95
ATmega8A [DATASHEET]
8159E–AVR–02/2013
17.11.2
TCCR1B – Timer/Counter 1 Control Register B
Bit 7 – ICNC1: Input Capture Noise Canceler
Setting this bit (to one) activates the Input Capture Noise Canceler. When the noise canceler is activated, the input
from the Input Capture Pin (ICP1) is filtered. The filter function requires four successive equal valued samples of
the ICP1 pin for changing its output. The Input Capture is therefore delayed by four Oscillator cycles when the
noise canceler is enabled.
Bit 6 – ICES1: Input Capture Edge Select
This bit selects which edge on the Input Capture Pin (ICP1) that is used to trigger a capture event. When the
ICES1 bit is written to zero, a falling (negative) edge is used as trigger, and when the ICES1 bit is written to one, a
rising (positive) edge will trigger the capture.
When a capture is triggered according to the ICES1 setting, the counter value is copied into the Input Capture Reg-
ister (ICR1). The event will also set the Input Capture Flag (ICF1), and this can be used to cause an Input Capture
Interrupt, if this interrupt is enabled.
When the ICR1 is used as TOP value (see description of the WGM13:0 bits located in the TCCR1A and the
TCCR1B Register), the ICP1 is disconnected and consequently the Input Capture function is disabled.
Bit 5 – Reserved Bit
This bit is reserved for future use. For ensuring compatibility with future devices, this bit must be written to zero
when TCCR1B is written.
Bit 4:3 – WGM13:2: Waveform Generation Mode
See TCCR1A Register description.
Bit 2:0 – CS12:0: Clock Select
The three clock select bits select the clock source to be used by the Timer/Counter, see
Figure 17-10 and
FigureIf external pin modes are used for the Timer/Counter1, transitions on the T1 pin will clock the counter even if the
pin is configured as an output. This feature allows software control of the counting.
Bit
7654
3210
ICNC1
ICES1
–
WGM13
WGM12
CS12
CS11
CS10
TCCR1B
Read/Write
R/W
R
R/W
Initial Value
0000
Table 17-6.
Clock Select Bit Description
CS12
CS11
CS10
Description
0
No clock source. (Timer/Counter stopped)
00
1
clk
I/O/1 (No prescaling)
01
0
clkI/O/8 (From prescaler)
01
1
clk
I/O/64 (From prescaler)
10
0
clk
I/O/256 (From prescaler)
10
1
clkI/O/1024 (From prescaler)
1
0
External clock source on T1 pin. Clock on falling edge.
1
External clock source on T1 pin. Clock on rising edge.