PIC18F23K22-ESLASHML datasheet(327/496 Pages) MICROCHIP

327 / 496 page

 2010 Microchip Technology Inc.

Preliminary

DS41412C-page 327

PIC18(L)F2X/4XK22

19.5

Measuring Time with the CTMU

Module

Time can be precisely measured after the ratio (C/I) is

measured from the current and capacitance calibration

step by following these steps:

Initialize the A/D Converter and the CTMU.

Set EDG1STAT.

Set EDG2STAT.

Perform an A/D conversion.

Calculate the time between edges as T = (C/I) * V,

where I is calculated in the current calibration step

(Section 19.3.1 “Current Source Calibration”),

C is calculated in the capacitance calibration step

(Section 19.3.2 “Capacitance Calibration”) and

V is measured by performing the A/D conversion.

It is assumed that the time measured is small enough

that the capacitance, COFFSET, provides a valid voltage

to the A/D Converter. For the smallest time measure-

ment, always set the A/D Channel Select register

(AD1CHS) to an unused A/D channel; the correspond-

ing pin for which is not connected to any circuit board

trace. This minimizes added stray capacitance, keep-

ing the total circuit capacitance close to that of the A/D

Converter itself (4-5 pF). To measure longer time

intervals, an external capacitor may be connected to an

A/D channel and this channel selected when making a

time measurement.

FIGURE 19-3:

TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME

MEASUREMENT

A/D Converter

CTMU

CTED1

CTED2

ANX

Output Pulse

EDG1

EDG2

CAD

RPR

Current Source

PIC18(L)FXXK22 Device

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