ATmega165 datasheet(77/336 Pages) ATMEL

77 / 336 page

ATmega165/V

2573G–AVR–07/09

Depending of the mode of operation used, the counter is cleared, incremented, or dec-

remented at each timer clock (clk

T0). clkT0 can be generated from an external or internal

clock source, selected by the Clock Select bits (CS02:0). When no clock source is

selected (CS02:0 = 0) the timer is stopped. However, the TCNT0 value can be accessed

by the CPU, regardless of whether clk

T0 is present or not. A CPU write overrides (has

priority over) all counter clear or count operations.

The counting sequence is determined by the setting of the WGM01 and WGM00 bits

located in the Timer/Counter Control Register (TCCR0A). There are close connections

between how the counter behaves (counts) and how waveforms are generated on the

Output Compare output OC0A. For more details about advanced counting sequences

and waveform generation, see “Modes of Operation” on page 80.

The Timer/Counter Overflow Flag (TOV0) is set according to the mode of operation

selected by the WGM01:0 bits. TOV0 can be used for generating a CPU interrupt.

Output Compare Unit

The 8-bit comparator continuously compares TCNT0 with the Output Compare Register

(OCR0A). Whenever TCNT0 equals OCR0A, the comparator signals a match. A match

will set the Output Compare Flag (OCF0A) at the next timer clock cycle. If enabled

(OCIE0A = 1 and Global Interrupt Flag in SREG is set), the Output Compare Flag gen-

erates an Output Compare interrupt. The OCF0A Flag is automatically cleared when the

interrupt is executed. Alternatively, the OCF0A Flag can be cleared by software by writ-

ing a logical one to its I/O bit location. The Waveform Generator uses the match signal

to generate an output according to operating mode set by the WGM01:0 bits and Com-

pare Output mode (COM0A1:0) bits. The max and bottom signals are used by the

Waveform Generator for handling the special cases of the extreme values in some

modes of operation (See “Modes of Operation” on page 80.).

Figure 29 shows a block diagram of the Output Compare unit.

Figure 29. Output Compare Unit, Block Diagram

OCFnx (Int.Req.)

= (8-bit Comparator )

OCRnx

OCnx

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMnx1:0

bottom

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