C8051F970-A-GM Datasheet, PDF (138/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
18.13.3. Adjusting Secondary Reset Timing: CS0DR
Adjustments for CS0IA and CS0DT should be set to their maximum value (CS0IA = 001b, CS0DT = 111b) while the
CS0DR adjustments are being performed.
Because the only function of DR is to reduce the effect of environmental noise, establishing the proper DR
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.
Increasing the CS0DR adjustment does not increase the level of possible noise rejection, it only changes the
amount of time that the CS0 module will wait for the secondary reset circuit to finish its noise-reduction operation.
Resistive sensors require longer CS0DR operating periods, and their CS0DR settings will be necessarily higher.
Higher settings for CS0DR cause the CS0 conversion process to slow substantially. The adjustment method is
intended to find the lowest (fastest) CS0DR setting that delivers full function.
1. Begin the adjustment with CS0DR set to maximum delay (CS0DR = 11b). Record a series of CS0 output
values for the sensor when it is being touched.
Note: If an ITO touch panel is being tested, this sensor touch should be performed at the sensor location at the end of the
longest ITO trace. The series of CS0 output values should be large enough that a reliably repeatable determination of
standard deviation can be made, a hundred samples or greater (but typically less than 10,000 samples).
2. For this test, the standard deviation of data in the series is the figure of merit used to define the level of
noise received by the CS0 converter. The secondary reset circuit reduces noise. An increase in standard
deviation indicates that the secondary reset circuit is no longer working optimally. The best adjustment
point for CS0DR is the lowest setting for which there is an acceptably-low standard deviation.
3. Decrease the value of the CS0DR setting by one (from 11b to 10b). Record a new data set and determine
its standard deviation. Repeat this process for CS0DR settings 01b and 00b. Compare the standard
deviations calculated for the four CS0DR settings. Select the lowest CS0DR setting for which there is not a
significant increase in standard deviation.
18.13.4. Adjusting CS0 Ramp Timing: CS0IA
In the presence of larger series resistors between the device pin and the capacitive sensor, it is necessary to also
adjust the ramp time for the CS0 conversion. This is done by using CS0IA to modify the source current used to
charge up the capacitive sensor. If this source current and the series impedance are both high, the CS0 module will
“see” less of the capacitor on the other side of the impedance. Reducing the current allows the pin voltage to more
directly reflect the voltage at the capacitive sensor.
The adjustment for CS0IA should be performed while CS0DR and CS0DT bits are set at their maximum values
(CS0DR = 11b, CS0DT = 111b).
1. Begin the adjustment with CS0IA set to minimum current (CS0IA = 001b). Measure the untouched average
CS0 result for the channel under test.
Note: When calibrating CS0 for use with an ITO panel, consider the use of an artificial finger: a small (¼” O.D.) washer (#2
regular, #4 narrow) wired through a 1000pF capacitor to ground. Select a touch point at the on the farthest end of the
longest row. Find the point where maximum response is returned from the CS0 conversion.
2. Record the average touched CS0 value with CS0IA = 001b. The touched value should be higher than the
untouched value. The magnitude of the difference between the touched and untouched average CS0
values is the figure of merit for touch sensitivity.
3. Increase the current control CS0IA by one and repeat the touched and untouched CS0 measurements.
Repeat this step until values have been recorded for all eight CS0IA settings (including CS0IA = 000b,
which is the highest current setting). As CS0IA is changed, the average sensitivity of the CS0 value may
begin to decrease significantly.
4. CS0IA should be chosen such that there is not a significant decrease in sensitivity due to resistance. Select
the CS0IA setting that occurred prior to the observed drop in CS0 touch sensitivity.
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