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NXP Semiconductors KE17Z Series User Manual Download Page 29

Figure 29. Parallel plate capacitor

Touch sensing capacitance can be calculated by 

Equation 13

:

∆ Cs = k*ε0*Ad

Equation 13.

•

∆ Cs

 is the touch sensing capacitance in farads (F).

• A is the touch area between finger and overlay
• d is the distance between finger and electrode in meter (m)
• k is the dielectric constant of the overlay material

•

ε0

 is the permittivity of the free space (8.85 × 1012 F/m)

Overlay can use different material, the dielectric constants of common material are as follows:

Table 13. Dielectric constants of common material

Material

Dielectric constant (k)

Acrylic (PMMA)

2.7 - 4.5

Air

1.0

Tempered Glass

7.2 - 8.0

Polyester (PET)

2.8 - 4.5

Air has the lowest dielectric constant, and other commonly used overlay materials have relatively high dielectric constants. 
According to 

Equation 13

, selecting an overlay with a high dielectric constant enhances the sensitivity of the touch electrode. 

Thicker overlay decreases the touch electrode resolution.

 

When using overlays with higher dielectric constants, remove any gap between touch electrode and 
overlay material.

  NOTE  

NXP Semiconductors

Hardware design guide

KE17Z Dual TSI User Guide, Rev. 0, 05 May 2022

User Guide

29 / 37

Summary of Contents for KE17Z Series

Page 1: ... with touch controls or touch keyboards the MCUs can support touchscreen designs scaling up to 98 touch electrodes 26 electrodes using self capacitance 72 electrodes using mutual channels 1 2 TSI model comparison of KE17Z parts Table 1 lists the number of TSI channels corresponding to different parts of KE17Z These parts all support dual TSI modules Contents 1 Introduction 1 1 1 KE17Z dual TSI 1 1...

Page 2: ...hree shield channels CH4 CH12 CH21 TX 0 5 RX 6 11 TX 0 5 RX 6 11 KE17Z 64LQFP 47 ch 22 channels TSI0 0 12 16 24 25 channels TSI1 0 24 No CH13 CH14 CH15 Three shield channels CH4 CH12 CH21 Three shield channels CH4 CH12 CH21 TX 0 5 RX 6 11 TX 0 5 RX 6 11 KE17Z 48LQFP 31 ch 15 channels No CH2 CH3 CH4 CH5 CH13 CH14 CH15 CH19 CH20 CH21 16 channels No CH8 CH9 CH10 CH13 CH14 CH19 CH20 CH21 CH22 One shie...

Page 3: ...duct TSI TSI0 TSI1 KE15Z 100LQFP MKE15Z256VLL7 MKE15Z128VLL7 25 ch 25 channels TSI0 0 24 One shield channel CH12 TX 0 5 RX 6 11 KE17Z 100LQFP MKE17Z256VLL7 50 ch 25 channels TSI0 0 24 25 channels TSI1 0 24 Three shield channels CH4 CH12 CH21 Three shield channels CH4 CH12 CH21 Table continues on the next page NXP Semiconductors Introduction KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 3 ...

Page 4: ...ield channel CH12 TX 0 5 RX 6 11 KE17Z 48LQFP MKE17Z256VLF7 MKE17Z128VLF7 47 ch 22 channels TSI0 0 12 16 24 25 channels TSI1 0 24 No CH13 CH14 CH15 Three shield channels CH4 CH12 CH21 Three shield channels CH4 CH12 CH21 TX 0 5 RX 6 11 TX 0 5 RX 6 11 For the KE17Z in 100LQFP 64LQFP and 48LQFP packages its pin assignment is compatible with the KE15Z That is the number and position of GPIO pins are t...

Page 5: ...signment comparison of 100 LQFP between KE15Z and KE17Z Figure 3 TSI pin assignment comparison of 64 LQFP between KE15Z and KE17Z NXP Semiconductors Introduction KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 5 37 ...

Page 6: ...ch sensing reference design including multiple touch patterns based on the 5 V Robust KE17Z MCU of NXP It has dual TSI modules and supports up to 50 touch channels all demonstrated on the board Figure 5 X KE17Z TSI EVB KE17Z dual TSI evaluation board NXP Semiconductors Introduction KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 6 37 ...

Page 7: ...ance exists between electrode to system ground Touch changes field through human body and creates extra capacitance Self cap touch sensor structure Cs Intrinsic self capacitance 10 50 pF as usual ΔCs Touch generated self capacitance 0 3 2 pF as usual Sensitivity of sensor ΔCs Cs 1 10 as usual 2 2 Self cap sensing mode There are three modes of self cap mode Basic self cap mode Noise cancellation mo...

Page 8: ...controls the sample phase When ph1 turns on the external touch electrode Cx is charged by vdd3v Charge phase The switch ph2 controls the charge phase When ph1 turns off and then ph2 turns on the charge on the capacitor Cx flows to the internal integrated capacitor Ci which generates the average current Icx Via the trans conductance amplifier the Icx are amplified to charge Ci and the voltage Vci r...

Page 9: ... touch event correctly To increase the sensitivity enable sensitivity boost feature by removing part parasitic capacitance virtually So touch works well under the thicker overlay with sensitivity boost enabled The capacitance to be removed cannot be configured more than the intrinsic capacitance of the touch key Otherwise it causes the sensitivity invalid NOTE 2 3 Clock generation The clock genera...

Page 10: ...mode is disabled the frequency is main clock SSC_PRESCALE_NUM 1 2 Keep other registers in TSIx_SSC0 TSIx_SSC1 and TSIx_SSC2 as default value Switcℎing Clock Main Clock Divider 1 2 16 65 MHz 8 1 2 1 04 MHz 2 3 2 Advanced clock generation spread spectrum clocking The Spread Spectrum Clocking SSC increases the noise immunity to RF interference and spreads the emissions With the SSC enabled TSIx_SSC0 ...

Page 11: ...clock using the Pseudo Random Binary Sequence PRBS method by setting TSI_SSC0 SSC_MODE 00 t2 is configured as the random width Table 6 TSI_SSC0 SSC_MODE 00 PRBS mode Variable Register Clock cycle Description t1 TSIx_SSC0 BASE_NOCHARGE_NUM 1 16 SSCHighWidth t2 TSIx_SSC0 PRBS_OUTSEL 2 15 SSCHighRandomWidth t3 TSIx_SSC0 CHARGE_NUM 1 16 SSCLowWidth Switching Clock can be generated in a configurable up...

Page 12: ...5 3 5 7 0 1 1 074 MHz Switching clock is spectrum spread pulse Switcℎing Clock Main Clock SSC_PRESCALE_NUM 1 SSCHigℎWidtℎ t1 SSCHigℎRandomWidtℎ t2 SSCLowWidtℎ t3 16 65MHz 0 1 5 3 5 7 1 074MHz 2 4 TSI scan time and scan result accumulation TSI supports multiple scans per channel That is to get better Signal to Noise Ratio SNR and resolution TSI performs multiple scans The final scan result is accum...

Page 13: ...end configured TSIx_MODE S_XIN TSIx_MODE S_XCH Fsw Configurable the switching clock frequency Cs The self capacitance of touch sensor 2 Equation 5 is a new equation to calculate NSTEP when noise cancellation mode is enabled NSTEP 2 Ci Vp Vm vdd3v vddlv Cs S_XIN S_XCH Equation 5 Vddlv is internal power supply voltage Typical 1 2 V 3 Calculate NSTEP in basic self cap mode The TSI self cap mode imple...

Page 14: ...scans per channel That is TSI performs single scan operation for many times from getting the trigger to end of scan Figure 12 Multiple scans per channel Each time the TSI is triggered multiple scans can be performed inside the TSI The scan round is set by the registers TSI_SINC DECIMATION ORDER and CUTOFF ranged from 1 to 322 When the TSI_SINC DECIMATION is set to 0 only once the single scan is en...

Page 15: ...the same digital scan result 2 5 Scan time and sensitivity boost tests in self cap mode 2 5 1 Scan time test in self cap mode This chapter shows the one self cap touch electrode scan time test results on X KE17Z TSI EVB As shown in Table 8 given the NSTEP is 110 and Tnstep is 239 us by the measurement result of single scan shown as Example 1 The theoretical value can be calculated by Equation 7 an...

Page 16: ...mate the time cost There are some inevitable small errors and differences between LPTMR measurement and real TSI scan time 2 5 2 Sensitivity test result when sensitivity boost feature enable When the sensitivity boost function is enabled NSTEP is calculated by Equation 6 and the calculation of TSICNT and scan time still uses Equation 7 and Equation 8 The sensitivity boost configurations include S_...

Page 17: ... 1 4 1 2 1 8 ON 1 2 12 5 12 5 15 43 8 Figure 13 Sensitivity curve From sensitivity calculated results we can find out that the Cremoved is the key configuration to the sensitivity boost feature As Cremoved increases the sensitivity becomes better That is it is to recognize touch event Therefore users can adjust the S_CTRIM and S_XDN to quickly adjust the sensitivity of touch electrode recognition ...

Page 18: ... and the other one is used as receive RX channel For the two TSI instances of KE17Z TSIx 5 0 can be used as TX channel by configuring TSIx_MUL0 M_SEL_TX TSIx 11 6 can be used as RX channel by configuring TSIx_MUL0 M_SEL_RX The mutual cap touch electrode design of TSI0 and TSI1 is independent Each TSI instance supports the design of 6 6 touch electrodes Touch changes field through human body and re...

Page 19: ... self mode clock the SSC must be enabled for mutual mode to generate switching clock because the TSI RX signal in the mutual mode depends on the TSI_SSC0 CHARGE_NUM In the mutual mode changing the SSC charge time changes the RX signal coupled from TX channel and affects the final scan result Figure 16 Clock generation in mutual cap mode spread spectrum clocking SSC is enabled the mutual mode share...

Page 20: ...P N STEP Comparator Vci Switching Clock ph2 OFF OFF ON discharge Tx Rx charge discharge Vpre V Vpre V VDD5V GND Figure 18 Mutual capacitive mode timing Vpre is selected by TSIx_MUL1 M_VPRE_CHOOSE ΔV signal voltage RX received decided by VDD5V Cm Cm Cs TX drive mode is controlled by TSIx_MUL1 M_MOD 5 5 V is selected in Figure 18 As shown in Figure 17 and Figure 18 there are two phases controlled by...

Page 21: ...ure 19 TX drive mode 3 3 1 Single scan in mutual cap mode The digital process in mutual mode is same as self cap mode as shown in Figure 11 and Figure 12 Ci ramp up steps in per cycle is detected and accumulated by digital SINC filter Digital SINC filter outputs totally counter which can be read from TSIx_DATA TSICNT NSTEP is the result of TSI single scan in mutual cap mode as shown in Equation 11...

Page 22: ... the scan number is configured by TSI_SINC DECIMATION ORDER and CUTOFF Table 11 shows the one mutual cap touch electrode scan time test results on X KE17Z TSI EVB The actual Scan Time is calculated by LPTMR module The Counter TSICNT is read from the register when debugging the code In Table 11 give the NSTEP as 388 and Tnstep is 315 μs by the measurement result of single scan as shown in Example 1...

Page 23: ... parasitic the sensitivity boost feature can be activated by setting M_SEN_BOOST 4 0 The basis average charge current is subtracted by boost current which enlarges the signal current Different to the self cap mode mutual cap mode implements sensitivity boost by changing the current It is configurable in register TSI_MUL0 M_SEN_BOOST The current value ranges from 0 μA to 62 μA Table 12 Sensitivity ...

Page 24: ...cap mode Due to the different internal structures of self cap and mutual cap mutual cap mode can be implemented without using shield channel 4 1 Principle of self cap mode to improve liquid tolerance by enabling shield channels Figure 21 Self cap touch key with water shield function If the shelf channel is enabled a parasitic mutual capacitance Cm is created between shield electrode and self cap s...

Page 25: ...d the three shield channels are opened the waveform of the shield channel can still follow the waveform of the self cap channel 3 shield channels on 2 shield channels on 1 shield channel on Figure 22 Driving capability comparison of shield channels 4 3 Principle of mutual cap mode to improve liquid tolerance Mutual cap mode does not need the shield electrode When there is a water drop between RX a...

Page 26: ...ectric field between RX and TX can be affected to the greatest extent Figure 25 is the recommended mutual key shape The electrode of TX wraps the electrode of RX which can prevent RX from being affected by noise The number of finger has much impact on the touch sensitivity In general more fingers result in stronger noise immunity but less touch sensitivity Customer must select the right finger num...

Page 27: ... bottom layer of PCB to avoid impact of fingers Avoid crossover with other signals For the mutual mode key keep the TX trace as far as possible from the RX trace 5 3 Ground plane A proper ground plane prevents the coupling of external electromagnetic interference to the touch sensing electrodes It also acts as a shield for undesired electric fields X hatch pattern ground is recommended instead of ...

Page 28: ...the touch electrode 5 4 Overlay of the touch electrode To protect the touch electrode from the interference of the external environment overlay material must be closely attached to the surface of the touch electrode There must not be air gap between the touch electrode and the overlay Using an overlay with a high dielectric constant or an overlay with a small thickness can improve the sensitivity ...

Page 29: ...ble 13 Dielectric constants of common material Material Dielectric constant k Acrylic PMMA 2 7 4 5 Air 1 0 Tempered Glass 7 2 8 0 Polyester PET 2 8 4 5 Air has the lowest dielectric constant and other commonly used overlay materials have relatively high dielectric constants According to Equation 13 selecting an overlay with a high dielectric constant enhances the sensitivity of the touch electrode...

Page 30: ...uch sensing signals If signals must go through the touch sensing traces let them go in a different layer and perpendicular Make sure to fill in ground between groups of traces analog digital and touch If possible fill in ground between touch sensing traces Traces as thin as the PCB or film technology allows Short traces 300 mm from electrode to MCU ideally 50 mm Electrode shape corners as rounded ...

Page 31: ...the different columns and the vertical slider created when a finger slides through the rows In the same manner a touch and release condition in a certain region of the touchpad is detected by the combination of one row and one column touched at a time NXP Semiconductors Hardware design guide KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 31 37 ...

Page 32: ...e is connected to a TSI touch channel Each electrode is independent of each other reducing interference between electrodes The hatched area around the 3 5 self cap touchpad is the common area of shield electrodes and GND The function of hatched area is switched through a 0 Ω resistors Figure 31 3 5 self cap touchpad For the 6 6 mutual cap touchpad it only cost 12 touch channels to get the 36 elect...

Page 33: ...ring The spring key keeps the PCB far away from overlay to get the enhanced liquid tolerance When there is water drop on the spring key the parasitic capacitance Cs between the liquid and the ground is relatively small No mis trigger happens NXP Semiconductors Hardware design guide KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 33 37 ...

Page 34: ...odes Document AN3863 Kinetis KE17Z 13Z 12Z with up to 256 kB Flash Reference Manual document KE1xZP100M72SF1RM NXP Touch Development Guide document AN12709 7 Revision history Rev Date Description 0 05 May 2022 Initial release NXP Semiconductors References KE17Z Dual TSI User Guide Rev 0 05 May 2022 User Guide 34 37 ...

Page 35: ...NXP Semiconductors products and NXP Semiconductors accepts no liability for any assistance with applications or customer product design It is customer s sole responsibility to determine whether the NXP Semiconductors product is suitable and fit for the customer s applications and products planned as well as for the planned application and use of customer s third party customer s Customers should p...

Page 36: ...rtex DesignStart DynamIQ Jazelle Keil Mali Mbed Mbed Enabled NEON POP RealView SecurCore Socrates Thumb TrustZone ULINK ULINK2 ULINK ME ULINK PLUS ULINKpro μVision Versatile are trademarks or registered trademarks of Arm Limited or its subsidiaries in the US and or elsewhere The related technology may be protected by any or all of patents copyrights designs and trade secrets All rights reserved Ai...

Page 37: ... described herein have been included in section Legal information NXP B V 2022 All rights reserved For more information please visit http www nxp com For sales office addresses please send an email to salesaddresses nxp com Date of release 05 May 2022 Document identifier KE17ZDTSIUG ...

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