PIC18F23K22-ESLASHML datasheet(320/496 Pages) MICROCHIP

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PIC18(L)F2X/4XK22

DS41412C-page 320

Preliminary

 2010 Microchip Technology Inc.

19.3

Calibrating the CTMU Module

The

CTMU

requires

calibration

for

precise

measurements 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

this type of application would include 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,

and the circuit being measured needs calibration to

measure and/or nullify all other capacitance other than

that to be measured.

19.3.1

CURRENT SOURCE CALIBRATION

The current source on board the CTMU module has a

range of ±60% nominal for each of three current

ranges. Therefore, 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 19-2. The current source measurement is

performed using the following steps:

Initialize the A/D Converter.

Initialize the CTMU.

Enable the current source by setting EDG1STAT

(CTMUCONL<0>).

Issue settling time delay.

Perform 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.

The CTMU current source may be trimmed with the

trim bits in CTMUICON using an iterative process to get

an exact desired current. Alternatively, the nominal

value without adjustment may be used; it may be

stored by the software for use in all subsequent

capacitive or time measurements.

To calculate the value for RCAL, the nominal current

must be chosen, and then the resistance can be

calculated. For example, if the A/D Converter reference

voltage is 3.3V, use 70% of full scale, or 2.31V as the

desired approximate voltage to be read by the A/D

Converter. If the range of the CTMU current source is

selected to be 0.55

A, the resistor value needed is cal-

culated as RCAL = 2.31V/0.55

A, for a value of 4.2 MΩ.

Similarly, if the current source is chosen to be 5.5

A,

RCAL would be 420,000Ω, and 42,000Ω if the current

source is set to 55

A.

FIGURE 19-2:

CTMU CURRENT SOURCE

CALIBRATION CIRCUIT

A value of 70% of full-scale voltage is chosen to make

sure that the A/D Converter was in a range that is well

above the noise floor. Keep in mind that if an exact cur-

rent is chosen, that is to incorporate the trimming bits

from CTMUICON, the resistor value of RCAL may need

to be adjusted accordingly. RCAL may also be adjusted

to allow for available resistor values. RCAL should be of

the highest precision available, keeping in mind the

amount of precision needed for the circuit that the

CTMU will be used to measure. A recommended

minimum would be 0.1% tolerance.

The following examples show one typical method for

performing a CTMU current calibration. Example 19-1

demonstrates how to initialize the A/D Converter and

the CTMU; this routine is typical for applications using

both modules. Example 19-2 demonstrates one

method for the actual calibration routine.

PIC18(L)FXXK22 Device

A/D Converter

CTMU

ANx

RCAL

Current Source

MUX

A/D

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