Datasheet

96
2586D–AVR–02/06
ATtiny25/45/85
16. 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.
16.1 Timer/Counter1 Prescaler
Figure 16-1 shows an ATtiny15 compatible prescaler. It has two prescaler units, the 3-bit pres-
caler for the the system clock (CK) and the 10-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 16-1. 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.
16.2 Timer/Counter1
Figure 16-2 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 pres-
caling 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 Com-
pare Registers serve as a stand-alone PWM. Refer to page 103 for a detailed description on this
function. Similarly, the high prescaling opportunities make this unit useful for lower speed func-
tions 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
CLEAR