ATmega16 datasheet(69/315 Pages) ATMEL

69 / 315 page

ATmega16(L)

2466E–AVR–10/02

BOTTOM

Signalize that TCNT0 has reached minimum value (zero).

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 (TCCR0). There are close connections

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

Output Compare output OC0. For more details about advanced counting sequences

and waveform generation, see “Modes of Operation” on page 71.

The Timer/Counter Overflow (TOV0) flag 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

(OCR0). Whenever TCNT0 equals OCR0, the comparator signals a match. A match will

set the Output Compare Flag (OCF0) at the next timer clock cycle. If enabled (OCIE0 =

1 and Global Interrupt Flag in SREG is set), the Output Compare Flag generates an out-

put compare interrupt. The OCF0 flag is automatically cleared when the interrupt is

executed. Alternatively, the OCF0 flag can be cleared by software by writing 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 Compare Output

mode (COM01:0) 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 71.).

Figure 29 shows a block diagram of the output compare unit.

Figure 29. Output Compare Unit, Block Diagram

OCFn (Int.Req.)

= (8-bit Comparator )

OCRn

OCn

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMn1:0

bottom

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