Datasheet

102

7701F–AVR–10/12

Atmel ATtiny24/44/84 [Preliminary]

Be aware that the COM1x1:0 bits are not double buffered together with the compare value.

Changing the COM1x1:0 bits will take effect immediately.

14.8 Compare Match Output Unit

The Compare Output mode (COM1x1:0) bits have two functions. The Waveform Generator

uses the COM1x1:0 bits for defining the Output Compare (OC1x) state at the next compare

match. Secondly the COM1x1:0 bits control the OC1x pin output source. Figure 14-5 on page

102 shows a simplified schematic of the logic affected by the COM1x1:0 bit settings. 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 COM1x1:0 bits are shown.

When referring to the OC1x state, the reference is for the internal OC1x register, not the OC1x

pin. If a system reset occurs, the OC1x register is reset to "0".

Figure 14-5. Compare Match Output Unit, Schematic

The general I/O port function is overridden by the Output Compare (OC1x) from the Waveform

Generator if either of the COM1x1:0 bits are set. However, the OC1x pin direction (input or

output) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direc-

tion Register bit for the OC1x pin (DDR_OC1x) must be set as output before the OC1x value is

visible on the pin. The port override function is generally independent of the Waveform Gener-

ation mode, but there are some exceptions. See Table 14-1 on page 113, Table 14-2 on page

113 and Table 14-3 on page 114 for details.

The design of the Output Compare pin logic allows initialization of the OC1x state before the

output is enabled. Note that some COM1x1:0 bit settings are reserved for certain modes of

operation. See “Register Description” on page 113

The COM1x1:0 bits have no effect on the Input Capture unit.

PORT

DDR

OCnx

Waveform

Generator

COMnx1

COMnx0

DATA BUS

FOCnx

clk

I/O