ATmega48/88/168
Table 49 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to fast
PWM mode.
Table 49. Compare Output Mode, Fast PWM Mode(1)
COM0B1
COM0B0
Description
0
0
1
1
0
1
0
1
Normal port operation, OC0B disconnected.
Reserved
Clear OC0B on Compare Match, set OC0B at TOP
Set OC0B on Compare Match, clear OC0B at TOP
Note:
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Fast PWM
Mode” on page 91 for more details.
Table 50 shows the COM0B1:0 bit functionality when the WGM02:0 bits are set to
phase correct PWM mode.
Table 50. Compare Output Mode, Phase Correct PWM Mode(1)
COM0B1
COM0B0
Description
0
0
1
0
1
0
Normal port operation, OC0B disconnected.
Reserved
Clear OC0B on Compare Match when up-counting. Set OC0B on
Compare Match when down-counting.
1
1
Set OC0B on Compare Match when up-counting. Clear OC0B on
Compare Match when down-counting.
Note:
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case,
the Compare Match is ignored, but the set or clear is done at TOP. See “Phase Cor-
rect PWM Mode” on page 92 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits in the ATmega48/88/168 and will always read as zero.
• Bits 1:0 – WGM01:0: Waveform Generation Mode
Combined with the WGM02 bit found in the TCCR0B Register, these bits control the
counting sequence of the counter, the source for maximum (TOP) counter value, and
what type of waveform generation to be used, see Table 51. Modes of operation sup-
ported by the Timer/Counter unit are: Normal mode (counter), Clear Timer on Compare
Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes (see
“Modes of Operation” on page 90).
Table 51. Waveform Generation Mode Bit Description
Timer/Count
er Mode of
Update of
OCRx at
TOV Flag
Mode WGM02 WGM01 WGM00 Operation
TOP
0xFF
0xFF
Set on(1)(2)
0
1
0
0
0
0
0
1
Normal
Immediate
TOP
MAX
PWM, Phase
Correct
BOTTOM
2
3
0
0
1
1
0
1
CTC
OCRA Immediate
0xFF TOP
MAX
MAX
Fast PWM
97
2545D–AVR–07/04
ATMEL [ ATMEL ]
