential. An overview
of the backup RTC is shown in Figure 22.1 (p. 566) .
Figure 22.1. BURTC Overview
Peripheral bus
LFXO
LFRCO
ULFRCO
2 ^-PRESC
Count er (CNT)
Clear
CLKSEL
COMP0
CNT = COMP0 *
RTC cont rol and st at us
Com pare m at ch
COMP0 TOP
* Num ber of bits evaluated
varies in low power mode
22.3.1 Counter
The Backup RTC is enabled by configuring MODE in the BURTC_CTRL register. This configuration of
MODE determines in which energy modes the backup RTC is operational. It will always be operational in
EM0-EM2, and optionally in EM3 and EM4. The Backup RTC is available when the system is in backup
mode if MODE is set to EM4EN. The counter is cleared by setting RSTEN in the control register. A
system reset will not clear the counter. The counter value can be read through the CNT register.
22.3.2 Clock source
The Backup RTC is clocked by LFXO, LFRCO, or ULFRCO, depending on the configuration of CLKSEL
in BURTC_CTRL. The PRESC bit-field in BURTC_CTRL controls the clock prescaling factor. Prescaler
is only available for LFXO and LFRCO. When using the ULFRCO as clock source, only two frequency
options are available; 2kHz and 1kHz. The 2kHz clock is selected when PRESC in BURTC_CTRL is set
to DIV1, and the 1kHz clock is selected when PRESC is set to any other value. Available frequencies
when using LFXO or LFRCO are given in Equation 22.1 (p. 566) . CLKSEL should not be changed
while the backup RTC is running.
BURTC Frequency Equation
f_BURTC = 32768/2^PRESC Hz, PRESC = 0..7
(22.1)
When the LFXO or LFRCO is enabled, the Backup RTC will not use the clock until the timeout defined
in the CMU has run out, i.e. the LFXORDY/LFRCORDY flag in CMU_STATUS is set. When an oscillator
first has been enabled and is used by the Backup RTC, the Backup RTC will keep the selected clock
source enabled, independent of both energy mode and CMU settings.
22.3.3 Compare channel
The backup RTC has one compare channel. The compare value is set by writing to the COMP0 register.
When the value of CNT equals the value of COMP0, the COMP0 interrupt flag is set. If COMP0TOP
in CTRL is set, the counter will wrap around when reaching the value in the compare register, COMP.
If COMP0TOP is cleared, the counter will continue counting, wrapping around when it overflows. On
overflow, the OF interrupt flag is set.
2013-05-08 - Wonder Gecko Family - d0233_Rev0.50
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