86
4317I–AVR–01/08
AT90PWM2/3/2B/3B
14. 8-bit Timer/Counter0 with PWM
Timer/Counter0 is a general purpose 8-bit Timer/Counter module, with two independent Output
Compare Units, and with PWM support. It allows accurate program execution timing (event man-
agement) and wave generation. The main features are:
•
Two Independent Output Compare Units
•
Double Buffered Output Compare Registers
•
Clear Timer on Compare Match (Auto Reload)
•
Glitch Free, Phase Correct Pulse Width Modulator (PWM)
•
Variable PWM Period
•
Frequency Generator
•
Three Independent Interrupt Sources (TOV0, OCF0A, and OCF0B)
14.1
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in
. For the actual
placement of I/O pins, refer to
. CPU accessible I/O Registers,
including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit loca-
tions are listed in the
“8-bit Timer/Counter Register Description” on page 96
The PRTIM0 bit in
“Power Reduction Register” on page 43
must be written to zero to enable
Timer/Counter0 module.
Figure 14-1. 8-bit Timer/Counter Block Diagram
14.1.1
Definitions
Many register and bit references in this section are written in general form. A lower case “n”
replaces the Timer/Counter number, in this case 0. A lower case “x” replaces the Output Com-
pare Unit, in this case Compare Unit A or Compare Unit B. 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.
Timer/Counter
D
ATA
B
U
S
=
TCNTn
Waveform
Generation
OCnA
Control Logic
count
clear
direction
TOVn
(Int.Req.)
OCRnx
TCCRnA
Clock Select
Tn
Edge
Detector
( From Prescaler )
clk
Tn
OCnA
(Int.Req.)
=
OCRnx
Waveform
Generation
OCnB
OCnB
(Int.Req.)
TCCRnB
=
Fixed
TOP
Values
=
0
TOP
BOTTOM
