AN1250 datasheet(6/22 Pages) MICROCHIP

6 / 22 page

AN1250

DS01250A-page 6

The software routine for reading each of the sensor

channels consists of the following 13 steps:

Set all of the A/D converter channels to digital

I/O pins and set them to logic zero (‘0’).

This has the effect of grounding all of the

capacitive touch sensor pads, so they are at

zero (0) volts potential.

Set the A/D channel to be read to an analog input.

Set the A/D converter to the selected channel.

Set the discharge bit, IDISSEN, of the CTMUCON

This step insures that the internal capacitance of

the A/D converter starts at zero (0) volts potential.

Wait for charge to drain (a few NOPs).

Clear the discharge bit, IDISSEN.

Set the sample bit, SAMP (AD1CON1), to begin

sampling the selected channel (PIC24F only).

Set either of the edge status bits of the CTMU.

For PIC24F:

EDG1STAT OR EDG2STAT (CTMUCON) bits

For PIC18F:

EDG1STAT OR EDG2STAT (CTMUCONL) bits

This step turns on the CTMU current source to

the selected A/D channel and begins charging

the connected capacitive touch sensor.

Wait for a fixed period of time. For most capacitive

touch applications, this is approximately 2-10

μS.

In this case, the fixed time period is a loop set up

to do nothing until the allotted time has expired.

10. Clear the edge status bit that was set above.

For PIC24F:

EDG1STAT OR EDG2STAT (CTMUCON) bits

For PIC18F:

EDG1STAT OR EDG2STAT (CTMUCONL) bits

This step turns off the CTMU current source and

stops charging the connected capacitive touch

sensor.

11. Initiate an A/D conversion on the selected

channel:

a) On PIC24F, clear the SAMP bit (AD1CON1).

b) On PIC18F, set the GO/DONE bit (ADCON0).

12. Wait for the conversion to complete.

13. Store the value read by the A/D converter.

Figure 3 provides a software flowchart of the CTMU

capacitive touch sensor, reading as outlined above.

FIGURE 3:

CTMU CAPACITIVE TOUCH SENSOR READ FLOWCHART

Start

Set all of ADC as

digital outputs (‘0’)

to insure all sensors

at zero potential

Set selected ADC

channel to analog

Set Discharge bit,

IDISSEN

Set ADC to sample

selected channel

Wait for charge to

drain (a few NOPs)

Clear Discharge bit,

IDISSEN

Begin sampling by

setting the SAMP

bit of the ADC

Set the Edge 1

Status bit of the

CTMU (EDG1STAT)

Wait for the

Capacitive Touch

Circuit to charge

Clear Edge 1

Status bit of the

CTMU (EDG1STAT)

Start ADC

Clearing the SAMP

Wait for conversion to

to complete:

Wait for ADC Interrupt

flag (PIC24F)

Move the data

from the ADC

conversion to a

temporary variable

End

(PIC24F only)

conversion by:

bit (PIC24F)

Setting GO/DONE

bit (PIC18F)

Wait for GO/DONE = 0

(PIC18F)

ADC Operation

CTMU Operation

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