As explain in the previous section, the absolute value of any measurement is impacted by VDD and VIH. The effect increases when VDD increases, and decreases when VDD decreases.
The figure below describes these effects on measures.
Another way to increase the noise immunity is to use the frequency-hopping algorithm. The charge-transfer frequency at the end of all groups acquisition can be changed by updating CTPH, CTPL, and DIV TSC registers fields. This spreads the charge-transfer frequency. Two, three or more frequency values can be used.
An example of this algorithm is given below. It can be used to update CTPH and CTPL at the end of each complete sensors acquisition. Cautions must be taken as the acquisition waveform must respect the criteria described previously:
MX_TSC_Init:// TSC Frequency Hopping
#define FREQ_VALUES 3
typedef struct{
int div;
int ctph;
int ctpl;
}tsc_setup_t;
tsc_setup_t FreqHopp[FREQ_VALUES]={0};
TSC_HandleTypeDef htsc;
TSC_HandleTypeDef htsc_freqhopp;
#define CTPH_TO_INT(a) ((uint32_t)( (a) >> 28) + 1) // From 1 to 16
#define INT_TO_CTPH(a) ((uint32_t)( (a-1) << 28)) // From 1 to 16
#define CTPL_TO_INT(a) ((uint32_t)( (a) >> 24) + 1 ) // From 1 to 16
#define INT_TO_CTPL(a) ((uint32_t)( (a-1) << 24)) // From 1 to 16
#define PRESC_TO_INT(a) ((uint32_t)( (a) >> 12) + 1) // 1/2/4/8/16/32/64/128
#define INT_TO_PRESC(a) ((uint32_t)( (a-1) << 12)) // 1/2/4/8/16/32/64/128OptimumTscCfgCTPH=x; //With:
OptimumTscCfgCTPL=x; //x = 1 to 16
OptimumTscCfgDIV=y; //y = 1 to 128 (power of 2)
...
htsc.Init.CTPulseHighLength = INT_TO_CTPH(OptimumTscCfgCTPH);
htsc.Init.CTPulseLowLength = INT_TO_CTPL(OptimumTscCfgCTPL);
htsc.Init.SpreadSpectrum = DISABLE;
...
htsc.Init.PulseGeneratorPrescaler = INT_TO_PRESC(OptimumTscCfgDIV);
...
if (HAL_TSC_Init(&htsc) != HAL_OK)
...for(int f=0; f<FREQ_VALUES; f++){
if(f!=0){
if(OptimunTscCfgCTPH+1 <=16 && OptimunTscCfgCTPL+1 <=16){
OptimunTscCfgCTPH++;
OptimunTscCfgCTPL++;
}else{
// We assume CTPH <= CTPL
OptimunTscCfgDIV *= 2;
OptimunTscCfgCTPH = 1 + (OptimunTscCfgCTPH/2);
OptimunTscCfgCTPL = 1 + (OptimunTscCfgCTPL/2);
}
}
FreqHopp[f].div = OptimunTscCfgDIV;
FreqHopp[f].ctph = OptimunTscCfgCTPH;
FreqHopp[f].ctpl = OptimunTscCfgCTPL;
}while(1) loopwhile (1)
{
// Process TSC acquisition
if(TSCAcqDone != 0){
TSCAcqDone = 0;
// Get TSC/TSL keys status
TSL_obj_GroupProcess(&MyObjGroup);
// Process DXS
…
// Process ECS algo
…
// activate led if key is press
…
//FTB, or Fast Transient Burst immunity: wait for 15ms
…
// We move to next frequency
MoveToNextFreq();
// After last bank acquisition we restart all bank acquisition
TSCidxGroup = 0;
TSCAcqDone = 0;
TSL_acq_BankConfig(TSCidxGroup);
TSL_acq_BankStartAcq_IT();
}else{
__WFI();
}
}* Api
void MoveToNextFreq(void){
static int f=0;
f++;
f %= FREQ_VALUES;
// Change TSC frequency DIV divider
htsc.Init.PulseGeneratorPrescaler = INT_TO_PRESC(FreqHopp[f].div);
htsc.Init.CTPulseHighLength = INT_TO_CTPH(FreqHopp[f].ctph);
htsc.Init.CTPulseLowLength = INT_TO_CTPL(FreqHopp[f].ctpl);
// Re Init TSC IP with New parameters...
HAL_TSC_Init(&htsc);
return;
}
Increasing Cs, sampling capacitor, increases the acquisition time but decreases the conducted-noise effects.
The configuration Rskey = 10 kΩ and Cskey = 47 nF gives good results on the immunity to conducted-noise.
See section 'Hardware timing' of the application note Tuning a touch sensing application on MCUs (AN4316) for more details Rskey and Cskey.
See section 'Hardware timing' of the application note Guidelines for designing touch sensing applications with projected sensors (AN4313) for more details Rsshield and Csshield.
/*============================================================================*/
/* Detection Exclusion System (DXS) */
/*============================================================================*/
/** @defgroup Common_Parameters_DXS 12 - DXS
* @{ */
/** Detection Exclusion System (0=No, 1=Yes)
*/
#define TSLPRM_USE_DXS (1)
/** @} Common_Parameters_DXS */
Usually all sensors are in TSL_STATEID_RELEASE state. When a sensor is pressed, its state moves to TSL_STATEID_DETECT.
SensorNbInStateId API as shown below./**
* @brief Count Sensor in stateId
* @param StateId Sensor state id, can NOT be OR-ed.
* @retval Sensor number in StateId state
*/
int SensorNbInState(TSL_StateId_enum_T StateId){
int cntStateId=0,cntTKey=0;
// Parse TKey sensors
#if TSLPRM_TOTAL_TOUCHKEYS > 0
for (cntTKey=0;cntTKey<TSLPRM_TOTAL_TOUCHKEYS;cntTKey++){
if(MyTKeys[cntTKey].p_Data->StateId == StateId){
cntStateId++;
}
}
#endif
// Parse LinRots sensors
#if TSLPRM_TOTAL_LINROTS > 0
for (cntTKey=0;cntTKey<TSLPRM_TOTAL_LINROTS;cntTKey++){
if(MyLinRots[cntTKey].p_Data->StateId == StateId){
cntStateId++;
}
}
#endif
return cntStateId;
}
Inside the main loop, sensors not in RELEASE state are counted and the acquisition is stopped if more than n sensors are not in a stable state. This may occur in case of incoming noise or when hands touch all sensors simultaneously.
#define NOISE_COMPUTE_WINDOWS 10 /* Use to monitor noise */
#define NOISE_COMPUTE_THRESHOLD 20 /* Threshold level to decide when noise is too high */
#define NOISE_SUSPEND_DELAY 5000 /* Suspend delay when we detect noise (5s) */
#define NOISE_SENSOR_NOT_IN_RELEASE 3 /* Suspend acquisition when sensors are not
in release state */
while (1)
{
if(TSCAcqDone != 0 && WaitEndOfNoiseDuration == 0){
TSCAcqDone = 0;
// Get TSC/TSL keys status
TSL_obj_GroupProcess(&MyObjGroup);
// Point to DXS first object
// Process ECS algo
// Application sensor processing
// For FTB test, wait for 15ms
// Handle Frequency Hopping
// Handle noise on application level
nbSensorInRelease = SensorNumberInState(TSL_STATEID_RELEASE);
// Handle next acquisition
if((((TSLPRM_TOTAL_TOUCHKEYS+TSLPRM_TOTAL_LINROTS)-nbSensorInRelease)
>= NOISE_SENSOR_NOT_IN_RELEASE)){
// Noise detect, we suspend sensors acquisition
WaitEndOfNoiseDuration = NOISE_SUSPEND_DELAY;
}else{
// After last bank acquisition we restart all bank acquisition
TSCidxGroup = 0;
TSCAcqDone = 0;
TSL_acq_BankConfig(TSCidxGroup);
TSL_acq_BankStartAcq_IT();
}
}else{
__WFI();
// We assume only 1ms tick interrupt occurs
if(WaitEndOfNoiseDuration > 0) {
WaitEndOfNoiseDuration--;
if(WaitEndOfNoiseDuration==0){
nbSensorInRelease = 0;
WaitEndOfNoiseDuration = 0;
// We restart all acquisition
TSCidxGroup = 0;
TSCAcqDone = 0;
TSL_acq_BankConfig(TSCidxGroup);
TSL_acq_BankStartAcq_IT();
}
}
}The GetNoiseLevel API can be used to get median noise level on all channels and decide to stop system when a value is too high. This API must be combined with the StateNbInSate one.
/**
* @brief Get noise level
* @param None
* @retval mean noise level
*/
int phaseMeasurementNoise=0;
int GetNoiseLevel(void){
int meanNoiseLevel=0, noiseLevelTKEYS=0, noiseLevelLROT=0;
uint32_t id = 0, channel=0, div=0;
//Reset array
if(phaseMeasurementNoise==0){
#if TSLPRM_TOTAL_TOUCHKEYS > 0
for (channel=0;channel<TSLPRM_TOTAL_TOUCHKEYS;channel++){
for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
NoiseTKEYS[channel][id] = 0;
}
}
#endif
#if TSLPRM_TOTAL_LINROTS > 0
for (channel=0;channel<TSLPRM_TOTAL_LINROTS;channel++){
for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
NoiseLROT[channel][id] = 0;
}
}
#endif
}
phaseMeasurementNoise++;
//Start to measure noise, level.....
#if TSLPRM_TOTAL_TOUCHKEYS > 0
// Parse TKey sensors
for (channel=0;channel<TSLPRM_TOTAL_TOUCHKEYS;channel++){
NoiseTKEYS[channel][phaseMeasurementNoise%NOISE_COMPUTE_WINDOWS]
=MyTKeys[channel].p_ChD->Delta;
}
#endif
#if TSLPRM_TOTAL_LINROTS > 0
// Parse LinRots sensors
for (channel=0;channel<TSLPRM_TOTAL_LINROTS;channel++){
NoiseLROT[channel][phaseMeasurementNoise%NOISE_COMPUTE_WINDOWS]
=MyLinRots[channel].p_ChD->Delta;
}
#endif
// Compute noise inside the rolling windows
#if TSLPRM_TOTAL_TOUCHKEYS > 0
for (channel=0;channel<TSLPRM_TOTAL_TOUCHKEYS;channel++){
for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
noiseLevelTKEYS += NoiseTKEYS[channel][id];
}
}
noiseLevelTKEYS = abs(noiseLevelTKEYS);
noiseLevelTKEYS /= NOISE_COMPUTE_WINDOWS;
div += TSLPRM_TOTAL_TOUCHKEYS; // Will be used to compute noise mean level
#endif
#if TSLPRM_TOTAL_LINROTS > 0
for (channel=0;channel<TSLPRM_TOTAL_LINROTS;channel++){
for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
noiseLevelLROT += NoiseLROT[channel][id];
}
}
noiseLevelLROT = abs(noiseLevelLROT);
noiseLevelLROT /= NOISE_COMPUTE_WINDOWS;
div += TSLPRM_TOTAL_LINROTS; // Will be used to compute noise mean level
#endif
// For debug purpose
//#if TSLPRM_TOTAL_TOUCHKEYS > 0
// for (channel=0;channel<TSLPRM_TOTAL_TOUCHKEYS;channel++){
// printf("Sensor TKEY %02d Delta(s) :",channel);
// for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
// printf(" %4d",NoiseTKEYS[channel][id]);
// }
// printf("\n");
// }
//#endif
//#if TSLPRM_TOTAL_LINROTS > 0
// for (channel=0;channel<TSLPRM_TOTAL_LINROTS;channel++){
// printf("Slider LROT %02d Delta(s) :",channel);
// for (id=0;id<NOISE_COMPUTE_WINDOWS;id++){
// printf(" %4d",NoiseLROT[channel][id]);
// }
// printf("\n");
// }
//#endif
// mean noise level take into account keys and sliders
meanNoiseLevel = (noiseLevelTKEYS + noiseLevelLROT)/div;
printf("==> Noise ..... noiseLevelTKEYS %d noiseLevelLROT %d [meanNoiseLevel %d]\n",
noiseLevelTKEYS,noiseLevelLROT,meanNoiseLevel);
return meanNoiseLevel;
}
Inside the main loop, noise is measured at the end of all groups acquisition. It is important to get the noise value only when all sensors are in RELEASE state.
#define NOISE_COMPUTE_WINDOWS 10 /* Use to monitor noise */
#define NOISE_COMPUTE_THRESHOLD 20 /* Threshold level to decide when noise is too high */
#define NOISE_SUSPEND_DELAY 5000 /* Suspend delay when we detect noise (5s) */
#define NOISE_SENSOR_NOT_IN_RELEASE 3 /* Suspend acquisition when sensors are not
in release state */
while (1)
{
if(TSCAcqDone != 0 && WaitEndOfNoiseDuration == 0){
TSCAcqDone = 0;
// Get TSC/TSL keys status
TSL_obj_GroupProcess(&MyObjGroup);
// Point to DXS first object
// Process ECS algo
// Application sensor processing
// For FTB test, wait for 15ms
// Handle Frequency Hopping
// Handle noise on application level
nbSensorInRelease = STMTBOX_SensorNumberInState(TSL_STATEID_RELEASE);
// We compute noise level only if all sensors are in Release state
if(nbSensorInRelease==(TSLPRM_TOTAL_TOUCHKEYS+TSLPRM_TOTAL_LINROTS)){
noiseLevel = STMTBOX_GetNoiseLevel();
if (noiseLevel >= NOISE_COMPUTE_THRESHOLD)
noiseIsDetected = 1;
else
noiseIsDetected = 0;
}
// Handle next acquisition
if((((TSLPRM_TOTAL_TOUCHKEYS+TSLPRM_TOTAL_LINROTS)-nbSensorInRelease)
>= NOISE_SENSOR_NOT_IN_RELEASE) || (noiseIsDetected)){
// Noise detect, we suspend sensors acquisition
WaitEndOfNoiseDuration = NOISE_SUSPEND_DELAY;
}else{
// After last bank acquisition we restart all bank acquisition
TSCidxGroup = 0;
TSCAcqDone = 0;
TSL_acq_BankConfig(TSCidxGroup);
TSL_acq_BankStartAcq_IT();
}
}else{
__WFI();
// We assume only 1ms tick interrupt occurs
if(WaitEndOfNoiseDuration > 0) {
WaitEndOfNoiseDuration--;
if(WaitEndOfNoiseDuration==0){
nbSensorInRelease = 0;
WaitEndOfNoiseDuration = 0;
noiseIsDetected = 0;
// We restart all acquisition
TSCidxGroup = 0;
TSCAcqDone = 0;
TSL_acq_BankConfig(TSCidxGroup);
TSL_acq_BankStartAcq_IT();
}
}
}Examples of software reading the ADC value are available in STM32CubeMX deliveries. Windowing can be used to trig the interrupts. Under the ADC interrupt routine, WaitEndOfNoiseDuration must be set to match with the algorithm detailed in the previous section.
// Noise detect, we suspend sensors acquisition
WaitEndOfNoiseDuration = NOISE_SUSPEND_DELAY;Examples: