Datasheet

116
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
16.2.1 Registers
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit registers. Interrupt
request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR0). All
interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK0). TIFR0 and TIMSK0 are not
shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T0 pin. The
Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment (or decrement)
its value. The Timer/Counter is inactive when no clock source is selected. The output from the Clock Select logic is
referred to as the timer clock (clk
T0
).
The double buffered Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter value
at all times. The result of the compare can be used by the Waveform Generator to generate a PWM or variable fre-
quency output on the Output Compare pins (OC0A and OC0B). See “Output Compare Unit” on page 117. for
details. The Compare Match event will also set the Compare Flag (OCF0A or OCF0B) which can be used to gener-
ate an Output Compare interrupt request.
16.2.2 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, that is, TCNT0 for accessing Timer/Counter0 counter value and so on.
The definitions in Table 16-1 are also used extensively throughout the document.
16.3 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 Reg-
ister (TCCR0B). For details on clock sources and prescaler, see “Timer/Counter 0, 1, 3, 4, and 5 Prescaler” on
page 164.
16.4 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 16-2 on page 117
shows a block diagram of the counter and its surroundings.
Table 16-1. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00.
MAX The counter reaches its MAXimum when it becomes 0xFF (decimal 255).
TOP 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.