8-bit Timer/Counter
Register Description
Timer/Counter Control
Register – TCCR0
Bit
7
FOC0
W
6
WGM00
R/W
0
5
COM01
R/W
0
4
COM00
R/W
0
3
WGM01
R/W
0
2
CS02
R/W
0
1
CS01
R/W
0
0
CS00
R/W
0
TCCR0
Read/Write
Initial Value
0
• Bit 7 – FOC0: Force Output Compare
The FOC0 bit is only active when the WGM00 bit specifies 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. Therefore 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 Com-
pare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes. See
Table 38 and “Modes of Operation” on page 73.
Table 38. Waveform Generation Mode Bit Description(1)
WGM01
(CTC0)
WGM00 Timer/Counter Mode
(PWM0) of Operation
Update of
OCR0
TOV0 Flag
Set-on
Mode
TOP
0xFF
0xFF
0
1
2
3
0
0
1
1
0
1
0
1
Normal
Immediate
TOP
MAX
PWM, Phase Correct
CTC
BOTTOM
MAX
OCR0 Immediate
0xFF BOTTOM
Fast PWM
MAX
Note:
1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 def-
initions. However, the functionality 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 corre-
sponding to the OC0 pin must be set in order to enable the output driver.
80
ATmega32(L)
2503J–AVR–10/06
ATMEL [ ATMEL ]
