219
8331B–AVR–03/12
Atmel AVR XMEGA AU
18. RTC – Real-Time Counter
18.1
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
18.2
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 config-
ured. With a 32.768kHz clock source, the maximum resolution is 30.5µs, and time-out periods
can range up to 2000 seconds. With a resolution of 1s, the maximum timeout 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 18-1.
Real-time counter overview.
32.768kHz Crystal Osc
32.768kHz Int. Osc
TOSC1
TOSC2
External Clock
DIV32
DIV32
32kHz int ULP (DIV32)
RTCSRC
10-bit
prescaler
clk
RTC
CNT
PER
COMP
=
=
”match”/
Compare
TOP/
Overflow
