Rainbow Electronics AT90LS4433 - Manual (Page 38)

38
AT90S/LS4433
1042G–AVR–09/02
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 the 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 Cap-
ture 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 AT90S4433 and always read as 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 Compare Match. If the CTC1 control bit is cleared, Timer/Counter1 contin-
ues counting and is unaffected by a Compare Match. Since the compare 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 Compare 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.
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