Note:
1. A special case occurs when OCR0B equals TOP and COM0B1 is set. In this case, the Com-
pare Match is ignored, but the set or clear is done at TOP. See “Phase Correct PWM Mode” on
page 109 for more details.
• Bits 3, 2 – Res: Reserved Bits
These bits are reserved bits in the AT90USB64/128 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 wave-
form generation to be used, see Table 13-7. Modes of operation supported 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 106).
Table 13-7. Waveform Generation Mode Bit Description
Timer/Counter
Mode of
Update of
OCRx at
TOV Flag
Mode WGM2 WGM1 WGM0 Operation
TOP
Set on(1)(2)
0
1
0
0
0
0
0
1
Normal
0xFF
Immediate
TOP
MAX
PWM, Phase
Correct
0xFF
BOTTOM
2
3
4
0
0
1
1
1
0
0
1
0
CTC
OCRA Immediate
MAX
MAX
–
Fast PWM
Reserved
0xFF
–
TOP
–
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
13.8.2
Timer/Counter Control Register B – TCCR0B
Bit
7
6
5
–
R
0
4
–
R
0
3
2
1
0
FOC0A
FOC0B
WGM02
R/W
0
CS02
R/W
0
CS01
R/W
0
CS00
R/W
0
TCCR0B
Read/Write
Initial Value
W
0
W
0
• Bit 7 – FOC0A: Force Output Compare A
The FOC0A 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
TCCR0B is written when operating in PWM mode. When writing a logical one to the FOC0A bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC0A output is
changed according to its COM0A1:0 bits setting. Note that the FOC0A bit is implemented as a
strobe. Therefore it is the value present in the COM0A1:0 bits that determines the effect of the
forced compare.
114
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
