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CSD measures a sensor’s self-capacitance; CSX measures mutual capacitance between transmit (Tx) and receive (Rx) electrodes. They are different sensing architectures, not interchangeable settings: device support, electrode layout, parasitic capacitance, noise, response time, power and tuning all affect which approach fits a design. For conventional buttons, sliders and touchpads, Infineon recommends considering SmartSense when its capacitance, scan-time and memory requirements are met; SmartSense currently supports CSD widgets, while CSX requires manual tuning.
What CSD and CSX measure
CSD: self-capacitance
CSD stands for CapSense sigma-delta. It measures the capacitance of an individual sensor. In the architecture described in Infineon’s AN85951 PSoC 4 and PSoC 6 MCU CapSense Design Guide, a switched-capacitance circuit converts sensor capacitance into current, an analog multiplexer selects the sensor current, and a current-to-digital converter generates the raw count.
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CSX: mutual capacitance
CSX means CapSense crosspoint. It measures mutual capacitance between Tx and Rx electrodes: a Tx signal is applied and the Rx response is measured. Infineon’s third- and fourth-generation description includes clocked Tx excitation, an 8-bit IDAC and sigma-delta converter, AMUXBUS A, clock generation, and external capacitors. Hardware details depend on the device generation, so those components should not be assumed for every PSoC 4 part.
How to choose between CSD and CSX
Start with the target MCU’s documentation and the intended electrode geometry, not just the PSoC family name. The sensor arrangement, electrode count, available pins, parasitic capacitance, board routing and CapSense component generation constrain the choice. For example, the guide says PSoC 4100 does not support the CSX implementation it describes; support must be checked for the exact part.
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- Consider CSD when the design uses self-capacitance sensors and the target part supports the required widget and configuration.
- Consider CSX when mutual-capacitance Tx/Rx sensing suits the electrode design and is supported by the exact device.
- Evaluate the whole system when liquid exposure, external noise, response time or power is important; the sensing method alone does not determine the result.
What Infineon’s generation figures do—and do not—show
AN85951 compares three CapSense generations using stated test conditions. Its SNR values are for VDD = 5 V, no firmware filter, Cp approximately 33 pF and Cf = 0.1 pF. These are guide comparison figures, not universal application guarantees or independent test results.
| CapSense generation | SNR under AN85951 table conditions | Sensor parasitic-capacitance range in AN85951 |
|---|---|---|
| Third | 5:1 | 5–45 pF |
| Fourth | 6.5:1 | 5–200 pF |
| Fifth | 48:1 | 2–200 pF |
Use these figures to understand the guide’s comparison, then verify the architecture and operating conditions for the selected MCU and validate performance on the intended PCB.
Choose a tuning method
SmartSense for supported CSD widgets
Infineon describes SmartSense as a tuning method that automatically sets sensing parameters based on user-specified finger-capacitance values and continuously compensates for system, manufacturing and environmental changes. The guide recommends it for conventional buttons, sliders and touchpads when sensor capacitance, scan time and memory requirements are suitable. It currently supports CSD widgets only; CSX widgets must be tuned manually.
Manual tuning for CSX or tighter control
Manual tuning takes more engineering effort but allows closer control of response time and power and supports specialized applications such as proximity and liquid-level sensing. SmartSense can also help establish initial hardware parameters before a switch to manual tuning. Infineon’s PSoC 4 code examples include manual CSD button and slider tuning and manual CSX button tuning with the CAPSENSE Tuner.
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Layout and noise considerations
Sensor construction and placement, pin assignment, trace routing, grounding, crosstalk and tuning all influence sensing performance. AN85951 advises keeping switching signals—including PWM, I2C and LED signals—away from CapSense traces; one stated guideline is at least 4 mm separation with hatched ground between the sensor traces and switching signals. Treat that as a design guideline to check against the device-specific guidance and validate on the actual board, rather than as a guarantee against interference.
Liquid and external-noise cases
For liquid-related false activations on mutual-capacitance sensors, Infineon’s example index documents a hybrid scan that measures self-capacitance on Tx and Rx nodes as well as mutual capacitance, using CSD and CSX modes. The index also provides multi-frequency scanning examples for CSD and CSX intended to mitigate false touches under external noise. These examples demonstrate possible approaches; they do not guarantee performance on every electrode layout or board.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Practical implementation path
- Confirm the exact MCU and component. Check device support, architecture generation, pin availability and widget limitations in the PSoC 4 Capacitive Sensing (CAPSENSE) ADC component documentation and the target device documentation.
- Match sensing mode to the electrode design. Determine whether the board implements self-capacitance sensors or Tx/Rx mutual-capacitance electrodes, and account for parasitic capacitance and routing.
- Select the tuning workflow. Use SmartSense only where its CSD support and capacitance, scan-time and memory requirements fit. Plan manual tuning for CSX, or when the application needs tighter control or specialized behavior.
- Lay out and validate the board. Apply the device-specific routing and grounding guidance, isolate switching signals where practical, then test under the intended operating conditions.
- Use an example that matches the problem. The official PSoC 4 code-example index includes CSD and CSX tuning, hybrid liquid-tolerant scans, multi-frequency scans, pipeline scans, proximity examples and low-power variants. The PSoC 4 application-note index lists AN85951 and related notes, including AN92239 on proximity sensing and AN234231 on lowest-power CapSense for PSoC 4000T.
- Check compatibility before migration. Infineon states that CAPSENSE ADC v3.0 and later are not backward compatible with CAPSENSE_CSD_P4 v2.40 or older. Verify the exact component version and required feature set before upgrading.
Official examples and hardware
The code-example index references CY8CKIT-040T and CY8CKIT-024 hardware in relevant examples. A compatible PSoC 4 CapSense development kit can help reproduce examples or prototype, but the current availability, package contents, board revision and fit for a particular MCU are not established here. Check the kit’s compatibility before purchase.
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