ATMEGA8U2_14 Datasheet, PDF (92/310 Pages) ATMEL Corporation – 125 Powerful Instructions – Most Single Clock Cycle Execution

ATmega8U2/16U2/32U2
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 Gen-
erator to generate a PWM or variable frequency output on the Output Compare pins (OC0A and
OC0B). See “Output Compare Unit” on page 93. 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.
15.2.2
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 Com-
pare 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 15-1 are also used extensively throughout the document.
Table 15-1. 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 depen-
dent on the mode of operation.
15.3
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 Register (TCCR0B). For details on clock sources and pres-
caler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on page 88.
15.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Figure
15-2 shows a block diagram of the counter and its surroundings.
Figure 15-2. Counter Unit Block Diagram
DATA BUS
TOVn
(Int.Req.)
TCNTn
count
clear
direction
Control Logic
clk_Tn
bottom
top
Clock Select
Edge
Detector
Tn
( From Prescaler )
Signal description (internal signals):
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