Rainbow-electronics ATmega8515L Manual de usuario descargar pdf (Pagina 79)

ATmega8515(L)

2512A–AVR–04/02

clk

Timer/Counterclock, referred to asclk

in the following.

top Signalizethat TCNT0 hasreached maximum value.

bottom Signalizethat TCNT0 hasreached minimum value (zero).

Depending of themodeof operation used, the counter iscleared, incremented, ordec-

remented at each timerclock (clk

). clk

can be generatedfrom an external or internal

clock source,selectedbythe Clock Select bits(CS02:0). When no clock sourceis

selected(CS02:0 = 0) thetimer isstopped. However, theTCNT0 value can beaccessed

by the CPU, regardless ofwhetherclk

ispresent or not. A CPUwrite overrides(has

priority over) all counterclear orcount operations.

The counting sequenceisdeterminedbythe setting of the WGM01 andWGM00 bits

located in theTimer/CounterControl Register(TCCR0).Thereare close connections

between how the counterbehaves(counts) andhowwaveforms aregenerated on the

Output Compareoutput OC0. For more details about advancedcounting sequences

andwaveform generation,see “Modes ofOperation” on page 82.

TheTimer/CounterOverflow(TOV0)flag isset accordingtothemodeof operation

selectedbythe WGM01:0 bits.TOV0 can beusedfor generating a CPU interrupt.

Output Compare Unit The8-bit comparatorcontinuously compares TCNT0 with the Output Compare Register

(OCR0). Whenever TCNT0 equals OCR0, the comparatorsignals amatch.Amatch will

set the Output Compare Flag (OCF0) at thenexttimerclock cycle. If enabled(OCIE0 =

1andGlobalInterrupt Flag in SREG isset), the Output Compare Flag generates an out-

put compareinterrupt. The OCF0 flag is automatically clearedwhen theinterruptis

executed.Alternatively, the OCF0 flag can be clearedbysoftware by writing a logical

onetoitsI/Obit location. The waveform generator uses thematch signal to generate an

output accordingtooperating mode set by the WGM01:0 bits andCompare Output

mode (COM01:0)bits.Themax andbottom signals areusedbythe waveform generator

forhandling the specialcases of theextreme values in some modes of operation. See

“Modes ofOperation” on page 82.

Figure 35 shows a block diagram of the output compare unit.

Figure 35. Output Compare Unit,Block Diagram

OCFn (Int.Req.)

(8-bit Comparator )

OCRn

OCn

DATA BUS

TCNTn

WGMn1:0

Waveform Generator

top

FOCn

COMn1:0

bottom

1 2 ... 74 75 76 77 78 79 80 81 82 83 84 ... 222 223

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