
102
ATmega64A [DATASHEET]
8160D–AVR–02/2013
15.11 Register Description
15.11.1
TCCR0 – Timer/Counter Control Register
Bit 7 – FOC0: Force Output Compare
The FOC0 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 TCCR0 is written when operating in PWM mode. When writing a
logical one to the FOC0 bit, an immediate Compare Match is forced on the waveform generation unit. The OC0
output is changed according to its COM01:0 bits setting. Note that the FOC0 bit is implemented as a strobe. There-
fore it is the value present in the COM01:0 bits that determines the effect of the forced compare.
A FOC0 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR0 as TOP.
The FOC0 bit is always read as zero.
Bit 6, 3 – WGM01:0: 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 operation 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
Note:
1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions. However, the function-
ality and location of these bits are compatible with previous versions of the timer.
Bit 5:4 – COM01:0: Compare Match Output Mode
These bits control the Output Compare pin (OC0) behavior. If one or both of the COM01:0 bits are set, the OC0
output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction
Register (DDR) bit corresponding to OC0 pin must be set in order to enable the output driver.
When OC0 is connected to the pin, the function of the COM01:0 bits depends on the WGM01:0 bit setting.
Table15-3 shows the COM01:0 bit functionality when the WGM01:0 bits are set to a Normal or CTC mode (non-PWM).
Bit
7
6
5
4
3
210
FOC0
WGM00
COM01
COM00
WGM01
CS02
CS01
CS00
TCCR0
Read/Write
W
R/W
Initial Value
0
Table 15-2.
Waveform Generation Mode Bit Descript
ionMode
WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter Mode of
Operation
TOP
Update of
OCR0 at
TOV0 Flag
Set on
0
Normal
0xFF
Immediate
MAX
1
0
1
PWM, Phase Correct
0xFF
TOP
BOTTOM
2
1
0
CTC
OCR0
Immediate
MAX
3
1
Fast PWM
0xFF
BOTTOM
MAX
Table 15-3.
Compare Output Mode, non-PWM Mode
COM01
COM00
Description
0
Normal port operation, OC0 disconnected.
0
1
Toggle OC0 on Compare Match.
1
0
Clear OC0 on Compare Match.
1
Set OC0 on Compare Match.