116
7593L–AVR–09/12
AT90USB64/128
Figure 15-1. 16-bit Timer/Counter block diagram
Note:
Timer/Counter1 and 3 and 3 pin placement and description.
15.1.1
Registers
The Timer/Counter (TCNTn), Output Compare Registers (OCRnA/B/C), and Input Capture Reg-
ister (ICRn) 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
117. The Timer/Counter Control Registers (TCCRnA/B/C) are 8-bit registers and have no CPU
access restrictions. Interrupt requests (shorten as Int.Req.) signals are all visible in the Timer
Interrupt Flag Register (TIFRn). All interrupts are individually masked with the Timer Interrupt
Mask Register (TIMSKn). TIFRn and TIMSKn 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 Tn 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
Tn).
The double buffered Output Compare Registers (OCRnA/B/C) are compared with the
Timer/Counter value at all time. The result of the compare can be used by the Waveform Gener-
ator to generate a PWM or variable frequency output on the Output Compare pin (OCnA/B/C).
ICFn (Int.Req.)
TOVn
(int.req.)
Clock select
Timer/Counter
DATABUS
ICRn
=
TCNTn
Waveform
generation
Waveform
generation
Waveform
generation
OCnA
OCnB
OCnC
Noise
canceler
ICPn
=
Fixed
TOP
values
Edge
detector
Control logic
= 0
TOP
BOTTOM
Count
Clear
Direction
OCFnA
(Int.Req.)
OCFnB
(Int.Req.)
OCFnC
(Int.Req.)
TCCRnA
TCCRnB
TCCRnC
( From Analog
Comparator Ouput )
Tn
Edge
detector
(From prescaler)
TCLK
OCRnC
OCRnB
OCRnA
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