ATMEGA8_14 Datasheet, PDF (69/331 Pages) ATMEL Corporation – High-performance, Low-power Atmel

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 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 loca-
tions are listed in the “8-bit Timer/Counter Register Description” on page 71.
Figure 26. 8-bit Timer/Counter Block Diagram
TCCRn
count
Control Logic
clk_Tn
Timer/Counter
TCNTn
= 0xFF
Clock Select
Edge
Detector
TOVn
(Int.Req.)
Tn
( From Prescaler )
Registers
Definitions
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 individ-
ually 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 (clk_T0).
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, that is, 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)
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