Forum Post: RE: ADS7846 ADC VALUES NOT CORRECT

Hi John,

First, if your customer is starting a new design based on ADS7846, I would recommend that your customer use TSC2046E instead of ADS7846. The TSC2046E is the next-generation version of the ADS7846 4-wire touch screen controller, 100% pin-compatible with the existing ADS7846, and drops into the same socket.

Second,  if you look a look at the Figure 9 in its datasheet, you can find the 11th bit is the MSB and the 1st bit is the LSB. 

To help your customer understand how to read the 12-bit X&Y coordinates from ADS7846 / TSC2046E, here is some example code, which is based on the timing in Figure 9: 

void tsc2046_get_position(unsigned int* px_pos, unsigned int* py_pos)
{
      // Get X position
      *px_pos = SendCommand(CMD_X_POSITION);

     // Get Y position
     *py_pos = SendCommand(CMD_Y_POSITION);

    // Switch to full power mode
    SendCommand(CMD_ENABLE_PENIRQ);
}

#define ADS_CTRL_PD0                              (1 << 0)    // PD0
#define ADS_CTRL_PD1                              (1 << 1)    // PD1
#define ADS_CTRL_DFR                              (1 << 2)   // SER/DFR
#define ADS_CTRL_EIGHT_BITS_MOD      (1 << 3)    // Mode
#define ADS_CTRL_START                         (1 << 7)    // Start Bit
#define ADS_CTRL_SWITCH_SHIFT          4               // Address setting

// Get X position command
#define CMD_Y_POSITION        ((1 << ADS_CTRL_SWITCH_SHIFT) | ADS_CTRL_START | ADS_CTRL_PD0 | ADS_CTRL_PD1)

// Get Y position command
#define CMD_X_POSITION        ((5 << ADS_CTRL_SWITCH_SHIFT) | ADS_CTRL_START | ADS_CTRL_PD0 | ADS_CTRL_PD1)

// Enable penIRQ
#define CMD_ENABLE_PENIRQ         ((1 << ADS_CTRL_SWITCH_SHIFT) | ADS_CTRL_START)

static unsigned int SendCommand(unsigned char bCmd)
{
int i;
AT91S_SPI *spi = BOARD_TSC_SPI_BASE;
unsigned int uResult = 0;
//unsigned int uTimeout = 0;
// (volatile declaration needed for code optimisation by compiler)
volatile unsigned char bufferRX[3];
volatile unsigned char bufferTX[3];

bufferRX[0] = 0;
bufferRX[1] = 0;
bufferRX[2] = 0;

bufferTX[0] = bCmd;
bufferTX[1] = 0;
bufferTX[2] = 0;

PIO_Clear(pinNss);
// Send command
i = 0;
while ((spi->SPI_SR & AT91C_SPI_TXEMPTY) == 0);
spi->SPI_TDR = bufferTX[i] | SPI_PCS(BOARD_TSC_NPCS);
while ((spi->SPI_SR & AT91C_SPI_TDRE) == 0);

// wait until the BUSY pin becomes low
while (PIO_Get(pinBusy) == 1);

while ((spi->SPI_SR & AT91C_SPI_RDRF) == 0);
bufferRX[i] = spi->SPI_RDR & 0xFFFF;

// Read data
for (i = 1; i < 3; i++)
{
while ((spi->SPI_SR & AT91C_SPI_TXEMPTY) == 0);
spi->SPI_TDR = bufferTX[i] | SPI_PCS(BOARD_TSC_NPCS);
while ((spi->SPI_SR & AT91C_SPI_TDRE) == 0);

while ((spi->SPI_SR & AT91C_SPI_RDRF) == 0);
bufferRX[i] = spi->SPI_RDR & 0xFFFF;
}
PIO_Set(pinNss);

uResult = ((unsigned int)bufferRX[1] << 8) | (unsigned int)bufferRX[2];
uResult = uResult >> 4;

return uResult;
}

Andy