Rainbow Electronics ATmega64M1 - Manual (Page 114)

114
8209A–AVR–08/09
ATmega16M1/32M1/64M1
16.8 Compare Match Output Unit
The Compare Output mode (COMnx1:0) bits have two functions. The Waveform Generator uses
the COMnx1:0 bits for defining the Output Compare (OCnx) state at the next compare match.
Secondly the COMnx1:0 bits control the OCnx pin output source.
Figure 16-5 shows a simplified
schematic of the logic affected by the COMnx1:0 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 COMnx1:0 bits are shown. When referring to the
OCnx state, the reference is for the internal OCnx Register, not the OCnx pin. If a system reset
occur, the OCnx Register is reset to “0”.
Figure 16-5. Compare Match Output Unit, Schematic
The general I/O port function is overridden by the Output Compare (OCnx) from the Waveform
Generator if either of the COMnx1:0 bits are set. However, the OCnx pin direction (input or out-
put) is still controlled by the Data Direction Register (DDR) for the port pin. The Data Direction
Register bit for the OCnx pin (DDR_OCnx) must be set as output before the OCnx value is visi-
ble on the pin. The port override function is generally independent of the Waveform Generation
mode, but there are some exceptions. Refer to
Table 16-1 , Table 16-2 and Table 16-3 for
details.
The design of the Output Compare pin logic allows initialization of the OCnx state before the out-
put is enabled. Note that some COMnx1:0 bit settings are reserved for certain modes of
operation.
See “Register Description” on page 124.
The COMnx1:0 bits have no effect on the Input Capture unit.
16.8.1 Compare Output Mode and Waveform Generation
The Waveform Generator uses the COMnx1:0 bits differently in normal, CTC, and PWM modes.
For all modes, setting the COMnx1:0 = 0 tells the Waveform Generator that no action on the
OCnx Register is to be performed on the next compare match. For compare output actions in the
PORT
DDR
D Q
D Q
OCnx
Pin OCnx
D Q
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCnx
clk
I/O