ATmega128(L)
8-bit
Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The main
features are:
• Single Channel Counter
• Clear Timer on Compare Match (Auto Reload)
• Glitch-free, Phase Correct Pulse Width Modulator (PWM)
• Frequency Generator
• 10-bit Clock Prescaler
• Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)
• Allows Clocking from External 32 kHz Watch Crystal Independent of the I/O Clock
Timer/Counter0
with PWM and
Asynchronous
Operation
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 34. For the actual place-
ment of I/O pins, refer to “Pin Configurations” on page 2. 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 locations are
listed in the “8-bit Timer/Counter Register Description” on page 104.
Figure 34. 8-bit Timer/Counter Block Diagram
TCCRn
count
clear
TOVn
(Int.Req.)
Control Logic
TOP
direction
clkTn
TOSC1
TOSC2
BOTTOM
T/C
Oscillator
Prescaler
Timer/Counter
TCNTn
= 0
= 0xFF
clk I/O
OCn
OCn
(Int.Req.)
Waveform
Generation
=
OCRn
clk I/O
Synchronized Status flags
Synchronization Unit
clk ASY
Status flags
ASSRn
asynchronous mode
select (ASn)
Registers
The Timer/Counter (TCNT0) and Output Compare Register (OCR0) are 8-bit registers. Interrupt
request (shorten as Int.Req.) 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 asynchronously clocked from
the TOSC1/2 pins, as detailed later in this section. The asynchronous operation is controlled by
the Asynchronous Status Register (ASSR). The Clock Select logic block controls which clock
source the Timer/Counter uses to increment (or decrement) its value. The Timer/Counter is inac-
93
2467P–AVR–08/07
ATMEL [ ATMEL ]
