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 21-16. 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:
• 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) and 32 kΩ (TSI_CS2[EXTCHRG]=101). Because of
this Rs change the amplitude of noise waveform change also.
Chapter 21 Touch Sense Input (TSI)
MC9S08PT60 Reference Manual, Rev. 4, 08/2014
Freescale Semiconductor, Inc.
601
Содержание MC9S08PT60
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