Datasheet

131
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
15.8.2 Timer/Counter Control Register B – TCCR2B
• 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 Atmel
®
ATmega48/88/168 and will always read as zero.
• Bit 3 – WGM22: Waveform Generation Mode
See the description in the Section 15.8.1 “Timer/Counter Control Register A – TCCR2A” on page 129.
• Bit 2:0 – CS22:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see Table 15-9.
Table 15-8. Waveform Generation Mode Bit Description
Mode WGM2 WGM1 WGM0
Timer/Counter Mode of
Operation TOP
Update of
OCRx at
TOV Flag
Set on
(1)(2)
0 0 0 0 Normal 0xFF Immediate MAX
1 0 0 1 PWM, phase correct 0xFF TOP BOTTOM
2 0 1 0 CTC OCRA Immediate MAX
3 0 1 1 Fast PWM 0xFF TOP MAX
4 1 0 0 Reserved – – –
5 1 0 1 PWM, phase correct OCRA TOP BOTTOM
6 1 1 0 Reserved – – –
7 1 1 1 Fast PWM OCRA TOP TOP
Notes: 1. MAX = 0xFF
2. BOTTOM= 0x00
Bit 7 6 5 4 3 210
FOC2A FOC2B – – WGM22 CS22 CS21 CS20 TCCR2B
Read/Write W W R R R R R/W R/W
Initial Value 0 0 0 0 0 0 0 0