Datasheet

121
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Depending of the mode of operation used, the counter is cleared, incremented, or dec-
remented at each timer clock (clk
T0
). clk
T0
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) 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 124.
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.
Output Compare Unit The 8-bit comparator continuously compares TCNT0 with the Output Compare Regis-
ters (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 Out-
put Compare Flag generates an Output Compare interrupt. The Output Compare Flag is
automatically cleared when the interrupt is executed. Alternatively, the flag can be
cleared by software by writing a logical one to its I/O bit location. The Waveform Gener-
ator 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 max and bottom sig-
nals are used by the Waveform Generator for handling the special cases of the extreme
values in some modes of operation (“Modes of Operation” on page 124).
Figure 39 shows a block diagram of the Output Compare unit.
Figure 39. Output Compare Unit, Block Diagram
OCFnx (Int.Req.)
= (8-bit Comparator )
OCRnx
OCnx
DATA BU S
TCNTn
WGMn1:0
Waveform Generator
top
FOCn
COMnX1:0
bottom