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Getting Started with CapSense

Document No. 001-64846 Rev. *I 17

measurement. The difference count should not be used in the measurement of SNR since it is a function of the

baseline update process, which involves filtering (filling the "bucket") and nonlinear threshold events.

Another factor to consider is how the signal is produced. The worst-case ON and OFF scenario should be used when

measuring SNR. If the system is designed to sense the presence of a finger, then measure SNR with a light touch of

the sensor area, and position the contact point slightly off-center. For automated testing, a worst-case finger touch

(0.1 pF) can often be simulated by an equivalent metal disc that is the size and shape of a small coin.

As an example of measuring SNR, consider the raw count waveform in Figure 2-9.

X0 = 5925 counts

X1 = 6055 counts

S = 130 counts

N = 5940 - 5925 = 15 counts

SNR = 130:15 = 8.6:1

2.4.4 SmartSense Auto-Tuning

2.4.4.1 What is SmartSense?

Tuning the touch sensing user interface is a critical step in ensuring proper system operation and a pleasant user

experience. The typical design flow involves tuning the sensor interface in the initial design phase, during system

integration, and finally production fine-tuning before the production ramp. Because tuning is an iterative process, it

can be time-consuming. SmartSense Auto-Tuning helps to simplify the user interface development cycle. Also, the

method is easy to use and reduces the design cycle time by eliminating the tuning process throughout the product

development cycle, from prototype to mass production.

2.4.4.2 What does SmartSense do?

SmartSense tunes each CapSense sensor automatically at power up and then monitors and maintains optimum

sensor performance during runtime. The number of parameters to be tuned is reduced from 17 in CSD to 4 with

SmartSense.

 Power-up tuning—SmartSense tunes the parameters of each sensor based on the individual sensor parasitic

capacitance to get the desired sensitivity for the sensor.

 Runtime tuning—Noise in the system is measured dynamically. The thresholds are adjusted accordingly for

each sensor to overcome false triggering due to dynamic variations in noise in the CapSense system.

2.4.4.3 How and where is SmartSense helpful?

SmartSense technology adapts for manufacturing variations in PCBs, overlays, and noise generators, such as LCD

inverters, AC line noise, and switch-mode power supplies and automatically tunes them out. SmartSense handles

changes in system environment, such as temperature, humidity, and noise sources such as RF, SMPS, LCD Inverter,

and AC line noise. In systems with special requirements or very high C

(greater than 45 pF), auto-tuning may not be

the ideal solution.

The following sections describe scenarios in which SmartSense is instrumental in adapting to the external noise. By

maintaining a robust signal-to-noise ratio, the false triggering of buttons is prevented.

2.4.4.3.1 Different Noise Levels in Different Designs

SmartSense technology dynamically tunes itself (adjusts noise and finger thresholds) for different noise

environments. In Figure 2-10, Design A and Design B have different noise levels. To maintain a minimum SNR of 5:1,

dynamic threshold adjustment is required. SmartSense does this automatically, which allows seamless transition from

one model to another with minimal or no tuning required.

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