AN1250 datasheet(8/22 Pages) MICROCHIP

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AN1250

DS01250A-page 8

EXAMPLE 4:

READING THE CAPACITIVE TOUCH SENSOR CIRCUIT SOFTWARE ALGORITHM

(PIC18F)

The main() function contains a while(1) loop

(endless) that checks for the dataReadyCTMU flag (set

by the Timer1 interrupt handler). If the dataReadyCTMU

flag is not set, other tasks are performed while it waits. In

the case of the PICDEM Touch Sense 2 Demo Board,

communication of data over the USB is serviced. If the

dataReadyCTMU

flag is set, the main() function clears

the flag and calls the routines necessary to process the

newly acquired data. The main() function temporarily

disables Timer1 to insure that no new data is introduced

in the middle of the routines that are handling data

processing. After all processing of the capacitive touch

sensor data is complete, and the LED display informa-

tion is also updated, Timer1 is re-enabled, and the whole

process begins again.

The structuring of the code in this manner is done to

make the system deterministic. Using a fixed timer to

read each capacitive touch sensor channel at a 1 ms

rate fixes the update rate to approximately 16 ms (1 ms

times 16 channels). The actual update rate is slightly

slower, since Timer1 is disabled while the new data is

processed.

//Read CTMU (Get the raw sensor reading)

ANCON0bits.PCFG= 0xFF;

//set all A/D channels to digital I/O pins

ANCON1bits.PCFG= 0x1F;

//set all A/D channels to digital I/O pins

TRISA = 0x00;

TRISB = 0x00;

TRISC = 0x00;

TRISE = 0x00;

LATA = 0x00;

LATB = 0x00;

LATC = 0x00;

LATE = 0x00;

PORTB = 0x00;

//set all channels to logical 0 outputs

Nop(); Nop(); Nop(); Nop();

//Wait for charge to drain

TRISA = ADPASELECT[Index];

TRISB = ADPBSELECT[Index];

TRISC = ADPCSELECT[Index];

TRISE = ADPESELECT[Index];

//set selected channel to input

ANCON0bits.PCFG = ~AD0SELECT[Index];

ANCON1bits.PCFG = ~AD1SELECT[Index];

//set selected channel to analog A/D input

Nop(); Nop(); Nop(); Nop();

//wait for A/D to connect to channel

ADCON0bits.CHS = Index;

//select A/D channel

CTMUCONHbits.IDISSEN =

//Drain any charge on the A/D circuit

Nop(); Nop(); Nop(); Nop(); Nop();

//Wait for charge to drain

CTMUCONHbits.IDISSEN =

//Stop discharge of A/D circuit

Nop(); Nop();

CTMUCONLbits.EDG2STAT = 0;

// Make sure edge2 is 0

CTMUCONLbits.EDG1STAT = 1;

// Set edge1 - Start Charge

for (count = 0; count < loopCount; count++);

// Delay for CTMU charge time

CTMUCONLbits.EDG1STAT = 0;

//Clear edge1 - Stop Charge

AD1CON0bits.GODONE =

//Set Go bit to begin A/D conversion

while(ANCON0bits.GODONE);

//Wait for the A/D conversion to finish

immediateValue = ADRESH;

immediateValue = immediateValue << 8;

immediateValue = immediateValue + ADRESL;

//Read the value from the A/D conversion

//End of CTMU read

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