ATMEGA261_10 Datasheet, PDF (89/226 Pages) ATMEL Corporation – High Performance, Low Power AVR® 8-Bit Microcontroller

ATtiny261/ATtiny461/ATtiny861
15. Timer/Counter1 Prescaler
Figure 15-1 shows the Timer/Counter1 prescaler that supports two clocking modes, a synchro-
nous clocking mode and an asynchronous clocking mode. The synchronous clocking mode uses
the system clock (CK) as a clock timebase and asynchronous mode uses the fast peripheral
clock (PCK) as a clock time base. The PCKE bit from the PLLCSR register enables the asyn-
chronous mode when it is set (‘1’).
Figure 15-1. Timer/Counter1 Prescaler
PCKE
PSR1
CK
S
PCK 64/32 MHz _A
T1CK
14-BIT
T/C PRESCALER
0
CS10
CS11
CS12
CS13
TIMER/COUNTER1 COUNT ENABLE
In the asynchronous clocking mode the clock selections are from PCK to PCK/16384 and stop,
and in the synchronous clocking mode the clock selections are from CK to CK/16384 and stop.
The clock options are described in Table 15-1 on page 91 and the Timer/Counter1 Control Reg-
ister, TCCR1B.
The frequency of the fast peripheral clock is 64 MHz or 32 MHz in Low Speed mode (the LSM bit
in PLLCSR register is set to one). The Low Speed Mode is recommended to use when the sup-
ply voltage below 2.7 volts are used.
15.0.1
Prescaler Reset
Setting the PSR1 bit in TCCR1B register resets the prescaler. It is possible to use the Prescaler
Reset for synchronizing the Timer/Counter to program execution.
15.0.2
Prescaler Initialization for Asynchronous Mode
To change Timer/Counter1 to the asynchronous mode follow the procedure below:
1. Enable PLL.
2. Wait 100 µs for PLL to stabilize.
3. Poll the PLOCK bit until it is set.
4. Set the PCKE bit in the PLLCSR register which enables the asynchronous mode.
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