Datasheet

74
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 17-1. 16-bit Timer/Counter Block Diagram
(1)
Note: 1. Refer to “Pin Configurations” on page 2, Table 13-2 on page 56, and Table 13-8 on page 61 for Timer/Counter1 pin
placement and description.
17.2.1 Registers
The Timer/Counter (TCNT1), Output Compare Registers (OCR1A/B), and Input Capture Register (ICR1) are all
16-bit registers. Special procedures must be followed when accessing the 16-bit registers. These procedures are
described in the section “Accessing 16-bit Registers” on page 75. The Timer/Counter Control Registers
(TCCR1A/B) are 8-bit registers and have no CPU access restrictions. Interrupt requests (abbreviated to Int.Req. in
the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR). All interrupts are individually masked
with the Timer Interrupt Mask Register (TIMSK). TIFR and TIMSK are not shown in the figure since these registers
are shared by other timer units.
The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on the T1 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
T
1
).
The double buffered Output Compare Registers (OCR1A/B) are compared with the Timer/Counter value at all time.
The result of the compare can be used by the waveform generator to generate a PWM or variable frequency output
on the Output Compare Pin (OC1A/B). See “Output Compare Units” on page 81. The Compare Match event will
also set the Compare Match Flag (OCF1A/B) which can be used to generate an Output Compare interrupt request.
Clock Select
Timer/Counter
DATA B US
OCRnA
OCRnB
ICRn
=
=
TCNTn
Waveform
Generation
Waveform
Generation
OCnA
OCnB
Noise
Canceler
ICPn
=
Fixed
TOP
Values
Edge
Detector
Control Logic
= 0
TOP BOTTOM
Count
Clear
Direction
TOVn
(Int. Req.)
OCFnA
(Int. Req.)
OCFnB
(Int.Req.)
ICFn (Int.Req.)
TCCRnA TCCRnB
( From Analog
Comparator Ouput )
Tn
Edge
Detector
( From Prescaler )
clk
Tn