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222 Rabbit 4000 Microprocessor User’s Manual
21.2 Dependencies
21.2.1 I/O Pins
Each input-capture channel can accept input from one of the following parallel port pins:
PC1, PC3, PC5, PC7, PD1, PD3, PD5, PD7, PE1, PE3, PE5, PE7. Use ICTxR to select
which input pin to trigger on.
Note that these pins can be used for other peripherals at the same time as the input-capture
peripheral. For example, you can use input capture to use measure the pulse width on a
serial port input to measure the baud rate.
21.2.2 Clocks
The 16-bit input-capture counters are clocked from the output of Timer A8, and can run at
rates from perclk/2 down to perclk/512 by writing the appropriate value to TAT8R.
21.2.3 Other Registers
21.2.4 Interrupts
Each input capture channel can generate an interrupt whenever a start/stop condition
occurs, or when the counter rolls over to zero. The interrupt request is cleared when
ICCSR is read.
The input capture interrupt vector is in the IIR at offset 0x1A0. It can be set as Priority 1,
2, or 3.
The input-capture channels synchronize their inputs to the peripheral clock (further
divided by Timer A8). Since the inputs are only sampled in synch with the peripheral
clock, any faster state faster changes cannot be detected, which is akin to a digital low-
pass filter functionality on the inputs. Because of this, there is some delay between the
input transition and when an interrupt is requested, as shown below. The status bits in
ICSxR are set coincident with the interrupt request and are reset when read from the
ICSxR.
Register Function
TAT8R Time constant for input-capture clock.
PERI CLOCK
TIMER A8
CPT INPUT
INTERRUPT