C8051F960-B-GM Datasheet, PDF (305/492 Pages) Silicon Laboratories – Ultra Low Power 128K, LCD MCU Family

C8051F96x
running (RTC0TR = 1) in order to set or capture the main timer. The transfer can take up to 2 smaRTClock
cycles to complete.
24.3.2. Setting a SmaRTClock Alarm
The SmaRTClock alarm function compares the 32-bit value of SmaRTClock Timer to the value of the
ALARMnBn registers. An alarm event is triggered if the SmaRTClock timer is equal to the ALARMnBn
registers. If Auto Reset is enabled, the 32-bit timer will be cleared to zero one SmaRTClock cycle after the
alarm 0 event.
The SmaRTClock alarm event can be configured to reset the MCU, wake it up from a low power mode, or
generate an interrupt. See Section “17. Interrupt Handler” on page 232, Section “19. Power Management”
on page 257, and Section “22. Reset Sources” on page 278 for more information.
The following steps can be used to set up a SmaRTClock Alarm:
1. Disable SmaRTClock Alarm Events (RTC0AEN = 0).
2. Set the ALARMn registers to the desired value.
3. Enable SmaRTClock Alarm Events (RTC0AEN = 1).
Notes:
1. The ALRM bit, which is used as the SmaRTClock Alarm Event flag, is cleared by disabling SmaRTClock Alarm
Events (RTC0AEN = 0).
2. If AutoReset is disabled, disabling (RTC0AEN = 0) then Re-enabling Alarm Events (RTC0AEN = 1) after a
SmaRTClock Alarm without modifying ALARMn registers will automatically schedule the next alarm after 2^32
SmaRTClock cycles (approximately 36 hours using a 32.768 kHz crystal).
24.3.3. Software Considerations for using the SmaRTClock Timer and Alarm
The SmaRTClock timer and alarm have two operating modes to suit varying applications. The two modes
are described below:
Mode 1:
The first mode uses the SmaRTClock timer as a perpetual timebase which is never reset to zero. Every 36
hours, the timer is allowed to overflow without being stopped or disrupted. The alarm interval is software
managed and is added to the ALRMnBn registers by software after each alarm. This allows the alarm
match value to always stay ahead of the timer by one software managed interval. If software uses 32-bit
unsigned addition to increment the alarm match value, then it does not need to handle overflows since
both the timer and the alarm match value will overflow in the same manner.
This mode is ideal for applications which have a long alarm interval (e.g., 24 or 36 hours) and/or have a
need for a perpetual timebase. An example of an application that needs a perpetual timebase is one
whose wake-up interval is constantly changing. For these applications, software can keep track of the
number of timer overflows in a 16-bit variable, extending the 32-bit (36 hour) timer to a 48-bit (272 year)
perpetual timebase.
Mode 2:
The second mode uses the SmaRTClock timer as a general purpose up counter which is auto reset to zero
by hardware after each alarm 0 event. The alarm interval is managed by hardware and stored in the
ALRM0Bn registers. Software only needs to set the alarm interval once during device initialization. After
each alarm 0 event, software should keep a count of the number of alarms that have occurred in order to
keep track of time. Alarm 1 and alarm 2 events do not trigger the auto reset.
This mode is ideal for applications that require minimal software intervention and/or have a fixed alarm
interval. This mode is the most power efficient since it requires less CPU time per alarm.
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