PCIF0 datasheet(92/294 Pages) ATMEL

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9219A–RFID–01/11

Atmel ATA5505 [Preliminary]

10.2.1

Definitions

The following definitions are used extensively throughout the section:

10.3

Timer/Counter Clock Sources

The Timer/Counter can be clocked by an internal synchronous or an external asynchronous

clock source. The clock source is selected by the clock select logic which is controlled by the

clock select (CS02:0) bits located in the Timer/Counter control register (TCCR0).The clock

source clk

T0 is by default equal to the MCU clock, clkI/O. When the AS0 bit in the ASSR Regis-

ter is written to logic one, the clock source is taken from the Timer/Counter Oscillator

connected to XTAL1 and XTAL2 or directly from XTAL1. For details on asynchronous opera-

tion, see “Asynchronous Status Register – ASSR” on page 106. For details on clock sources

and prescaler, see “Timer/Counter0 Prescaler” on page 103.

10.4

Counter Unit

The main part of the 8-bit Timer/Counter is the programmable bi-directional counter unit. Fig-

ure 10-2 shows a block diagram of the counter and its surrounding environment.

Figure 10-2. Counter Unit Block Diagram

Signal description (internal signals):

count

Increment or decrement TCNT0 by 1.

direction

Selects between increment and decrement.

clear

Clear TCNT0 (set all bits to zero).

clk

Timer/Counter0 clock.

top

Signalizes that TCNT0 has reached maximum value.

bottom

Signalizes that TCNT0 has reached minimum value (zero).

BOTTOM

The counter reaches the BOTTOM when it becomes zero (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 is

dependent on the mode of operation.

DATA BUS

TCNTn

Control Logic

count

TOVn

(Int.Req.)

top

bottom

direction

clear

XTAL2

Oscillator

XTAL1

Prescaler

clk

I/O

clk

TnS

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