AT90PWM2 datasheet(91/365 Pages) ATMEL

91 / 365 page

AT90PWM2/3

4317B–AVR–02/05

Be aware that the COM0x1:0 bits are not double buffered together with the compare

value. Changing the COM0x1:0 bits will take effect immediately.

Compare Match Output

Unit

The Compare Output mode (COM0x1:0) bits have two functions. The Waveform Gener-

ator uses the COM0x1:0 bits for defining the Output Compare (OC0x) state at the next

compare match. Also, the COM0x1:0 bits control the OC0x pin output source. Figure 34

shows a simplified schematic of the logic affected by the COM0x1:0 bit setting. The I/O

Registers, I/O bits, and I/O pins in the figure are shown in bold. Only the parts of the

general I/O port control registers (DDR and PORT) that are affected by the COM0x1:0

bits are shown. When referring to the OC0x state, the reference is for the internal OC0x

Figure 34. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC0x) from the

Waveform Generator if either of the COM0x1:0 bits are set. However, the OC0x pin

direction (input or output) is still controlled by the Data Direction Register (DDR) for the

port pin. The Data Direction Register bit for the OC0x pin (DDR_OC0x) must be set as

output before the OC0x value is visible on the pin. The port override function is indepen-

dent of the Waveform Generation mode.

The design of the Output Compare pin logic allows initialization of the OC0x state before

the output is enabled. Note that some COM0x1:0 bit settings are reserved for certain

modes of operation. See “8-bit Timer/Counter Register Description” on page 97.

Compare Output Mode and

Waveform Generation

The Waveform Generator uses the COM0x1:0 bits differently in Normal, CTC, and PWM

modes. For all modes, setting the COM0x1:0 = 0 tells the Waveform Generator that no

action on the OC0x Register is to be performed on the next compare match. For com-

pare output actions in the non-PWM modes refer to Table 38 on page 97. For fast PWM

mode, refer to Table 39 on page 98, and for phase correct PWM refer to Table 40 on

page 98.

A change of the COM0x1:0 bits state will have effect at the first compare match after the

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

by using the FOC0x strobe bits.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

FOCn

clk

I/O

AT90PWM2 Datasheet(PDF) 91 Page - ATMEL Corporation

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