Rainbow-electronics ATmega8515L Instrukcja Użytkownika Pobierz pdf (Strona 80)

ATmega8515(L)

2512A–AVR–04/02

The OCR0 Register isdouble bufferedwhen using any of thePulse WidthModulation

(PWM) modes. For thenormal andClear Timer on Compare (CTC) modes of operation,

the double buffering isdisabled.The double buffering synchronizes theupdate of the

OCR0 CompareRegister to either top orbottom of the counting sequence. The synchro-

nization prevents theoccurrenceof odd-length, non-symmetrical PWM pulses, thereby

making the output glitch-free.

The OCR0 Register access mayseem complex, but this is not case. When the double

buffering is enabled, the CPUhas access to the OCR0 Buffer Register, and ifdouble

buffering isdisabled the CPUwill access the OCR0 directly.

Force Output Compare Innon-PWM waveform generation modes, thematch output of the comparatorcan be

forcedbywriting a one to the Force Output Compare (FOC0)bit. Forcing compare

match will not set the OCF0 flag orreload/clear thetimer, but the OC0 pin will be

updated as if a realcomparematch had occurred(the COM01:0 bitssettingsdefine

whether the OC0 pin isset,cleared or toggled).

Compare Match Blocking by

TCNT0 Write

All CPUwrite operations to theTCNT0 Registerwill block anycomparematch that

occur in thenexttimerclock cycle, even when thetimer isstopped.Thisfeatureallows

OCR0 to beinitialized to the same value as TCNT0 without triggering an interrupt when

theTimer/Counterclock is enabled.

Using the Output Compare

Unit

Since writing TCNT0 in any modeof operation will block all comparematchesfor one

timerclock cycle, thereare risks involvedwhen changing TCNT0 when using the output

compare channel, independently ofwhether theTimer/Counter isrunning or not. If the

value written to TCNT0 equals the OCR0 value, the comparematch will bemissed,

resulting in incorrect waveformgeneration. Similarly, donotwrite theTCNT0 value

equal to BOTTOM when the counter isdowncounting.

The setup of the OC0 should be performedbefore setting the Data Direction Registerfor

the port pin to output. Theeasiest way ofsetting the OC0value is to usethe Force Out-

put Compare (FOC0)strobe bits in Normal mode. The OC0 Registerkeeps its value

even when changing between Waveform Generation modes.

Beawarethat the COM01:0 bits are not double buffered togetherwith the compare

value. Changing the COM01:0 bitswill takeeffectimmediately.

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