Rainbow Electronics ATtiny43U - Manual (Page 83)

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8048B–AVR–03/09
generation. Similarly, do not write the TCNTn value equal to BOTTOM when the counter is
down-counting.
The setup of the OCnx should be performed before setting the Data Direction Register for the
port pin to output. The easiest way of setting the OCnx value is to use the Force Output Com-
pare (nx) strobe bits in Normal mode. The OCnx Registers keep their values even when
changing between Waveform Generation modes.
Be aware that the COMnx1:0 bits are not double buffered together with the compare value.
Changing the COMnx1:0 bits will take effect immediately.
12.6 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.
Also, the COMnx1:0 bits control the OCnx pin output source.
Figure 12-4 on page 83 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 12-4. Compare Match Output Unit, Schematic (non-PWM Mode)
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 independent of the Waveform Generation mode.
PORT
DDR
D Q
D Q
OCn
Pin OCnx
D Q
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCn
clk
I/O