Table 15-7. Waveform Generation Mode Bit Description
Timer/Counter
Mode of
Mode WGM2 WGM1 WGM0 Operation
Update of
OCRx at
TOV Flag
TOP
Set on(1)(2)
3
4
0
1
1
0
1
0
Fast PWM
Reserved
0xFF
–
TOP
–
MAX
–
PWM, Phase
Correct
5
1
0
1
OCRA
TOP
BOTTOM
6
7
1
1
1
1
0
1
Reserved
Fast PWM
–
–
–
OCRA
TOP
TOP
Notes: 1. MAX= 0xFF
2. BOTTOM= 0x00
15.8.2
Timer/Counter Control Register B – TCCR2B
Bit
7
6
5
–
R
0
4
–
R
0
3
2
1
0
FOC2A
FOC2B
WGM22
R/W
0
CS22
R/W
0
CS21
R/W
0
CS20
TCCR2B
Read/Write
Initial Value
W
0
W
0
R/W
0
• Bit 7 – FOC2A: Force Output Compare A
The FOC2A bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR2B is written when operating in PWM mode. When writing a logical one to the FOC2A bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC2A output is
changed according to its COM2A1:0 bits setting. Note that the FOC2A bit is implemented as a
strobe. Therefore it is the value present in the COM2A1:0 bits that determines the effect of the
forced compare.
A FOC2A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR2A as TOP.
The FOC2A bit is always read as zero.
• Bit 6 – FOC2B: Force Output Compare B
The FOC2B bit is only active when the WGM bits specify a non-PWM mode.
However, for ensuring compatibility with future devices, this bit must be set to zero when
TCCR2B is written when operating in PWM mode. When writing a logical one to the FOC2B bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC2B output is
changed according to its COM2B1:0 bits setting. Note that the FOC2B bit is implemented as a
strobe. Therefore it is the value present in the COM2B1:0 bits that determines the effect of the
forced compare.
A FOC2B strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR2B as TOP.
The FOC2B bit is always read as zero.
• Bits 5:4 – Res: Reserved Bits
These bits are reserved bits in the AT90USB64/128 and will always read as zero.
164
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
