Datasheet

104
2490R–AVR–02/2013
ATmega64(L)
8-bit
Timer/Counter
Register
Description
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 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.
Note: 1. The CTC0 and PWM0 bit definition names are now obsolete. Use the WGM01:0 definitions.
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 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. Table 53 shows the COM01:0 bit functionality when the WGM01:0 bits are set to a
Normal or CTC mode (non-PWM).
Bit 76543210
0x33 (0x53) FOC0 WGM00 COM01 COM00 WGM01 CS02 CS01 CS00 TCCR0
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 52. Waveform Generation Mode Bit Description
(1)
Mode
WGM01
(CTC0)
WGM00
(PWM0)
Timer/Counter Mode of
Operation TOP
Update of
OCR0 at
TOV0 Flag
Set on
0 0 0 Normal 0xFF Immediate MAX
1 0 1 PWM, Phase Correct 0xFF TOP BOTTOM
2 1 0 CTC OCR0 Immediate MAX
31 1Fast PWM 0xFFBOTTOMMAX