Datasheet

121
2466T–AVR–07/10
ATmega16(L)
Compare Match
Output Unit
The Compare Output mode (COM21:0) bits have two functions. The Waveform Generator uses
the COM21:0 bits for defining the Output Compare (OC2) state at the next compare match. Also,
the COM21:0 bits control the OC2 pin output source. Figure 56 shows a simplified schematic of
the logic affected by the COM21:0 bit setting. The I/O Registers, I/O bits, and I/O pins in the fig-
ure are shown in bold. Only the parts of the general I/O Port Control Registers (DDR and PORT)
that are affected by the COM21:0 bits are shown. When referring to the OC2 state, the reference
is for the internal OC2 Register, not the OC2 pin.
Figure 56. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OC2) from the waveform
generator if either of the COM21:0 bits are set. However, the OC2 pin direction (input or output)
is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction Regis-
ter bit for the OC2 pin (DDR_OC2) must be set as output before the OC2 value is visible on the
pin. The port override function is independent of the Waveform Generation mode.
The design of the output compare pin logic allows initialization of the OC2 state before the out-
put is enabled. Note that some COM21:0 bit settings are reserved for certain modes of
operation. See “8-bit Timer/Counter Register Description” on page 128.
Compare Output Mode
and Waveform
Generation
The waveform generator uses the COM21:0 bits differently in Normal, CTC, and PWM modes.
For all modes, setting the COM21:0 = 0 tells the Waveform Generator that no action on the OC2
Register is to be performed on the next compare match. For compare output actions in the non-
PWM modes refer to Table 51 on page 129. For fast PWM mode, refer to Table 52 on page 129,
and for phase correct PWM refer to Table 53 on page 129.
A change of the COM21:0 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
FOC2 strobe bits.
PORT
DDR
DQ
DQ
OCn
Pin
OCn
DQ
Waveform
Generator
COMn1
COMn0
0
1
DATA BU S
FOCn
clk
I/O