Peripheral Touch Controller (PTC) Operation Manual
Block Diagram
The Peripheral Touch Controller (PTC) block diagram can be observed in Figures 1 and 2.
Figure 1: PTC Block Diagram for Self-Capacitance
Figure 2: PTC Block Diagram for Mutual Capacitance
- CX/CXY → Sensor Capacitance
- CC → Internal Compensation Capacitance
- CINT → Integration Capacitor
- Rx/Rsx/Rsy → Series Resistance
Self-Capacitance
Waveform Description
The capacitive self-sensing waveform of the PTC is shown in Figure 3, captured by probing the touch pin with an oscilloscope. The complete PTC acquisition waveform, invoking sensor and CC, is shown in Figure 4. The CC waveform cannot be observed by probing.
Figure 3: Sensor waveform as observed in an oscilloscope
Figure 4: Waveform on CC and Sensor
*The explanation of the waveforms provided in this document is for the PTC associated with the TinyAVR1-series device family. There may be minor variations in the PTCs present in other devices.
Operation
Capacitive self-sensing measurements on the PTC are done in phases.
Figure 5: 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 the 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 the 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.
Mutual Capacitance
Waveform Description
Mutual sensing measures the capacity between a mutual TX and a mutual RX. On probing the X-line (the mutual TX), the waveform shown in Figure 6 will be observed. Figure 7 shows, again, the waveform on the sensor as well as CC. The sensor waveform can be viewed on an oscilloscope, but the CC waveform cannot. This line is the mutual RX or Y line.
Figure 6: Sensor waveform as observed in an oscilloscope
Figure 7: Waveform on CC and Sensor
*The explanation of the waveforms provided in this document is for the PTC associated with the TinyAVR1-series device family. There may be minor variations in the PTCs present in other devices.
Operation
Capacitive mutual-sensing measurements on the PTC are done in phases.
Figure 8: 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 the 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 the same, then the charge proportional to the difference is integrated into CINT. The Analog-to-Digital Converter (ADC) measurement is performed. |
After these phases, the total charge accumulated on CINT will be proportional to the difference between CXY and CC.
