Datasheet

152
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
synchronized to the internal I/O clock domain. Synchronization takes place for every rising TOSC1 edge. When
waking up from Power-save mode, and the I/O clock (clk
I/O
) again becomes active, TCNT2 will read as the
previous value (before entering sleep) until the next rising TOSC1 edge. The phase of the TOSC clock after
waking up from Power-save mode is essentially unpredictable, as it depends on the wake-up time. The
recommended procedure for reading TCNT2 is thus as follows:
1. Write any value to either of the registers OCR2A or TCCR2A.
2. Wait for the corresponding Update Busy Flag to be cleared.
3. Read TCNT2.
• During asynchronous operation, the synchronization of the Interrupt Flags for the asynchronous timer takes
three processor cycles plus one timer cycle. The timer is therefore advanced by at least one before the processor
can read the timer value causing the setting of the Interrupt Flag. The Output Compare pin is changed on the
timer clock and is not synchronized to the processor clock
18.9.1 Timer/Counter prescaler
Figure 18-12. Prescaler for Timer/Counter2.
The clock source for Timer/Counter2 is named clk
T2S
. clk
T2S
is by default connected to the main system I/O clock
clk
IO
. By setting the AS2 bit in ASSR, Timer/Counter2 is asynchronously clocked from the TOSC1 pin. This
enables use of Timer/Counter2 as a Real Time Counter (RTC). When AS2 is set, pins TOSC1 and TOSC2 are dis-
connected from Port C. An XTAL can then be connected between the TOSC1 and TOSC2 pins to serve as an
independent clock source for Timer/Counter2. The Oscillator is optimized for use with a 32.768kHz XTAL. If apply-
ing an external clock on TOSC1, the EXCLK bit in ASSR must be set.
For Timer/Counter2, the possible prescaled selections are: clk
T2S
/8, clk
T2S
/32, clk
T2S
/64, clk
T2S
/128, clk
T2S
/256, and
clk
T2S
/1024. Additionally, clk
T2S
as well as 0 (stop) may be selected. Setting the PSR2 bit in GTCCR resets the
prescaler. This allows the user to operate with a predictable prescaler.
10-BIT T/C PRESCALER
TIMER/COUNTER2 CLOCK SOURCE
clk
I/O
clk
T2S
TOSC1
AS2
CS20
CS21
CS22
clk
T2S
/8
clk
T2S
/64
clk
T2S
/128
clk
T2S
/1024
clk
T2S
/256
clk
T2S
/32
0
PSR2
Clear
clk
T2