Compare pins (OC0A and OC0B). See “Output Compare Unit” on page 119. for details.
The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which
can be used to generate an Output Compare interrupt request.
Definitions
Many register and bit references in this section are written in general form. A lower case
“n” replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the
Output Compare Unit, in this case Compare Unit A or Compare Unit B. However, when
using the register or bit defines in a program, the precise form must be used, i.e.,
TCNT0 for accessing Timer/Counter0 counter value and so on.
The definitions in Table 69 are also used extensively throughout the document.
Table 69. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00.
MAX
TOP
The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
The counter reaches the TOP when it becomes equal to the highest
value in the count sequence. The TOP value can be assigned to be the
fixed value 0xFF (MAX) or the value stored in the OCR0A Register. The
assignment is dependent on the mode of operation.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal or an external clock source. The clock
source is selected by the Clock Select logic which is controlled by the Clock Select
(CS02:0) bits located in the Timer/Counter Control Register (TCCR0B). For details on
clock sources and prescaler, see “Timer/Counter0, Timer/Counter1, Timer/Counter3,
Timer/Counter4, and Timer/Counter5 Prescalers” on page 169.
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 39 shows a block diagram of the counter and its surroundings.
Figure 39. Counter Unit Block Diagram
TOVn
(Int.Req.)
DATA BUS
Clock Select
count
clear
Edge
Detector
Tn
clkTn
TCNTn
Control Logic
direction
( From Prescaler )
bottom
top
Signal description (internal signals):
count Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear
clkTn
top
Clear TCNT0 (set all bits to zero).
Timer/Counter clock, referred to as clkT0 in the following.
Signalize that TCNT0 has reached maximum value.
Signalize that TCNT0 has reached minimum value (zero).
bottom
118
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
