2 Self-Capacitance

Waveform Description

The capacitive self-sensing waveform of the PTC as shown in Fig 2-1 by probing the touch pin with an oscilloscope. The complete PTC acquisition waveform invoking sensor and Cc is shown in Fig 2-2. The Cc waveform cannot be observed by probing.

Figure 2-1. Sensor waveform as observed in an oscilloscope

Figure 2-2. Waveform on Cc and Sensor

*The explanation of the waveforms provided in this document are for PTC associated with TinyAVR1-series device family. There may be minor variations in PTC's present in other devices.

Operation

Capacitive self-sensing measurements on the Peripheral Touch Controller is done in phases.

Figure 2-3. Self-Cap PTC Block Diagram

* Point 3 is the free space earth to ground return path.

Phases

1

Pre-charge

2

Distribute

3

Integrate

4

Discharge

5

Precharge & Distribute

6

Integrate
Connections
  • 1→Vcc
  • 2→Vcc
  • 1→NC
  • 2→Gnd
  • 1→NC
  • 2→Gnd
  • 1→Gnd
  • 2→Gnd
  • 1→NC
  • 2→Vcc
  • 1→NC
  • 2→Vcc
Explanation

Current flows from Vcc to Cx to fully charge the external capacitor.

Current flows from Cx to Cc thereby sharing the charge.

If Cc and Cx are not same, then the charge proportional to the difference is integrated into Cint.

Both Cx and Cc are discharged.

The current flows from Vcc to Cc and Cx, thereby charging both the capacitors (which are in Series).

If Cc and Cx are not same, then the charge proportional to the difference is integrated into Cint.

The ADC measurement is performed.

*NC → Not connected

The total charge accumulated on Cint will be proportional to the difference between Cx and Cc.