ATmega8(L)
Table 38 shows the COM1x1:0 bit functionality when the WGM13:0 bits are set to the phase cor-
rect or the phase and frequency correct, PWM mode.
Table 38. Compare Output Mode, Phase Correct and Phase and Frequency Correct PWM(1)
COM1A1/
COM1B1
COM1A0/
COM1B0
Description
0
0
0
1
Normal port operation, OC1A/OC1B disconnected.
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
1
0
1
Clear OC1A/OC1B on Compare Match when up-counting. Set
OC1A/OC1B on Compare Match when downcounting.
Set OC1A/OC1B on Compare Match when up-counting. Clear
OC1A/OC1B on Compare Match when downcounting.
Note:
1. A special case occurs when OCR1A/OCR1B equals TOP and COM1A1/COM1B1 is set. See
“Phase Correct PWM Mode” on page 90 for more details
• Bit 3 – FOC1A: Force Output Compare for channel A
• Bit 2 – FOC1B: Force Output Compare for channel B
The FOC1A/FOC1B bits are only active when the WGM13:0 bits specifies a non-PWM mode.
However, for ensuring compatibility with future devices, these bits must be set to zero when
TCCR1A is written when operating in a PWM mode. When writing a logical one to the
FOC1A/FOC1B bit, an immediate Compare Match is forced on the waveform generation unit.
The OC1A/OC1B output is changed according to its COM1x1:0 bits setting. Note that the
FOC1A/FOC1B bits are implemented as strobes. Therefore it is the value present in the
COM1x1:0 bits that determine the effect of the forced compare.
A FOC1A/FOC1B strobe will not generate any interrupt nor will it clear the timer in Clear Timer
on Compare Match (CTC) mode using OCR1A as TOP.
The FOC1A/FOC1B bits are always read as zero.
• Bit 1:0 – WGM11:0: Waveform Generation Mode
Combined with the WGM13:2 bits found in the TCCR1B Register, these bits control the counting
sequence of the counter, the source for maximum (TOP) counter value, and what type of wave-
form generation to be used, see Table 39. Modes of operation supported by the Timer/Counter
unit are: Normal mode (counter), Clear Timer on Compare Match (CTC) mode, and three types
of Pulse Width Modulation (PWM) modes (see “Modes of Operation” on page 87).
Table 39. Waveform Generation Mode Bit Description
WGM12
(CTC1)
WGM11
WGM10
Timer/Counter Mode of
Update of
OCR1x
TOV1 Flag
Set on
Mode WGM13
(PWM11) (PWM10) Operation(1)
TOP
0
1
2
3
4
5
6
0
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
Normal
0xFFFF Immediate
MAX
PWM, Phase Correct, 8-bit
PWM, Phase Correct, 9-bit
PWM, Phase Correct, 10-bit
CTC
0x00FF
0x01FF
0x03FF
TOP
TOP
TOP
BOTTOM
BOTTOM
BOTTOM
MAX
OCR1A Immediate
Fast PWM, 8-bit
0x00FF
0x01FF
BOTTOM
BOTTOM
TOP
Fast PWM, 9-bit
TOP
97
2486AA–AVR–02/2013
ATMEL [ ATMEL ]
