PIC18F4450T-I/ML【PDF 67/241 页】IC PIC MCU FLASH 8KX16 44QFN

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159

XMEGA A [MANUAL]

8077I–AVR–11/2012

configuration for the pin must be set to generate an event on both edges. Figure 14-13 on page 159 shows and example

where the pulse width is measured twice for an external signal.

Figure 14-13.Pulse width capture of an external signal.

14.7.4 32-bit Input Capture

Two timer/counters can be used together to enable true 32-bit input capture. In a typical 32-bit input capture setup, the

overflow event of the least-significant timer is connected via the event system and used as the clock input for the most-

significant timer.

The most-significant timer will be updated one peripheral clock period after an overflow occurs for the least-significant

timer. To compensate for this, the capture event for the most-significant timer must be equally delayed by setting the

event delay bit for this timer.

14.7.5 Capture Overflow

The timer/counter can detect buffer overflow of the input capture channels. When both the buffer valid flag and the

capture interrupt flag are set and a new capture event is detected, there is nowhere to store the new timestamp. If a

buffer overflow is detected, the new value is rejected, the error interrupt flag is set, and the optional interrupt is

generated.

14.8

Compare Channel

Each compare channel continuously compares the counter value (CNT) with the CCx register. If CNT equals CCx, the

comparator signals a match. The match will set the CC channel's interrupt flag at the next timer clock cycle, and the

event and optional interrupt are generated.

The compare buffer register provides double buffer capability equivalent to that for the period buffer. The double

buffering synchronizes the update of the CCx register with the buffer value to either the TOP or BOTTOM of the counting

sequence according to the UPDATE condition. The synchronization prevents the occurrence of odd-length, non-

symmetrical pulses for glitch-free output.

14.8.1 Waveform Generation

The compare channels can be used for waveform generation on the corresponding port pins. To make the waveform

visible on the connected port pin, the following requirements must be fulfilled:

A waveform generation mode must be selected.

Event actions must be disabled.

The CC channels used must be enabled. This will override the corresponding port pin output register.

Pulsewitdh (tp)

external signal

events

CNT

MAX

BOTTOM

"capture"

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