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Sheet目录3861

154页 155页 156页 157页 [158页]159页 160页 161页 162页

PIC18F66K80 FAMILY

DS39977F-page 240

2010-2012 Microchip Technology Inc.

18.2.5

INTERRUPTS

The CTMU sets its interrupt flag (PIR3<3>) whenever

the current source is enabled, then disabled. An inter-

rupt is generated only if the corresponding interrupt

enable bit (PIE3<3>) is also set. If edge sequencing is

not enabled (i.e., Edge 1 must occur before Edge 2), it

is necessary to monitor the edge status bits and

determine which edge occurred last and caused the

interrupt.

18.3

CTMU Module Initialization

The following sequence is a general guideline used to

initialize the CTMU module:

Select the current source range using the

IRNGx bits (CTMUICON<1:0>).

Adjust the current source trim using the ITRIMx

bits (CTMUICON<7:2>).

Configure the edge input sources for Edge 1 and

Edge 2 by setting the EDG1SEL and EDG2SEL

bits (CTMUCONL<3:2> and <6:5>, respectively).

Configure the input polarities for the edge inputs

using the EDG1POL and EDG2POL bits

(CTMUCONL<4,7>).

The default configuration is for negative edge

polarity (high-to-low transitions).

Enable edge sequencing using the EDGSEQEN

bit (CTMUCONH<2>).

By default, edge sequencing is disabled.

Select the operating mode (Measurement or Time

Delay) with the TGEN bit (CTMUCONH<4>).

The

default

mode

Time/Capacitance

Measurement.

Configure the module to automatically trigger

an A/D conversion when the second edge

event has occurred using the CTTRIG bit

(CTMUCONH<0>).

The conversion trigger is disabled by default.

Discharge the connected circuit by setting the

IDISSEN bit (CTMUCONH<1>).

After waiting a sufficient time for the circuit to

discharge, clear the IDISSEN bit.

10. Disable the module by clearing the CTMUEN bit

(CTMUCONH<7>).

11. Clear the Edge Status bits, EDG2STAT and

EDG1STAT (CTMUCONL<1:0>).

Both bits should be cleared simultaneously, if

possible, to avoid re-enabling the CTMU current

source.

12. Enable both edge inputs by setting the EDGEN

bit (CTMUCONH<3>).

13. Enable the module by setting the CTMUEN bit.

Depending on the type of measurement or pulse

generation being performed, one or more additional

modules may also need to be initialized and configured

with the CTMU module:

Edge Source Generation: In addition to the

external edge input pins, ECCP1/CCP2 Special

Event Triggers can be used as edge sources for

the CTMU.

Capacitance or Time Measurement: The CTMU

module uses the A/D Converter to measure the

voltage across a capacitor that is connected to one

of the analog input channels.

Pulse Generation: When generating system clock

independent, output pulses, the CTMU module

uses Comparator 2 and the associated

comparator voltage reference.

18.4

Calibrating the CTMU Module

The CTMU requires calibration for precise measure-

ments of capacitance and time, as well as for accurate

time delay. If the application only requires measurement

of a relative change in capacitance or time, calibration is

usually not necessary. An example of a less precise

application is a capacitive touch switch, in which the

touch circuit has a baseline capacitance and the added

capacitance of the human body changes the overall

capacitance of a circuit.

If actual capacitance or time measurement is required,

two hardware calibrations must take place:

The current source needs calibration to set it to a

precise current.

The circuit being measured needs calibration to

measure or nullify any capacitance other than that

to be measured.

18.4.1

CURRENT SOURCE CALIBRATION

The current source on board the CTMU module has a

range of ±62% nominal for each of three current

ranges. For precise measurements, it is possible to

measure and adjust this current source by placing a

high-precision resistor, RCAL, onto an unused analog

channel. An example circuit is shown in Figure 18-2.

To measure the current source:

Initialize the A/D Converter.

Initialize the CTMU.

Enable the current source by setting EDG1STAT

(CTMUCONL<0>).

Issue time delay for voltage across RCAL to

stabilize and A/D sample/hold capacitor to charge.

Perform the A/D conversion.

Calculate the current source current using

I=V/RCAL, where RCAL is a high-precision

resistance and V is measured by performing an

A/D conversion.

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