ADC touchscreen readings

Hi,

I'm working on a touchscreen driver for a LM3S9B95 Stellaris.

I have an ADC sample sequence set up and I am able to read data out of the adc channels.

But the data does not seem to be of high enough resolution to convert to XY screen coordinates.

The touchscreen is a TTW4028001 on a 320X240 pixels LCD.

The data I'm getting from the ADC channel 11 (ADC_CTL_CH11), when I press the screen, is ranging from 0 to 30 for the Y. Which correlates to the 240 pixels width, on the bottom of the screen. And from 0 to 20 on the top of the screen. Which are quite promising results although quite low resolution of course.

The real problem lies in the reading of the ADC channel 10 (ADC_CTL_CH10), for the X values. That is constant around 43, when I press the screen it goes down by 5 to 38 at maximum.

This is with the reference voltage set to 1 volt external, connected at pin B6. With internal or external 3V, the resolution of the readings go down significantly.

Right now I'm using processor triggered interrupts, fired from a loop with interval. Eventually when I can make sensible readings this will be changed to timer based interrupts.

Another issue for me is getting valid conversion parameters to get valid XY coordinates. Like g_plParmSet in the touchscreen example driver for dk-lm3s9d96, touch.c. I don't know how to produce these values for my touchscreen, no clues in the datasheet.

If anyone can shed some light here, it would be greatly appreciated

Here is my code:

#define TS_ADC_PERIPH_B        SYSCTL_PERIPH_GPIOB // the adc pins
#define TS_ADC_PERIPH_E        SYSCTL_PERIPH_GPIOE // the drive pins

#define TS_ADC_BASE            GPIO_PORTB_BASE
#define TS_DRIVE_BASE        GPIO_PORTE_BASE

#define TS_X_ADC_PIN        GPIO_PIN_4
#define TS_X_DRIVE_PIN        GPIO_PIN_0

#define TS_Y_DRIVE_PIN        GPIO_PIN_1
#define TS_Y_ADC_PIN        GPIO_PIN_5

#define ADC_CTL_CH_XP         ADC_CTL_CH10
#define ADC_CTL_CH_YP         ADC_CTL_CH11

// from touch.c in the dk-lm3s9d96 example
const long g_plParmSet[7] =
{ -864,           // M0
        -79200,         // M1
        70274016,       // M2
        -85088,         // M3
        1056,           // M4
        80992576,       // M5
        199452,         // M6
        };

void ts_Init()
{
    SysCtlPeripheralEnable (SYSCTL_PERIPH_ADC0);
    SysCtlPeripheralEnable (TS_ADC_PERIPH_B);
    SysCtlPeripheralEnable (TS_ADC_PERIPH_E);

    GPIOPinTypeADC(TS_ADC_BASE, TS_X_ADC_PIN | TS_Y_ADC_PIN);
    GPIOPinTypeADC(GPIO_PORTB_BASE, GPIO_PIN_6);

    ADCSequenceDisable(ADC0_BASE, 1);
    ADCReferenceSet(ADC0_BASE, ADC_REF_EXT_1V); // yields best results
    //ADCReferenceSet(ADC0_BASE, ADC_REF_INT);
    //ADCReferenceSet(ADC0_BASE, ADC_REF_EXT_3V);
    //ADCHardwareOversampleConfigure(ADC0_BASE, 4);
    ADCSequenceConfigure(ADC0_BASE, 1, ADC_TRIGGER_PROCESSOR, 0);
    ADCSequenceStepConfigure(ADC0_BASE, 1, 0, ADC_CTL_CH_XP);
    ADCSequenceStepConfigure(ADC0_BASE, 1, 1, ADC_CTL_CH_YP | ADC_CTL_IE | ADC_CTL_END);

    ADCIntRegister(ADC0_BASE, 1, ts_IntHandler);
    ADCIntEnable(ADC0_BASE, 1);
    ADCSequenceEnable(ADC0_BASE, 1);

    GPIOPinWrite (TS_DRIVE_BASE, TS_X_DRIVE_PIN, TS_X_DRIVE_PIN);
    GPIOPinWrite (TS_DRIVE_BASE, TS_Y_DRIVE_PIN, 0);

    // Changing high low on the pins seems to have no effect

    /*GPIOPinWrite (TS_ADC_BASE, TS_Y_ADC_PIN, 0);
    GPIOPinWrite (TS_DRIVE_BASE, TS_X_DRIVE_PIN, TS_X_DRIVE_PIN);

    GPIOPinWrite (TS_ADC_BASE, TS_X_ADC_PIN, 0);
    GPIOPinWrite (TS_DRIVE_BASE, TS_Y_DRIVE_PIN, 0);*/
}

void ts_IntHandler(void)
{
    unsigned long adc_buffer[4]; // Sequence 1 has a FIFO of 4 entries

    ADCIntClear(ADC0_BASE, 1);
    ADCSequenceDataGet(ADC0_BASE, 1, adc_buffer);

    tRectangle sRect;
    sRect.sXMin = ((GrContextDpyWidthGet(&g_sContext)) / 2) - 75;
    sRect.sYMin = ((GrContextDpyHeightGet(&g_sContext)) / 2) - 40;
    sRect.sXMax = ((GrContextDpyWidthGet(&g_sContext)) / 2) + 75;
    sRect.sYMax = ((GrContextDpyHeightGet(&g_sContext)) / 2) + 40;
    GrContextForegroundSet(&g_sContext, ClrBlack);
    GrRectFill(&g_sContext, &sRect);

    char buf[16];
    GrContextForegroundSet(&g_sContext, ClrWhite);

    // adc_buffer[0] (x) is constant at 43, when screen press about 5 down to 38 max
    // adc_buffer[1] (y) has a range of 0 to 30

    usprintf(buf, "%d", adc_buffer[0]);
    GrStringDrawCentered(&g_sContext, buf, -1, GrContextDpyWidthGet(&g_sContext) / 2 - 40, ((GrContextDpyHeightGet(&g_sContext)) / 2) - 25, 0);

    usprintf(buf, "%d", adc_buffer[1]);
    GrStringDrawCentered(&g_sContext, buf, -1, GrContextDpyWidthGet(&g_sContext) / 2 + 40, ((GrContextDpyHeightGet(&g_sContext)) / 2) - 25, 0);
}

// main.cpp
int main(void)
{
    SysCtlClockSet (SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_OSC_MAIN | SYSCTL_XTAL_8MHZ);
    IntMasterEnable ();

    ...

    // Here the touchscreen init and interrupt loop
    ts_Init();

    while (1)
    {
        ADCProcessorTrigger(ADC0_BASE, 1);
        DelayMs(500);
    }
}