Rainbow-electronics ATmega32L Manual de usuario descargar pdf (Pagina 114)

114

ATmega32(L)

2503C–AVR–10/02

top Signalizes that TCNT2 has reached maximum value.

bottom Signalizes that TCNT2 has reached minimum value (zero).

Depending on the mode of operation used, the counter is cleared, incremented, or dec-

remented at each timer clock (clk

). clk

can be generated from an external or internal

clock source, selected by the Clock Select bits (CS22:0). When no clock source is

selected (CS22:0 = 0) the timer is stopped. However, the TCNT2 value can be accessed

by the CPU, regardless of whether clk

is present or not. A CPU write overrides (has

priority over) all counter clear or count operations.

The counting sequence is determined by the setting of the WGM21 and WGM20 bits

located in the Timer/Counter Control Register (TCCR2). There are close connections

between how the counter behaves (counts) and how waveforms are generated on the

Output Compare output OC2. For more details about advanced counting sequences

and waveform generation, see “Modes of Operation” on page 116.

The Timer/Counter Overflow (TOV2) flag is set according to the mode of operation

selected by the WGM21:0 bits.

TOV2 can be used for generating a CPU interrupt.

Output Compare Unit The 8-bit comparator continuously compares TCNT2 with the output compare register

(OCR2). Whenever TCNT2 equals OCR2, the comparator signals a match. A match will

set the Output Compare Flag (OCF2) at the next timer clock cycle. If enabled (OCIE2 =

1), the output compare flag generates an output compare interrupt. The OCF2 flag is

automatically cleared when the interrupt is executed. Alternatively, the OCF2 flag can

be cleared by software by writing a logical one to its I/O bit location. The waveform gen-

erator uses the match signal to generate an output according to operating mode set by

the WGM21:0 bits and Compare Output mode (COM21:0) bits. The max and bottom sig-

nals are used by the waveform generator for handling the special cases of the extreme

values in some modes of operation (“Modes of Operation” on page 116). Figure 55

shows a block diagram of the output compare unit.

Figure 55. Output Compare Unit, Block Diagram

OCFn (Int.Req.)

= (8-bit Comparator )

OCRn

OCxy

DATA BU S

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMn1:0

bottom

1 2 ... 109 110 111 112 113 114 115 116 117 118 119 ... 312 313

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