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2588F–AVR–06/2013
ATtiny261/461/861
12.3.1.1
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.
12.3.1.2
Prescaler Initialization for Asynchronous Mode
To change Timer/Counter1 to the asynchronous mode follow the procedure below:
1. Enable PLL.
2. Wait 100 s 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.
12.4
Counter Unit
The main part of the Timer/Counter1 is the programmable bi-directional counter unit.
Figure 12-4 shows a block diagram of the counter and its surroundings.
Figure 12-4. Counter Unit Block Diagram
Signal description (internal signals):
count
TCNT1 increment or decrement enable.
direction
Select between increment and decrement.
clear
Clear TCNT1 (set all bits to zero).
clk
Tn
Timer/Counter clock, referred to as clk
T1 in the following.
top
Signalize that TCNT1 has reached maximum value.
bottom
Signalize that TCNT1 has reached minimum value (zero).
Depending on the mode of operation used, the counter is cleared, incremented, or decremented
at each timer clock (clk
T1). The timer clock is generated from an synchronous system clock or an
asynchronous PLL clock using the Clock Select bits (CS13:0) and the PCK Enable bit (PCKE).
When no clock source is selected (CS13:0 = 0) the timer is stopped. However, the TCNT1 value
can be accessed by the CPU, regardless of whether clk
T1 is present or not. A CPU write over-
rides (has priority over) all counter clear or count operations.
The counting sequence of the Timer/Counter1 is determined by bits WGM11:10, PWM1A and
PWM1B, located in the Timer/Counter1 Control Registers (TCCR1A, TCCR1C and TCCR1D).
For more details about advanced counting sequences and waveform generation, see
“Modes ofof operation and can be used for generating a CPU interrupt.
DATA BUS
TCNT1
Control Logic
count
TOV1
top
Timer/Counter1 Count Enable
( From Prescaler )
bottom
direction
clear
PCK
CK
PCKE
clk
T1