capacitive touch

Dear Walter,

Thank you for sending a detailed reply to my query. Will send it to my hardware team to review the issue ...

Would like you to share a code snippet of touch interface done by my predecessor , I was given the task of adding UART communication without modifying the existing hardware and additionally adding an eeprom to the i2c line.

#define NUM_SEN 5 // Defines number of sensors
#define THRESHOLD 100 // Threshold for Switch sensing

void main()

{

...

swithc_init();

while(1)

{

..

switch_read();

..

}

}

void switch_init(void)
{
unsigned char count1, count2;

P3SEL &= ~( BIT6 | BIT7); // Touch switch pin selections
P2SEL &= ~(BIT3); // Touch switch pin selections
P3SEL2 &= ~(BIT6 | BIT7); // Touch switch pin selections
P2SEL2 &= ~(BIT3); // Touch switch pin selections
P4SEL &= ~( BIT0 | BIT7); // Touch switch pin selections
P4SEL2 &= ~(BIT0 | BIT7); // Touch switch pin selections

measure_count(); // Getting base count

for (count1 = 0; count1 < NUM_SEN; count1++) // reading base count for all the channels
base_cnt[count1] = meas_cnt[count1];

for (count1 = 15; count1 > 0; count1--) // 15 sample average base count calculation for all channels
{
measure_count();
for (count2 = 0; count2 < NUM_SEN; count2++)
base_cnt[count2] = (meas_cnt[count2] + base_cnt[count2]) >> 1; // averaging
}
}

void measure_count(void)
{
char key;

TA1CTL = TASSEL_3 + MC_2; // Timer configuration for touch
TA1CCTL2 = CM_3 + CCIS_2 + CAP; // timer configuration for touch

for(key=0;key<2;key++)
{
P4DIR &= ~electrode_bit[key];
P4SEL &= ~electrode_bit[key];
P4SEL2 |= electrode_bit[key];
__bis_SR_register(GIE);
TA1CTL |= TACLR; // Clear Timer_A TAR
delay(40);
TA1CCTL2 ^= CCIS0;
_nop();
_nop();
TA1CCTL2 ^= CCIS0; // Create SW capture of CCR1
meas_cnt[key] = TA1CCR2; // Counter value storing
P4SEL2 &= ~electrode_bit[key];
}
for (key = 2; key < 4; key++) // touch count calculation for 4 switches
{
P3DIR &= ~electrode_bit[key];
P3SEL &= ~electrode_bit[key];
P3SEL2 |= electrode_bit[key];
__bis_SR_register(GIE);
TA1CTL |= TACLR; // Clear Timer_A TAR
delay(40);
TA1CCTL2 ^= CCIS0;
_nop();
_nop();
TA1CCTL2 ^= CCIS0; // Create SW capture of CCR1
meas_cnt[key] = TA1CCR2; // Counter value storing
P3SEL2 &= ~electrode_bit[key];
}
if (key == 4) // touch count calculation for 5th switch
{
P2DIR &= ~electrode_bit[key];
P2SEL &= ~electrode_bit[key];
P2SEL2 |= electrode_bit[key];
__bis_SR_register(GIE);
TA1CTL |= TACLR; // Clear Timer_A TAR
delay(50); //16
TA1CCTL2 ^= CCIS0;
_nop();
_nop();
TA1CCTL2 ^= CCIS0; // Create SW capture of CCR1
meas_cnt[key] = TA1CCR2; // Counter value storing
P2SEL2 &= ~electrode_bit[key];
key++;
}
}

void switch_read(void)
{
unsigned char count1;

for (count1 = 0; count1 < NUM_SEN; count1++) // calculation of key press for 5 keys
{

delta_cnt[count1] = base_cnt[count1] - meas_cnt[count1]; // Calculate delta: c_change

if (delta_cnt[count1] < 0) // If negative: result increased
{ // beyond baseline, i.e. cap dec
base_cnt[count1] = (base_cnt[count1] + meas_cnt[count1]) >> 1; // Re-average quickly
delta_cnt[count1] = 0;
cnt[count1] = 1;
}
if (delta_cnt[count1] > d_threshold[count1]) // Determine if key is pressed
{
cnt[count1]++; // debounse count
if (cnt[count1] >= 20) // debounse check
{
cnt[count1] = 1;
key_press[count1] = 1; // key pressed
}

// d_Threshold = delta_cnt[count1];
key_pressed = count1 + 1; // key pressed
}
else
{
cnt[count1] = cnt[count1] - 2;

if(cnt[count1] < 2)
cnt[count1] = 0;
key_press[count1] = 0; // removing the key press if delata count is not enough
}
}

if (!key_pressed)
{ // if no keys are touched
for (count1 = 0; count1 < NUM_SEN; count1++)
base_cnt[count1] = base_cnt[count1] - 1; // Adjust baseline count
d_repeat_delay=0;
}

}

Varun,

I would also like to point out that timing issues might also be giving you trouble.  Looking through your code, I'm not sure what happens in the delay() function call, but it is absolutely critical that this time be consistent for each sample that is measured.  If the delay time varies, the results will vary, and that can look like a detection.  The most common thing to corrupt a measurement in a scenario like this is another interrupt (say, a UART interrupt) occurring during the delay period, which could potentially cause the delay time to increase.  Timing issues can look just like noise issues with this measurement mechanism.

Walter