Datasheet

93
ATmega8A [DATASHEET]
8159E–AVR–02/2013
pin it is connected to. If one or both of the COM1B1:0 bit are written to one, the OC1B output overrides the normal
port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corre-
sponding to the OC1A or OC1B pin must be set in order to enable the output driver.
When the OC1A or OC1B is connected to the pin, the function of the COM1x1:0 bits is dependent of the WGM13:0
bits setting. Table 17-2 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to a normal or a CTC
mode (non-PWM).
Table 17-3 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the fast PWM mode.
Note: 1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. In this case the Compare
Match is ignored, but the set or clear is done at BOTTOM. See “Fast PWM Mode” on page 85. for more details.
Table 17-4 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase correct or the phase
and frequency correct, PWM mode.
Note: 1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See “Phase Correct PWM
Mode” on page 87. for more details.
Table 17-2. Compare Output Mode, Non-PWM
COM1A1/
COM1B1
COM1A0/
COM1B0 Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 Toggle OC1A/OC1B on Compare Match
1 0 Clear OC1A/OC1B on Compare Match (Set output to low level)
1 1 Set OC1A/OC1B on Compare Match (Set output to high level)
Table 17-3. Compare Output Mode, Fast PWM
(1)
COM1A1/
COM1B1
COM1A0/
COM1B0 Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM13:0 = 15: Toggle OC1A on Compare Match, OC1B disconnected
(normal port operation). For all other WGM1 settings, normal port
operation, OC1A/OC1B disconnected.
1 0 Clear OC1A/OC1B on Compare Match, set OC1A/OC1B at BOTTOM,
(non-inverting mode)
1 1 Set OC1A/OC1B on Compare Match, clear OC1A/OC1B at BOTTOM,
(inverting mode)
Table 17-4. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM
(1)
COM1A1/
COM1B1
COM1A0/
COM1B0 Description
0 0 Normal port operation, OC1A/OC1B disconnected.
0 1 WGM13:0 = 9 or 14: Toggle OC1A on Compare Match, OC1B
disconnected (normal port operation). For all other WGM1 settings,
normal port operation, OC1A/OC1B disconnected.
1 0 Clear OC1A/OC1B on Compare Match when up-counting. Set
OC1A/OC1B on Compare Match when downcounting.
1 1 Set OC1A/OC1B on Compare Match when up-counting. Clear
OC1A/OC1B on Compare Match when downcounting.