PIC17C752 datasheet(101/320 Pages) MICROCHIP

101 / 320 page

Preliminary

DS30264A-page 101

PIC17C75X

This mode (3 Capture, 1 Period) is selected if control bit

CA1/PR3 is clear. In this mode, the Capture1 register,

consisting of high byte (PR3H/CA1H) and low byte

(PR3L/CA1L), is conﬁgured as the period control regis-

ter for TMR3. Capture1 is disabled in this mode, and

the corresponding Interrupt bit CA1IF is never set.

TMR3 increments until it equals the value in the period

clock.

All other Captures are active in this mode.

13.2.1.1

CAPTURE OPERATION

The CAxED1 and CAxED0 bits determine the event on

which capture will occur. The possible events are:

• Capture on every falling edge

• Capture on every rising edge

• Capture every 4th rising edge

• Capture every 16th rising edge

When a capture takes place, an interrupt ﬂag is latched

into the CAxIF bit. This interrupt can be enabled by set-

ting the corresponding mask bit CAxIE. The Peripheral

Interrupt Enable bit (PEIE) must be set and the Global

Interrupt Disable bit (GLINTD) must be cleared for the

interrupt to be acknowledged. The CAxIF interrupt ﬂag

bit is cleared in software.

When the capture prescale select is changed, the pres-

caler is not reset and an event may be generated.

Therefore, the ﬁrst capture after such a change will be

ambiguous. However, it sets the time-base for the next

capture. The prescaler is reset upon chip reset.

The capture pin, CAPx, is a multiplexed pin. When

used as a port pin, the capture is not disabled. How-

ever, the user can simply disable the Capture interrupt

by clearing CAxIE. If the CAPx pin is used as an output

pin, the user can activate a capture by writing to the

port pin. This may be useful during development phase

to emulate a capture interrupt.

The input on the capture pin CAPx is synchronized

internally to internal phase clocks. This imposes certain

restrictions on the input waveform (see the Electrical

Speciﬁcation section for timing).

The capture overﬂow status ﬂag bit is double buffered.

The master bit is set if one captured word is already

residing in the Capture register (CAxH:CAxL) and

another “event” has occurred on the CAPx pin. The

new event will not transfer the TMR3 value to the

capture register, protecting the previous unread

capture value. When the user reads both the high and

the low bytes (in any order) of the Capture register, the

master overﬂow bit is transferred to the slave overﬂow

bit (CAxOVF) and then the master bit is reset. The user

can then read TCONx to determine the value of

CAxOVF.

The recommended sequence to read capture registers

and

capture

overﬂow

ﬂag

bits

shown

Example 13-1.

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