Datasheet AT90S8515 (Atmel) - 37

AT90S8515
Timer/Counter1 Control
Register B – TCCR1B Bit 7 6 5 4 3 2 1 0 $2E ($4E) ICNC1 ICES1 – – CTC1 CS12 CS11 CS10 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 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, 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 ICR1 on
the rising edge of the ICP.
• Bits 5, 4 – Res: Reserved Bits
These bits are reserved bits in the AT90S8515 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 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 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 | .
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 10. Clock 1 Prescale Select
CS12 CS11 CS10 Description 0 0 0 Stop, the Timer/Counter1 is stopped. 0 0 1 CK 0 1 0 CK/8 0 1 1 CK/64 1 0 0 CK/256 1 0 1 CK/1024 1 1 0 External Pin T1, falling edge 1 1 1 External Pin T1, rising edge 37
0841G–09/01