The double buffered Output Compare Register (OCR0A) is compared with the Timer/Counter
value at all times. The result of the compare can be used by the Waveform Generator to gener-
ate a PWM or variable frequency output on the Output Compare pin (OC0A). See ”Output
Compare Unit” on page 93. for details. The compare match event will also set the Compare Flag
(OCF0A) which can be used to generate an Output Compare interrupt request.
13.1.2
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 Com-
pare unit number, in this case unit A. 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 13-1 are also used extensively throughout the document.
Table 13-1. Timer/Counter Definitions
BOTTOM
MAX
The counter reaches the BOTTOM when it becomes 0x00.
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.
13.2 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 (TCCR0A). For details on clock sources and pres-
caler, see ”Timer/Counter0 and Timer/Counter1 Prescalers” on page 135.
13.3 Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
13-2 shows a block diagram of the counter and its surroundings.
Figure 13-2. 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
92
ATmega169P
8018A–AVR–03/06
ATMEL [ ATMEL ]
