Cypress CapSense CY8C20x36 Guía de usuario descargar pdf (Pagina 41)

Getting Started with CapSense

Document No. 001-64846 Rev. *I 41

3.4 Software Filtering

Software filters are one of the techniques of dealing with high levels of system noise. Table 3-5 lists the types of filters

that are useful for CapSense.

Table 3-5. CapSense Filter Types 49

Type

Description

Application

Average

Finite impulse response filter (no feedback) with

equally weighted coefficients

Periodic noise from power supplies

IIR

Infinite impulse response filter (feedback) with a step

response similar to an RC filter

High frequency white noise (1/f noise)

Median

Nonlinear filter that computes median input value

from a buffer of size N

Noise spikes from motors and switching power supplies

Jitter

Nonlinear filter that limits current input based on

previous input

Noise from thick overlay (SNR < 5:1), especially useful for

slider centroid data

Event-Based

Nonlinear filter that causes a predefined response to

a pattern observed in the sensor data

Commonly used to block generation or posting of

nonexistent events

Rule-Based

Nonlinear filter that causes a predefined response to

a pattern observed in the sensor data

Commonly used during normal operation of the touch

surface to respond to special scenarios such as accidental

multi-button selection

3.4.1 Average Filter

An average filter is a finite impulse response (FIR) filter with equal-weighted coefficients. The average filters work well

with periodic noise that is attenuated by spacing the samples out over one noise cycle. Sample spacing is not critical.

For example, power line noise can be anywhere from 50 Hz to 60 Hz. Without adjusting the sampling rate, the

average filter works well for 50-Hz and 60-Hz noise. Figure 3-12 shows a sample rate that is synchronized with a

simple periodic waveform. There is no feedback path in this filter.

Figure 3-12. Synchronized Sample Rate

The general equation for an average filter is:























 



  



     Equation 16

Figure 3-13 and Figure 3-14 illustrate the results of using an average filter on real CapSense data using the 16-

sample filter equation:























 



  



   Equation 17

1 2 ... 36 37 38 39 40 41 42 43 44 45 46 ... 74 75

Comentarios a estos manuales

Sin comentarios

Cypress CapSense CY8C20x36 Guía de usuario Pagina 41

Comentarios a estos manuales