Rainbow Electronics ATmega3290P_V - Manual (Page 90)

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ATmega329/3290/649/6490
2552H–AVR–11/06
event will also set the Compare Flag (OCF0A) which can be used to generate an Output
Compare interrupt request.
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 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 54 are also used extensively throughout the document.
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 prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on
page 103.
Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 29 shows a block diagram of the counter and its surroundings.
Figure 29. Counter Unit Block Diagram
Signal description (internal signals):
count Increment or decrement TCNT0 by 1.
direction Select between increment and decrement.
clear Clear TCNT0 (set all bits to zero).
clk
Tn
Timer/Counter clock, referred to as clk
T0
in the following.
top Signalize that TCNT0 has reached maximum value.
bottom Signalize that TCNT0 has reached minimum value (zero).
Table 54. Definitions of Timer/Counter values.
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.
DATA BUS
TCNTn Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear