Rainbow Electronics ATmega162V - Manual (Page 92)

92
ATmega162/V
2513E–AVR–09/03
Using the Output Compare
Unit Since writing TCNT0 in any mode of operation will block all compare matches for one
timer clock cycle, there are risks involved when changing TCNT0 when using the output
compare channel, independently of whether the Timer/Counter is running or not. If the
value written to TCNT0 equals the OCR0 value, the Compare Match will be missed,
resulting in incorrect waveform generation. Similarly, do not write the TCNT0 value
equal to BOTTOM when the counter is down-counting.
The setup of the OC0 should be performed before setting the Data Direction Register for
the port pin to output. The easiest way of setting the OC0 value is to use the Force Out-
put Compare (FOC0) strobe bits in Normal mode. The OC0 Register keeps its value
even when changing between Waveform Generation modes.
Be aware that the COM01:0 bits are not double buffered together with the compare
value. Changing the COM01:0 bits will take effect immediately.
Compare Match Output
Unit The Compare Output mode (COM01:0) bits have two functions. The Waveform Genera-
tor uses the COM01:0 bits for defining the Output Compare (OC0) state at the next
Compare Match. Also, the COM01:0 bits control the OC0 pin output source. Figure 36
shows a simplified schematic of the logic affected by the COM01:0 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 COM01:0
bits are shown. When referring to the OC0 state, the reference is for the internal OC0
Register, not the OC0 pin. If a System Reset occur, the OC0 Register is reset to “0”.
Figure 36. Compare Match Output Unit, Schematics
The general I/O port function is overridden by the Output Compare (OC0) from the
waveform generator if either of the COM01:0 bits are set. However, the OC0 pin direc-
tion (input or output) is still controlled by the Data Direction Register (DDR) for the port
pin. The Data Direction Register bit for the OC0 pin (DDR_OC0) must be set as output
before the OC0 value is visible on the pin. The port override function is independent of
the Waveform Generation mode.
PORT
DDR
D Q
D Q
OCn
Pin OCn
D Q
Waveform
Generator
COMn1
COMn0
0
1
D
ATA
B
U
S
FOCn
clk
I/O