ATmega8(L)
8-bit Timer/Counter0 Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The
main features are:
• Single Channel Counter
• Frequency Generator
• External Event Counter
• 10-bit Clock Prescaler
Overview
A simplified block diagram of the 8-bit Timer/Counter is shown in Figure 26. For the
actual placement 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 70.
Figure 26. 8-bit Timer/Counter Block Diagram
TCCRn
TOVn
(Int.Req.)
count
Control Logic
Clock Select
clkTn
Edge
Detector
Tn
Timer/Counter
TCNTn
( From Prescaler )
=
0xFF
Registers
The Timer/Counter (TCNT0) is an 8-bit register. Interrupt request (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 or 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 (clkT0).
Definitions
Many register and bit references in this document are written in general form. A lower
case “n” replaces the Timer/Counter number, in this case 0. 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.
The definitions in Table 33 are also used extensively throughout this datasheet.
Table 33. Definitions
BOTTOM The counter reaches the BOTTOM when it becomes 0x00
MAX
The counter reaches its MAXimum when it becomes 0xFF (decimal 255)
67
2486M–AVR–12/03
ATMEL [ ATMEL ]
