AT90S/LS4434 and AT90S/LS8535
35
Timer/Counter1 Control Register B – TCCR1B
•
Bit 7 – ICNC1: Input Capture1 Noise Canceler (4 CKs)
When the ICNC1 bit is cleared (zero), the Input Capture Trigger Noise Canceler function is disabled. The input capture is
triggered at the first rising/falling edge sampled on the ICP (input capture pin) as specified. When the ICNC1 bit is set (one),
four successive samples are measured on the ICP (input capture pin), and all samples must be high/low according to the
input capture trigger specification in the ICES1 bit. The actual sampling frequency is XTAL clock frequency.
•
Bit 6 – ICES1: Input Capture1 Edge Select
While the ICES1 bit is cleared (zero), the Timer/Counter1 contents are transferred to the Input Capture Register (ICR1) on
the falling edge of the input capture pin (ICP). While the ICES1 bit is set (one), the Timer/Counter1 contents are transferred
to the Input Capture Register (ICR1) on the rising edge of the input capture pin (ICP).
•
Bits 5, 4 – Res: Reserved Bits
These bits are reserved bits in the AT90S4434/8535 and always read zero.
•
Bit 3 – CTC1: Clear Timer/Counter1 on Compare Match
When the CTC1 control bit is set (one), the Timer/Counter1 is reset to $0000 in the clock cycle after a compareA match. If
the CTC1 control bit is cleared, Timer/Counter1 continues counting and is unaffected by a compare match. Since the com-
pare match is detected in the CPU clock cycle following the match, this function will behave differently when a prescaling
higher than 1 is used for the timer. When a prescaling of 1 is used and the compareA register is set to C, the timer will count
as follows if CTC1 is set:
... | C-2 | C-1 | C | 0 | 1 |...
When the prescaler is set to divide by 8, the timer will count like this:
... | C-2, C-2, C-2, C-2, C-2, C-2, C-2, C-2 | C-1, C-1, C-1, C-1, C-1, C-1, C-1, C-1 | C, 0, 0, 0, 0, 0, 0, 0 |1,1,1,1,1,1,1,1|...
In PWM mode, this bit has no effect.
•
Bits 2, 1, 0 – CS12, CS11, CS10: Clock Select1, Bits 2, 1 and 0
The Clock Select1 bits 2, 1 and 0 define the prescaling source of Timer/Counter1.
Table 13.
PWM Mode Select
PWM11
PWM10
Description
0
0
PWM operation of Timer/Counter1 is disabled
0
1
Timer/Counter1 is an 8-bit PWM
1
0
Timer/Counter1 is a 9-bit PWM
1
1
Timer/Counter1 is a 10-bit PWM
Bit
7
6
5
4
3
2
1
0
$2E ($4E)
ICNC1
ICES1
–
–
CTC1
CS12
CS11
CS10
TCCR1B
Read/Write
R/W
R/W
R
R
R/W
R/W
R/W
R/W
Initial value
0
0
0
0
0
0
0
0