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XMEGA A [MANUAL]
8077I–AVR–11/2012
17.
RTC – Real-Time Counter
17.1
Features
16-bit resolution
Selectable clock source
– 32.768kHz external crystal
– External clock
– 32.768kHz internal oscillator
– 32kHz internal ULP oscillator
Programmable 10-bit clock prescaling
One compare register
One period register
Clear counter on period overflow
Optional interrupt/event on overflow and compare match
17.2
Overview
The 16-bit real-time counter (RTC) is a counter that typically runs continuously, including in low-power sleep modes, to
keep track of time. It can wake up the device from sleep modes and/or interrupt the device at regular intervals.
The reference clock is typically the 1.024kHz output from a high-accuracy crystal of 32.768kHz, and this is the
configuration most optimized for low power consumption. The faster 32.768kHz output can be selected if the RTC needs
a resolution higher than 1ms. The RTC can also be clocked from an external clock signal, the 32.768kHz internal
oscillator or the 32kHz internal ULP oscillator.
The RTC includes a 10-bit programmable prescaler that can scale down the reference clock before it reaches the
counter. A wide range of resolutions and time-out periods can be configured. With a 32.768kHz clock source, the
maximum resolution is 30.5s, and time-out periods can range up to 2000 seconds. With a resolution of 1s, the
maximum time-out period is more than18 hours (65536 seconds). The RTC can give a compare interrupt and/or event
when the counter equals the compare register value, and an overflow interrupt and/or event when it equals the period
register value.
Figure 17-1. Real-time counter overview.
17.2.1 Clock Domains
The RTC is asynchronous, operating from a different clock source independently of the main system clock and its
derivative clocks, such as the peripheral clock. For control and count register updates, it will take a number of RTC clock
and/or peripheral clock cycles before an updated register value is available in a register or until a configuration change
32.768kHz Crystal Osc
32.768kHz Int. Osc
TOSC1
TOSC2
External Clock
DIV32
32kHz int ULP (DIV32)
RTCSRC
10-bit
prescaler
clkRTC
CNT
PER
COMP
=
”match”/
Compare
TOP/
Overflow
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