124
ATmega169V/L
2514A–AVR–08/02
ment) 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
T2
).
The double buffered Output Compare Register (OCR2A) is compared with the
Timer/Counter value at all times. The result of the compare can be used by the Wave-
form Generator to generate a PWM or variable frequency output on the Output Compare
pin (OC2A). See “Output Compare Unit” on page 125. for details. The compare match
event will also set the compare flag (OCF2A) which can be used to generate an Output
Compare interrupt request.
Definitions Many register and bit references in this document are written in general form. A lower
case “n” replaces the Timer/Counter number, in this case 2. However, when using the
register or bit defines in a program, the precise form must be used, i.e., TCNT2 for
accessing Timer/Counter2 counter value and so on.
The definitions in Table 60 are also used extensively throughout the section.
Timer/Counter Clock
Sources
The Timer/Counter can be clocked by an internal synchronous or an external asynchro-
nous clock source. The clock source clk
T2
is by default equal to the MCU clock, clk
I/O
.
When the AS2 bit in the ASSR Register is written to logic one, the clock source is taken
from the Timer/Counter Oscillator connected to TOSC1 and TOSC2. For details on
asynchronous operation, see “Asynchronous Status Register – ASSR” on page 137. For
details on clock sources and prescaler, see “Timer/Counter Prescaler” on page 141.
Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 53 shows a block diagram of the counter and its surrounding environment.
Figure 53. Counter Unit Block Diagram
Signal description (internal signals):
count Increment or decrement TCNT2 by 1.
direction Selects between increment and decrement.
clear Clear TCNT2 (set all bits to zero).
clk
T2
Timer/Counter clock.
Table 60. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes zero (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 OCR2A Register. The
assignment is dependent on the mode of operation.
DATA B U S
TCNTn Control Logic
count
TOVn
(Int.Req.)
topbottom
direction
clear
TOSC1
T/C
Oscillator
TOSC2
Prescaler
clk
I/O
clk
Tn
(27 pages)
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