NXP Semiconductors MKW01Z128, Reference Manual

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The following flow chart shows how to detect touch with noise sense and normal
capacitive sense.
Start Channels
Scan
Set first channel
Noise detection/
sense algorithm
Change channel
Last channel?
Detected noise
low for capacitive
scan?
End Channels
Scan
Perform
capacitive scan
Yes No
No
Yes
Figure 35-8. Detection of touch with noise sense and normal capacitive sense flow chart
One example of noise detection mode is shown in the following figure. in this figure the
TSI is working in capacitive mode until 28 µs (T1) when it is changed to noise detection
mode. In noise detection mode the selected pad is biased with 0.815V and all AC
waveform in this pad is caused by a noise source external to the MCU.
It is possible to observe in the following figure that, in noise detection mode, the clkref
output has the peak detection and the number of detected peaks can be counted or used by
digital block. The clkext output has the internal oscillator output and can be used to set
the maximum noise detection time window.
The waveform of the following figure shows two operations during noise detection mode.
Again, this waveform is captured from Freescale internal design simulation data, the
actual useful points for noise detection should be consisted with the table provided above.
• The V(vp) and V(vm) thresholds are changed in 34.4 µs (T2).
• The Rs series resistance value is changed between 184 kΩ
(TSI_CS2[EXTCHRG]=011b), 77 kΩ (TSI_CS2[EXTCHRG]=100) and 32
kΩ(TSI_CS2[EXTCHRG] = 101). Because of this Rs change the amplitude of noise
waveform change also.
Chapter 35 Touch Sensing Input (TSI)
MKW01Z128 MCU Reference Manual, Rev. 3, 04/2016
Freescale Semiconductor, Inc. 707