Datasheet
82
2586N–AVR–04/11
ATtiny25/45/85
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 73).
Notes: 1. MAX = 0xFF
2. BOTTOM = 0x00
11.9.3 TCCR0B – Timer/Counter Control Register B
• 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 COM0A[1:0] bits setting. Note that the FOC0A bit is implemented as a
strobe. Therefore it is the value present in the COM0A[1:0] 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 COM0B[1:0] bits setting. Note that the FOC0B bit is implemented as a
strobe. Therefore it is the value present in the COM0B[1:0] bits that determines the effect of the
forced compare.
Table 11-5. Waveform Generation Mode Bit Description
Mode
WGM
02
WGM
01
WGM
00
Timer/Counter Mode
of Operation TOP
Update of
OCRx at
TOV Flag
Set on
0 0 0 0 Normal 0xFF Immediate MAX
(1)
1 0 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
(2)
2 0 1 0 CTC OCRA Immediate MAX
(1)
3 0 1 1 Fast PWM 0xFF BOTTOM
(2)
MAX
(1)
4 1 0 0 Reserved – – –
5 1 0 1 PWM, Phase Correct OCRA TOP BOTTOM
(2)
6 1 1 0 Reserved – – –
7 1 1 1 Fast PWM OCRA BOTTOM
(2)
TOP
Bit 7 6 5 4 3 2 1 0
0x33 FOC0A FOC0B – – WGM02 CS02 CS01 CS00 TCCR0B
Read/Write W W R R R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0