ATtiny261A datasheet(70/292 Pages) ATMEL

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8197A–AVR–10/09

ATtiny261A/461A/861A

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.

In 16-bit mode one more 8-bit register is available, the Timer/Counter0 High Byte Register

(TCNT0H). Also, in 16-bit mode, there is only one output compare unit as the two Output Com-

pare Registers, OCR0A and OCR0B, are combined to one, 16-bit Output Compare Register,

where OCR0A contains the low byte and OCR0B contains the high byte of the word. When

accessing 16-bit registers, special procedures described in section “Accessing Registers in 16-

bit Mode” on page 78 must be followed.

11.2.2

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. TCNT0L for accessing

Timer/Counter0 counter value, and so on.

The definitions in Table 11-1 are also used extensively throughout the document.

11.3 Clock Sources

The Timer/Counter can be clocked internally, via the prescaler, or by an external clock source on

the T0 pin. The Clock Select logic is controlled by the Clock Select (CS02:0) bits located in the

Timer/Counter Control Register 0 B (TCCR0B), and 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).

11.3.1

Prescaler

The Timer/Counter can be clocked directly by the system clock (by setting the CSn2:0 = 1). This

provides the fastest operation, with a maximum Timer/Counter clock frequency equal to system

clock frequency (fCLK_I/O). Alternatively, one of four taps from the prescaler can be used as a

clock source.

See Figure 11-2 for an illustration of the prescaler unit.

Table 11-1.

Definitions

Constant

Description

BOTTOM

The counter reaches BOTTOM when it becomes 0x00

MAX

The counter reaches its MAXimum when it becomes 0xFF (decimal 255)

TOP

The counter reaches the TOP when it becomes equal to the highest value in the count

sequence. The TOP value can be assigned to be the fixed value 0xFF (MAX) or the

value stored in the OCR0A Register. The assignment depends on the mode of operation

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