Datasheet

14 Document No. 001-66271 Rev. *B CY8C21x34/B CapSense

Design Guide

Figure 2-4. CSD Raw Counts during a Finger Touch

2.2.2 CSD with ADC Functionality (CSDADC)

The CSDADC CapSense method functions in an identical manner as CSD except that it is augmented with the ADC

functionality in addition to capacitance measurement.

2.2.2.1 ADC Features



Absolute and ratiometric ADC input modes with run-time mode switching, which allows you to easily scan

different sensor types

 Single-slope ADC for absolute voltage input mode

 Built-in calibration mechanism for the single-slope ADC

 Integrating incremental ADC for ratiometric input mode

2.2.3 SmartSense™ Auto-tuning

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

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

integration, and finally production fine-tuning before the production ramp. Tuning is an iterative process and can be

time consuming. SmartSense Auto-tuning was developed to simplify the user interface development cycle. It is easy

to use and significantly reduces the design cycle time by eliminating the tuning process throughout the entire product

development cycle, from prototype to mass production. SmartSense tunes each CapSense sensor automatically at

power up and then monitors and maintains optimum sensor performance during run time. This technology adapts for

manufacturing variation in PCBs, overlays, and noise generators such as LCD inverters, AC line noise, and switch-

mode power supplies, and automatically tunes them out. SmartSense Auto-tuning requires between 2.1 KB -3.7 KB

of flash depending on configuration and 29B of RAM per sensor.

2.2.3.1 Process Variation

The SmartSense User Module (UM) for the CY8C21x34B is designed to work with sensor parasitic capacitance in the

range of 5 pF to 45 pF (typical sensor C

values are in the range of 10 pF to 20 pF). The sensitivity parameter for

each sensor is set automatically, based on the characteristics of that particular sensor. This improves the yield in

mass production, as consistent response is maintained from every sensor regardless of C

variation between sensors

within the specified range of 5 to 41 pF. Parasitic capacitance of the individual sensors can vary due to PCB layout,

PCB manufacturing process variation, or with vendor-to-vendor PCB variation within a multisourced supply chain.

The sensitivity of a sensor depends on its parasitic capacitance; higher C

values decrease the sensor sensitivity and

result in decreased finger touch signal amplitude. In some cases, the change in C

value detunes the system,

resulting in less than optimum sensor performance (either too sensitive or not sensitive enough) or, worst case, a

nonoperational sensor. In either situation, you must retune the system, and in some cases requalify the UI

subsystem. SmartSense Auto-tuning solves these issues.