136
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
When the ICRn is used as TOP value (see description of the WGMn3:0 bits located in the
TCCRnA and the TCCRnB Register), the ICPn is disconnected and consequently the Input Cap-
ture function is disabled.
Bit 5 – Reserved
This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be
written to zero when TCCRnB is written.
Bit 4:3 – WGMn3:2: Waveform Generation Mode
Bit 2:0 – CSn2:0: Clock Select
The three Clock Select bits select the clock source to be used by the Timer/Counter, see
FigureIf external pin modes are used for the Timer/Countern, transitions on the Tn pin will clock the
counter even if the pin is configured as an output. This feature allows software control of the
counting.
16.12.3
TCCRnC – Timer/Counter n Control Register C
Bit 7 – FOCnA: Force Output Compare for Channel A
Bit 6 – FOCnB: Force Output Compare for Channel B
The FOCnA/FOCnB bits are only active when the WGMn3:0 bits specifies a non-PWM mode.
However, for ensuring compatibility with future devices, these bits must be set to zero when
TCCRnA is written when operating in a PWM mode. When writing a logical one to the
FOCnA/FOCnB bit, an immediate compare match is forced on the Waveform Generation unit.
The OCnA/OCnB output is changed according to its COMnx1:0 bits setting. Note that the
FOCnA/FOCnB bits are implemented as strobes. Therefore it is the value present in the
COMnx1:0 bits that determine the effect of the forced compare.
Table 16-6.
Clock Select bit description.
CSn2
CSn1
CSn0
Description
0
No clock source (Timer/Counter stopped).
001
clkI/O/1 (No prescaling)
010
clk
I/O/8 (From prescaler)
011
clkI/O/64 (From prescaler)
100
clkI/O/256 (From prescaler)
101
clk
I/O/1024 (From prescaler)
1
0
External clock source on Tn pin. Clock on falling edge.
1
External clock source on Tn pin. Clock on rising edge.
Bit
765
4
3
2
1
0
FOCnA
FOCnB
–
TCCRnC
Read/Write
R/W
R
Initial Value000
0