PIC18F44J50-I/SPSQTP 데이터 시트보기 (PDF) - Microchip Technology

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PIC18F44J50-I/SPSQTP

28/44-Pin, Low-Power, High-Performance USB Microcontrollers with nanoWatt XLP Technology

Microchip Technology 

PIC18F46J50 FAMILY

26.5 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:

1. Initialize the A/D Converter and the CTMU.

2. Set EDG1STAT.

3. Set EDG2STAT.

4. Perform an A/D conversion.

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

where I is calculated in the current calibration step

(Section 26.3.1 “Current Source Calibration”),

C is calculated in the capacitance calibration step

(Section 26.3.2 “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, CAD + CEXT, provides a valid volt-

age to the A/D Converter. For the smallest time

measurement, always set the A/D Channel Select reg-

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

corresponding pin which is not connected to any circuit

board trace. This minimizes added stray capacitance,

keeping the total circuit capacitance close to that of the

A/D Converter itself. 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 26-3:

TYPICAL CONNECTIONS AND INTERNAL CONFIGURATION FOR TIME

MEASUREMENT

CTED1

CTED2

PIC18F46J50 Device

CTMU

EDG1 Current Source

EDG2

A/D Voltage

ANX

CEXT

A/D Converter

CAD

 2011 Microchip Technology Inc.

DS39931D-page 411

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