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ATmega8A [DATASHEET]
8159E–AVR–02/2013
17.7.2
Compare Match Blocking by TCNT1 Write
All CPU writes to the TCNT1 Register will block any Compare Match that occurs in the next timer clock cycle, even
when the timer is stopped. This feature allows OCR1x to be initialized to the same value as TCNT1 without trigger-
ing an interrupt when the Timer/Counter clock is enabled.
17.7.3
Using the Output Compare Unit
Since writing TCNT1 in any mode of operation will block all compare matches for one timer clock cycle, there are
risks involved when changing TCNT1 when using any of the Output Compare channels, independent of whether
the Timer/Counter is running or not. If the value written to TCNT1 equals the OCR1x value, the Compare Match will
be missed, resulting in incorrect waveform generation. Do not write the TCNT1 equal to TOP in PWM modes with
variable TOP values. The Compare Match for the TOP will be ignored and the counter will continue to 0xFFFF.
Similarly, do not write the TCNT1 value equal to BOTTOM when the counter is downcounting.
The setup of the OC1x should be performed before setting the Data Direction Register for the port pin to output.
The easiest way of setting the OC1x value is to use the Force Output Compare (FOC1x) strobe bits in Normal
mode. The OC1x Register keeps its value even when changing between Waveform Generation modes.
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.
17.8
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 17-5 shows a simplified schematic of the logic affected by the COM1x1:0 bit set-
ting. 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 occur, the OC1x Regis-
ter is reset to “0”.
Figure 17-5. Compare Match Output Unit, Schematic
PORT
DDR
DQ
OCnx
Pin
OCnx
DQ
Waveform
Generator
COMnx1
COMnx0
0
1
D
ATA
B
U
S
FOCnx
clk
I/O