KW40Z_Connectivity_Software_1.0.1 (KSDK_1.3.0); IAR 7.50;
On a MKW30Z requiring one pin for capacitive touch TSI sensing circuit PTC3 muxed to TSI0_CH15
The TSI plate is not mounted on PCB, but via a small coax cable less than 10 cm to a brass metal knob on the metal case isolated from the case.
The reference article AN3863: Designing Touch Sensing does not give references for outside pcb sensors.
http://www.nxp.com/assets/documents/data/en/application-notes/AN3863.pdf
There are some specifications on how to adapt the button ?
Pull-up resistor is required ?
There is a need for additional parallel capacitance ?
Hi Michael,
And some extra questions:
There is some how to set these values ?
/* Set up the HW configuration for normal mode of TSI */
/* FSL_FEATURE_TSI_VERSION == 4 */
static const tsi_config_t tsi_HwConfig =
{
.ps = kTsiElecOscPrescaler_2div, /*!< Electrode oscillator frequency divided by 2 */
.extchrg = kTsiExtOscChargeCurrent_16uA, /*!< External oscillator charge current is 16uA */
.refchrg = kTsiRefOscChargeCurrent_16uA, /*!< Reference oscillator charge current is 16uA */
.nscn = kTsiConsecutiveScansNumber_8time, /*!< 8 times consecutive scan */
.mode = kTsiAnalogModeSel_Capacitive, /*!< Active TSI capacitive sensing mode */
.dvolt = kTsiOscVolRails_Dv_029, /*!< DV = 0.29 V; VP = 0.95 V; Vm = 0.67 V */
.thresh = 100,
.thresl = 200,
};
Also about Fig B I see some other references:
AN0040.pdf
A series resistor of 100 Ohm - 1 kOhm
A typical resistor value is 1 kΩ, but the value can range from 100 Ω to 10 kΩ
What about Envelope detector techniques ?
Thank you,
Lucian
Hi Michael,
On my layout I can cope with a few topologies like: Fig A (R2 0-10R)
Or can add additional protection to Vcc 3.3
Or can I fully decouple DC component from touch button ?
Hi Michael,
My HW circuit looks like this:
My sensor size is disc 19.45mm diameter; surface 297mm2 - brass.
(~Optimal 15 mm x 15 mm - diagonal 21,213mm; surface 225 mm2)
Sensor traces length to connector 23mm. Coax wire to sensor 150mm.
The sensor is isolated from metal case:
R2 is to protect from earth injection to gnd.
How should the earth be connected ?
Techniques for Robust Touch Sensing Design | DigiKey
Main goals keep sensitivities high and noise low.
Sensitivities is inverse proportional to offset (system) capacitance measured by sensor.
I can see some possible SW approaches :
(KSDK demo)
1. after reset, some untouched value (low) can be measured and considered reference avgUntouch,
than the threshold constant (10) is added for the comparing process (avgMeasure > avgUntouch+10).
Here the moving average refers to all active channels.
But I only have one channel.
2. via Bluetooth connection the client could set the threshold constant - like a sensitivity value.
3. via Bluetooth the user can activate a calibration for low (un-touch), then for high (touched)
Then the threshold can be updated to 1/2*(low+high)
( or using the sensitivity 1/sensitivity*(low+high) )
4. Sensitivity is strong correlated to with temperature ? - the chip can measure the temperature.
What about humidity like rainy /sunny days ?
5. Also the sampling rate can be varied - 50ms 100ms or 150ms - as we need short push and long push events.
6. threshold hysteresis
7. jittering and filtering ?
Best Regards,
Lucian
Hi Michael,
Ok R13 will not be mounted.
R81 should act like a fuse 1R ~ 10R (protect for external ESD injection)
The environment is quite nosy - did not have any other better choice -
we use all pins of the MKW30Z (has less fewer pins than MKW40Z ...)
Yes the physical electrode size is large made of brass, isolated from metal head.
(like a brass knob)
A coax wire is use between the knob and PCB.
R14 and R18 can changed to capacitors so the knob is decoupled by the circuit ?
Thank you,
Lucian.
