Datasheet

44 Document No. 001-66271 Rev. *B CY8C21x34/B CapSense
®
Design Guide
Figure 6-2. Capacitance Measurement with Water Drop
 C
WD
– Capacitance between the water drop and shield electrode
The purpose of the shield electrode is to set up an electric field around the touch sensors that helps attenuate the
effects of water. The shield electrode works by mirroring the voltage of the touch sensor on the shield.
Follow these guidelines to ensure proper shield operation:
 Schematic
 Layout
 Firmware Development
6.1.1.1 Schematic
Select the proper pin to drive the shield electrode out signal. The following pins should NOT be used to drive the
shield electrode out signal.
 Modulator capacitor pin (C
MOD
): P0[1] or P0[3]
 Feedback resistor pin (R
B
): P1[1] or P1[5]
 Other port pins: P0[0], P0[4], P1[0], P1[4], P2[0], P2[4], and P3[0]
6.1.1.2 Layout
Follow the layout guidelines given in Getting Started with CapSense.
6.1.1.3 Firmware Development
Step 1: Select the shield electrode signal source from one of the free digital row buses (Row_0_Output_1 to
Row_0_Output_3):
 Select Row_0_Output_1 if shield pin is P0[5], P1[5], P2[1], P2[5]
 Select Row_0_Output_2 if shield pin is P0[2], P0[6],P1[2], P1[6], P2[2], P2[6], P3[2]
 Select Row_0_Output_3 if shield pin is P0[7],P1[3], P1[7], P2[3], P2[7]
Step 2: Route the shield electrode out to the shield pin. Follow these steps:
1. Select the shield electrode out.
2. Route the Row output signal to Global out Odd/Even.
3. Connect the Global out Odd/Even to Port pin. For example, if P0[2] is selected as the shield pin:
From the user module properties, select the shield electrode out on Select Row_0_Output_2.
4. Click on Row_0_Output_2 and open the digital interconnect view as shown in Figure 6-3.