C8051F970-A-GM Datasheet, PDF (139/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
18.13.5. Low-Pass Filter Adjustments
A programmable active low-pass filter is provided to limit external noise interference in CS0 operation. The filter is
programmable in two ways. The filter can be tailored for optimal performance with slow-rising signals by adjusting
the low-pass filter ramping control, CS0RP. Another control, CS0LP, allows the user to adjust the filter’s corner
frequency. For most applications, the default settings for the filter controls (CS0LP = 000b, CS0RP = 00b) should
be used.
18.13.6. Adjusting CS0 Ramp Timing: CS0RP
Determining the appropriate setting for CS0RP is one of the last in a series of related adjustments to be made for
high-resistance loads. It requires that the adjustments for gain (CS0CG), output current (CS0IA), and the reset
timing (CS0DT and CS0DR) have already been made. The adjustment values determined for those settings should
be programmed into the CS0 module when performing the CS0RP adjustment.
Configure the CS0 module to perform continuously repeated capacitance sensing operations. Using an
oscilloscope, measure the maximum rise time seen on the CS0 sensor pin. Subtract 200 ns. This is the CS0 ramp
time for this channel.
18.13.7. Adjusting CS0LP for Non-Default CS0RP Settings
The default setting for the low-pass filter corner frequency (CS0LP = 000b) gives the best sensing response for all
applications using default ramp timing (CS0RP = 00b). For applications with slower ramp timing, the corner
frequency should always be modified to match the edge-rate of the input ramp. For all non-default settings of
CS0RP (CS0RP = 01b, 10b or 11b), set CS0LP = 001b.
18.13.8. Other Options for Adjusting CS0LP
In some circumstances, it may be preferable to trade CS0 sensitivity for increased noise filtering. Decreasing the
filter’s corner frequency below the natural ramp rate of the converter will cause a lower capacitance value to be
reported. The change in capacitance due to a touch event will also be attenuated. As a result, lowering the corner
frequency will not necessarily increase the signal-to-noise ratio for capacitive touch events.
Although signal-to-noise is the figure of merit for this adjustment, there may be acceptable trade-offs in the
adjustment of CS0LP which result in an overall lower SNR but better operation over a wide range of environmental
conditions. Some applications may call for adaptive changes to the corner frequency based on measurements of
input noise, trading sensitivity for noise rejection only when necessary. Because this optional adjustment requires a
subjective trade-off between noise rejection and sensitivity, the ultimate determination of acceptable results for this
adjustment will be determined by the end application.
When performing these tests, all other CS0 configuration registers should be properly adjusted for the channel
under test. CS0LP operation can only be analyzed when the CS0 is otherwise optimally adjusted. CS0LP
adjustments should only be performed during performance tuning for a specific application in a well-defined noise
environment.
CS0LP settings adjust the CS0 response to environmental noise. As in the adjustment of CS0DR settings, CS0LP
adjustment can only be performed in a test environment with the highest expected level of ambient noise while
connected to the sensor which is specific to the intended application.
Higher settings for CS0LP cause the low-pass filter corner frequency to drop. Noise will be reduced and reported
capacitance will be reduced. This adjustment process incrementally increases CS0LP settings to determine which,
if any, of the settings provide a higher SNR. For this test, the optimum setting for CS0LP will provide higher SNR
results for the system in this high-interference environment, although the same setting is likely to reduce SNR for
the same system in a low-interference environment.
1. Begin the adjustment with CS0LP set to maximum corner frequency (CS0LP = 000b). Record a series of
CS0 output values for the sensor when it is untouched and record another series of CS0 output values from
the sensor when it is being touched. If an ITO touch panel is being tested, this sensor touch should be
performed at a sensor location on the shortest ITO trace at a location closest to the panel connector.
Calculation of SNR may be performed as described in application note AN367, “Understanding Capacitive
Sensing Signal to Noise Ratios and Setting Reliable Thresholds”.
2. Increase the value of the CS0LP setting by one (from 000b to 001b). Record a new data set and determine
its SNR. Repeat this process for all remaining CS0LP settings.
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