Rainbow-electronics ATmega8515L Manuel d'utilisateur télécharger pdf (Page 81)

ATmega8515(L)

2512A–AVR–04/02

Compare Match Output

Unit

The Compare Output mode (COM01:0)bitshave two functions.The Waveform Genera-

tor uses the COM01:0 bitsfordefining the Output Compare (OC0)state at thenext

comparematch.Also, the COM01:0 bitscontrol the OC0 pin output source. Figure 36

shows a simplified schematic of the logic affectedbythe COM01:0 bit setting. The I/O

Registers, I/Obits, andI/Opins in the figureare showninbold. Only the parts of the

generalI/Oport Control Registers (DDRand PORT) that areaffectedbythe COM01:0

bits are shown. When referring to the OC0 state, the referenceisfor theinternalOC0

Figure 36. Compare Match Output Unit,Schematics

The generalI/Oport function is overridden by the output compare (OC0)from the Wave-

form Generator if either of the COM01:0 bits are set. However, the OC0 pin direction

(input or output) isstill controlledbythe Data Direction Register(DDR)for the port pin.

The Data Direction Registerbit for the OC0 pin (DDR_OC0) must be set as output

beforethe OC0value is visibleonthe pin. The portoverride function is independent of

the Waveform Generation mode.

The design of theoutput compare pin logic allows initialization of the OC0 state before

the output is enabled. Note that some COM01:0 bit settings are reservedforcertain

modes of operation. See “8-bit Timer/Counter RegisterDescription” on page 87.

Compare Output Mode and

Waveform Generation

The waveformgenerator uses the COM01:0 bitsdifferently in Normal, CTC, and PWM

modes. For all modes, setting the COM01:0 = 0tells the Waveform Generator that no

action on the OC0Register is to be performed on thenext comparematch. Forcompare

output actions in the non-PWM modesrefer to Table45onpage 88. ForfastPWM

mode,refer to Table46 on page 89, andforphase correctPWM refer to Table47 on

page 89.

A change of the COM01:0 bitsstate will have effectatthe first comparematch after the

bits are written. For non-PWM modes, theaction can be forced to have immediate effect

by using the FOC0 strobe bits.

PORT

DDR

OCn

Waveform

Generator

COMn1

COMn0

DATA BU S

FOCn

clk

I/O

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 222 223

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Rainbow-electronics ATmega8515L Manuel d'utilisateur Page 81

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