1.1. CSA Sensing Method
CSA stands for CapSense with Successive Approximation. CSA is only implemented in the CY8C20x34 PSoC device family.
Figure 3.CSA Configuration of CapSense
A block diagram of the CSA configuration is shown in Figure 3. CSA operates as follows: Switches SW1 and SW2 and the CapSense sensor CX form a switched capacitor network with an equivalent circuit of a resistor to ground. With the iDAC set to a calibrated level, and SW1 and SW2 switching, the average voltage on CMOD settles at a level that varies with the value of CX. Setting the iDAC to a low current level with SW2 open, the voltage on CMOD ramps up. The time for the ramp voltage on CMOD to reach VREF is an indication of the value of CX. The timer on the output of the comparator converts the ramp time to a digital value. Self-calibration of the system is accomplished through a successive approximation binary search to determine iDAC setting necessary to keep voltage on CMOD at VREF when no finger is present. Individual calibrated iDAC settings are stored for all sensors. When a finger is present, the voltage on CMOD settles at a lower voltage, requiring more time to reach the threshold voltage VREF, as shown in Figure 4. If (t2-t1) is long enough, the sensor state is in Finger-Present state, otherwise the button is in the Finger-Absent state.
An internal capacitor, programmable up to 100 pF, can be used for CMOD, but a larger external capacitor improves performance: 1000 pF for buttons and sliders and 10 nF for proximity sensors. Series resistors, 560 ohms, are recommended in-line with all CapSense inputs to improve RF immunity.
Figure 4. CSA Waveform Changes With Finger Absent/Present
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