48.5.14 Spread spectrum clocking
Figure 48-8. Spread spectrum clocking
In Capacitance touch sense systems, the baseband signal is narrow band and it is nearing
the DC, while the noise is a wideband noise. For lower cost, the cut-off frequency of the
anti-aliasing low pass filter at analog front end is not low enough comparing with the
sampling frequency, so when sampling, the noises that frequency is nearing sampling
frequency will be overlapped to baseband. For solving this problem in Capacitance touch
sense systems, a low cost SSC can be involved. T he SSC’s center frequency and
frequency span range should be flexible enough for handling various frequency noises.
With this SSC, the noises that frequency is nearing sampling frequency can be spanned to
a wider frequency range instead of a single peak frequency noise, and only parts of the
noise is overlapped into baseband (because baseband is a narrow band), so SNR can be
promoted.
This SSC is composed of 5 components, they work together and generate the
configurable center frequency and configurable span frequency range, they are:
1. Configurable registers, generating all configurable settings for component 3/4/5 using
TSI_SSC0/ TSI_SSC1/ TSI_SSC2 registers;
2. State machine engine, controlling and monitoring the component 3/4/5;
3. A configurable counter for generating “1”, the max value of the counter is controlled
by TSI_SSC0[BASE_NOCHARGE_NUM];
Chapter 48 Touch Sensing Input (TSI)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
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