100
7679H–CAN–08/08
AT90CAN32/64/128
12.2.1
Registers
The Timer/Counter (TCNT0) and Output Compare Register (OCR0A) are 8-bit registers. Inter-
rupt request (abbreviated to Int.Req. in the figure) signals are all visible in the Timer Interrupt
Flag Register (TIFR0). All interrupts are individually masked with the Timer Interrupt Mask Reg-
ister (TIMSK0). TIFR0 and TIMSK0 are not shown in the figure.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on
the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter
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 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).
for details. The compare match event will also set the Compare
Flag (OCF0A) which can be used to generate an Output Compare interrupt request.
12.2.2
Definitions
The following definitions are used extensively throughout the section:
12.3
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 (TCCR0A). For details on clock sources and pres-
caler, see
“Timer/Counter3/1/0 Prescalers” on page 96
12.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
shows a block diagram of the counter and its surroundings.
Figure 12-2.
Counter Unit Block Diagram
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.
DATA BUS
TCNTn
Control Logic
count
TOVn
(Int.Req.)
Clock Select
top
Tn
Edge
Detector
( From Prescaler )
clk
Tn
bottom
direction
clear
