AT90SCR100 datasheet(90/433 Pages) ATMEL

90 / 433 page

TPR0327AY–SMS–30Jan09

AT90SCR100

whether clkT0 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) and the WGM02 bit located in the Timer/Counter

Control Register B (TCCR0B). There are close connections between how the counter behaves

(counts) and how waveforms are generated on the Output Compare outputs OC0A and OC0B.

For more details about advanced counting sequences and waveform generation, see “Modes of

Operation” on page 92.

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

the WGM02:0 bits. TOV0 can be used for generating a CPU interrupt.

13.1.4

Output Compare Unit

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

(OCR0A and OCR0B). Whenever TCNT0 equals OCR0A or OCR0B, the comparator signals a

match. A match will set the Output Compare Flag (OCF0A or OCF0B) at the next timer clock

cycle. If the corresponding interrupt is enabled, the Output Compare Flag generates an Output

Compare interrupt. The Output Compare Flag is automatically cleared when the interrupt is exe-

cuted. Alternatively, the 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 WGM02:0 bits and Compare Output mode (COM0x1:0) bits. The

TOP and BOTTOM signals are used by the Waveform Generator for handling the special cases

that occur in some modes of operation when maximum and minimum values are reached

(“Modes of Operation” on page 92).

Figure 13-3 shows a block diagram of the Output Compare unit.

Figure 13-3.

Output Compare Unit, Block Diagram

The OCR0x Registers are double buffered when using any of the Pulse Width Modulation

(PWM) modes. For the normal and Clear Timer on Compare (CTC) modes of operation, the dou-

OCF0x (Int.Req.)

= (8-bit Comparator )

OCR0x

OC0x

DATA BUS

TCNT0

WGM02:0

Waveform Generator

top

FOC0x

COM0x1:0

bottom

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