Figure 44. Timer/Counter Timing Diagram, Clear Timer on Compare Match Mode, with Pres-
caler (fclk_I/O/8)
clkI/O
clkTn
(clkI/O/8)
TCNTn
(CTC)
TOP - 1
TOP
BOTTOM
BOTTOM + 1
OCRn
TOP
OCFn
8-bit
Timer/Counter
Register
Description
Timer/Counter Control
Register – TCCR0
Bit
7
FOC0
W
6
5
4
COM00
R/W
0
3
2
CS02
R/W
0
1
CS01
R/W
0
0
CS00
R/W
0
WGM00
R/W
0
COM01
R/W
0
WGM01
R/W
0
TCCR0
Read/Write
Initial Value
0
• Bit 7 – FOC0: Force Output Compare
The FOC0 bit is only active when the WGM bits specify a non-PWM mode. However, for ensur-
ing 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 Compare match (CTC) mode, and
two types of Pulse Width Modulation (PWM) modes. See Table 52 and “Modes of Operation” on
page 98.
104
ATmega128(L)
2467P–AVR–08/07
ATMEL [ ATMEL ]
