The ATMEL QTouch Library API provides a function pointer called “qt_filter_callback”. The user can use this hook to apply filter functions to the measured signal values.
If the pointer is non-NULL, the library calls the function after library has made capacitive channel measurements, but before the library has processed the channel information and determining the sensor states.
Figure 1. Block diagram to represent usage of filter callback
function>
Figure 2. Averaging the Last Four Signal Values
/* 1. Add a static variable in the main module: */
/* filter for channel signals */
static uint16_t filter[QT_NUM_CHANNELS][4];
/* 2. Add a filter function prototype to the main module: */
/* example signal filtering function */
static void filter_data_mean_4( void );
/* 3. When configuring the ATMEL QTouch library, set the
callback function pointer: */
/* set callback function */
qt_filter_callback = filter_data_mean_4;
/* 4. Add the filter function: */
void filter_data_mean_4( void )
{
uint8_t i;
/*
* Shift previously stored channel signal data.
* Store new channel signal data.
* Set library channel signal data = mean of last 4 values.
*/
for( i = 0u; i < QT_NUM_CHANNELS; i++ )
{
filter[i][0] = filter[i][1];
filter[i][1] = filter[i][2];
filter[i][2] = filter[i][3];
filter[i][3] = qt_measure_data.channel_signals[i];
qt_measure_data.channel_signals[i] = (filter[i][0] +
filter[i][1] +
filter[i][2] +
filter[i][3]) / 4u;
}
}
The signal values processed by the ATMEL QTouch Library are now the mean of the last four actual signal values.