Figure 20. TSI sensitivity change curve when current boost increase
4 Shield channels
Shielding methods are used to eliminate or the environmental influences, such as, temperature drifts, humidity on PCB, or water
droplets on the touch control panel.
Shield electrode can reduce mis-trigger risk induced by water drop, oil, steam, and other environmental influence. For KE17Z,
each TSI module has three shield channels on CH4, CH12, and CH21. These shield channels can be enabled and selected by
configuring TSIx_MOD[S_W_SHIELD]. Shield channels can work as touch channels when turned off.
Shield electrode is only used in self-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 (C
m
) is created between shield electrode and self-cap sensing
electrode. When PH1 is turned on, C
x
is charged and C
m
is cleared. When PH2 is turned on, C
x
charges C
m
and C
i
.
• When the hand touches self-cap electrode, C
x
becomes larger and C
m
decreases. When PH2 is on, the current charging
C
i
becomes larger and the number of charging times decreases, so the count number decreases.
NXP Semiconductors
Shield channels
KE17Z Dual TSI User Guide, Rev. 0, 05 May 2022
User Guide
24 / 37