Datasheet

138
ATmega325/3250/645/6450
2570K–AVR–04/07
8-bit Timer/Counter
Register Description
TCCR2A – Timer/Counter
Control Register A
• 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 TCCR2A
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, 3 – WGM21:0: Waveform Generation Mode
These bits control the counting sequence of the counter, the source for the maximum
(TOP) counter value, and what type of waveform generation to be used. Modes of oper-
ation supported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare
match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes. See Table
59 and “Modes of Operation” on page 132.
Note: 1. The CTC2 and PWM2 bit definition names are now obsolete. Use the WGM21:0 def-
initions. However, the functionality and location of these bits are compatible with
previous versions of the timer.
Bit 7 6 5 4 3 2 1 0
(0xB0) FOC2A WGM20 COM2A1 COM2A0 WGM21 CS22 CS21 CS20 TCCR2A
Read/Write W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 59. Waveform Generation Mode Bit Description
(1)
Mode
WGM21
(CTC2)
WGM20
(PWM2)
Timer/Counter Mode
of Operation TOP
Update of
OCR2A at
TOV2 Flag
Set on
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR2A Immediate MAX
3 1 1 Fast PWM 0xFF BOTTOM MAX