User guide

12 Document No. 001-64846 Rev. *A Getting Started with CapSense®

Figure 2-7. CSD Equivalent Circuit

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Equation 3

Where:

= switching frequency

For an in-depth discussion of Cypress’s CSD sensing method refer to

= sensor capacitance

PSoC® CY8C20x66, CY8C20x66A,

CY8C20x46/96, CY8C20x46A/96A, CY8C20x36, CY8C20x36A Technical Reference Manual (TRM).

2.3.4 CapSense Successive Approximation Electromagnetic Compatible (CSA_EMC)

Cypress’s CSA_EMC method also uses a switched capacitor circuit on the front end of the system to convert the

sensor capacitance to an equivalent resistor. An internal constant current source called the iDAC is calibrated with a

successive approximation procedure until a preset voltage develops across the equivalent resistor. This baseline

voltage is measured using a single slope ADC. When a finger is on the sensor the capacitance increases and the

equivalent resistance decreases. This causes the voltage across the resistor to decrease and the ADC output

increases. This results in an increase in the digital count.

The CSA_EMC method requires a single dedicated pin and a single external component, C

mod

Figure 2-8. Block Diagram of CSA_EMC

. This is an Integration

capacitor that is used by the single slope ADC.

iDAC

Oscillator

16-bit

Timer

Data

Processing

MOD

External Components

Internal Low-pass Filter

REF