95
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
13. 8-bit Timer/Counter1 in ATtiny15 Mode
The ATtiny15 compatibility mode is selected by writing the code “0011” to the CKSEL fuses (if any other code is
written, the Timer/Counter1 is working in normal mode). When selected the ATtiny15 compatibility mode provides
an ATtiny15 backward compatible prescaler and Timer/Counter. Furthermore, the clocking system has same clock
frequencies as in ATtiny15.
13.1
Timer/Counter1 Prescaler
Figure 13-1 shows an ATtiny15 compatible prescaler. It has two prescaler units, a 10-bit prescaler for the system
clock (CK) and a 3-bit prescaler for the fast peripheral clock (PCK). The clocking system of the Timer/Counter1 is
always synchronous in the ATtiny15 compatibility mode, because the same RC Oscillator is used as a PLL clock
source (generates the input clock for the prescaler) and the AVR core.
Figure 13-1. Timer/Counter1 Prescaler
The same clock selections as in ATtiny15 can be chosen for Timer/Counter1 from the output multiplexer, because
the frequency of the fast peripheral clock is 25.6 MHz and the prescaler is similar in the ATtiny15 compatibility
mode. The clock selections are PCK, PCK/2, PCK/4, PCK/8, CK, CK/2, CK/4, CK/8, CK/16, CK/32, CK/64,
CK/128, CK/256, CK/512, CK/1024 and stop.
13.2
Counter and Compare Units
Figure 13-2 shows Timer/Counter 1 synchronization register block diagram and synchronization delays in between
registers. Note that all clock gating details are not shown in the figure. The Timer/Counter1 register values go
through the internal synchronization registers, which cause the input synchronization delay, before affecting the
counter operation. The registers TCCR1, GTCCR, OCR1A and OCR1C can be read back right after writing the
register. The read back values are delayed for the Timer/Counter1 (TCNT1) register and flags (OCF1A and TOV1),
because of the input and output synchronization.
The Timer/Counter1 features a high resolution and a high accuracy usage with the lower prescaling opportunities.
It can also support an accurate, high speed, 8-bit Pulse Width Modulator (PWM) using clock speeds up to 25.6
MHz. In this mode, Timer/Counter1 and the Output Compare Registers serve as a stand-alone PWM. Refer to
ing opportunities make this unit useful for lower speed functions or exact timing functions with infrequent actions.
TIMER/COUNTER1 COUNT ENABLE
PSR1
CS10
CS11
CS12
PCK (25.6 MHz)
0
CS13
3-BIT T/C PRESCALER
PCK/2
PCK
PCK/4
PCK/8
CK/2
CK/4
CK/8
CK/16
CK/32
CK/64
CK/128
CK/256
CK/512
CK/1024
10-BIT T/C PRESCALER
CK (1.6 MHz)
CK
CLEAR