81
8008H–AVR–04/11
ATtiny48/88
ter 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 11-1 are used extensively throughout the document.
11.2.2
Registers
The Timer/Counter (TCNT0) and Output Compare Registers (OCR0A and OCR0B) are 8-bit
registers. Interrupt 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 Inter-
rupt Mask Register (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 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 Output Compare Registers (OCR0A and OCR0B) are compared with the Timer/Counter
value at all times. The compare match event will also set the Compare Flag (OCF0A or OCF0B)
which can be used to generate an Output Compare interrupt request.
11.3
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 (CS0[2:0]) bits
located in the Timer/Counter Control Register (TCCR0A). For details on clock sources and pres-
11.4
Counter Unit
The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit.
Figure11-2 shows a block diagram of the counter and its surroundings.
Figure 11-2. Counter Unit Block Diagram
Table 11-1.
Definitions
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
clear