3 Mutual Capacitance

Waveform Description

Mutual sensing measure the capacity between and mutual TX and a mutual RX. On probing the X-line (the mutal TX), the waveform from Fig 3-1 will be observed. Fig. 3-2 shows again the waveform on sensor as well as Cc. The sensor waveform is scopeable, the Cc waveform not. This line is the mutal RX or Y line.

Figure 3-1. Sensor waveform as observed in an oscilloscope

Figure 3-2. 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 mutual-sensing measurements on the Peripheral Touch Controller is done in phases.

Figure 3-3. Mutual Cap PTC Block Diagram

Phases

1

Y Line Pre-charge

2

Distribute and Integrate

3

Distribute and Integrate

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

The current flows from Vcc to Cc and Cxy, thereby charging both the capacitors.

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

The current flows from Vcc to Cxy and Cc, thereby charging both the capacitors.

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

The ADC measurement is performed.

After these phases, the total charge accumulated on Cint will be proportional to the difference between Cxy and Cc.

The waveform timing for different PTC configurations can be found here.