12.1.1 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 12-1 are also used extensively throughout the document.
Table 12-1. Definitions
Definitions
BOTTOM
MAX
The counter reaches the BOTTOM when it becomes 0x00.
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.
TOP
12.1.2 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 (clkT0).
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 frequency output on
the output compare pins (OC0A and OC0B). See Section 13.6.3 “Using the Output Compare Unit” on page 101 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.
12.2 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 Section 11. “Timer/Counter0 and Timer/Counter1 Prescalers” on page 75.
12.3 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure 12-2 shows a block diagram
of the counter and its surroundings.
Figure 12-2. Counter Unit Block Diagram
TOVn
(Int. Req.)
DATA BUS
Clock Select
count
Edge
Tn
clkTn
Detector
clear
TCNTn
Control Logic
direction
(from Prescaler)
bottom
top
78
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
