Rainbow Electronics ATmega8515L - Manual (Page 78)

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ATmega8515(L)
2512A–AVR–04/02
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 Register (OCR0) is com pared 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 (OC0). See “Output Compare Unit” on page 79. for details. The compare match
event will also set the Compare Flag (OCF0) 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 0. 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 43 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 (TCCR0). For details on
clock sources and prescaler, see “Timer/Counter0 and Timer/Counter1 Prescalers” on
page 92.
Counter Unit The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure 34 shows a block diagram of the counter and its surroundings.
Figure 34. 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).
Table 43. 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 OCR0 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