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 14-8. 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 90).
Table 14-8. Waveform Generation Mode Bit Description
Timer/Count
er Mode of
Update of
OCRx at
TOV Flag
Mode WGM02 WGM01 WGM00 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
14.8.2
Timer/Counter Control Register B – TCCR0B
Bit
7
FOC0A
W
6
FOC0B
W
5
–
4
–
3
2
CS02
R
1
0
CS00
R/W
0
WGM02
CS01
R/W
0
TCCR0B
Read/Write
Initial Value
R
0
R
0
R
0
0
0
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.
A FOC0A strobe will not generate any interrupt, nor will it clear the timer in CTC mode using
OCR0A as TOP.
The FOC0A bit is always read as zero.
• Bit 6 – FOC0B: Force Output Compare B
The FOC0B 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 FOC0B bit,
an immediate Compare Match is forced on the Waveform Generation unit. The OC0B output is
changed according to its COM0B1:0 bits setting. Note that the FOC0B bit is implemented as a
98
AT90PWM2/3/2B/3B
4317J–AVR–08/10
ATMEL [ ATMEL ]
