Chapter 7:
Designing a realistic touch sensing application using STM32 MCU
7.2 Connecting the STM32 MCU and performing analysis
Connecting the touch sensor with the STM32 MCU
Now that our physical model of the touch sensor is ready, we must connect it to the controller pins, in order to be able to simulate the entire system. This step is also known as System Configuration. I’ll spare you the details (which you can find here) but what you essentially have to do is connect the appropriate GPIO pins of the controller with the traces of the touch sensor.
Since Fieldscale SENSE offers STM32 controllers, with a few clicks you can choose the controller you want to use (in our case STM32F071C8Tx), and connect its pins with the traces. In Figure 3 you can see how the system configuration looks once everything is set up.
Figure 3. The system configuration.
Running analysis on the controller - touch sensor system
We can now perform the holy grail of capacitive system analysis: counts analysis. Having created the sensor and the stackup, selected the placement of the pointer, selected the controller and connected it to the touch sensor, you can now perform counts analysis. counts analysis in SENSE shows you exactly what the controller measures in each case. So, by performing the counts analysis, you will get 5 measurements for each controller pin (that is 20 measurements in total, since we have 4 touch buttons connected to the controller’s pins).
But as we discussed in previous chapters, what’s more important than counts is delta counts. When the pointer is near a button, its counts become lower than when it is not. This difference in counts is what matters for detecting touch events. With SENSE, you can access this information with a few clicks once you have performed the counts analysis. In Figure 4, you can see a visualization of the delta counts for our layout.
Figure 4. Visualization of delta counts
As a general rule, delta counts between 100-200 are regarded as acceptable values that ensure touch functionality. This means that values lower than 100 cannot be detected by the controller, whereas values higher than 200 result in too high sensitivity.
In the sensor of the example, you can see that the delta counts are higher than the acceptable value of 200, and therefore this sensor is prone to false touches. This means that we need to redesign our sensor, even though we followed the design guidelines indicated in STM’s documentation.
Without SENSE, one would have to redesign the touch sensor, create physical prototypes, and then perform lab measurements to verify their design. With SENSE, none of this is needed. You can simply tweak your design, run your analysis and have new results in a matter of minutes.
This again proves that there is no one-fits-all solution in touch sensor design to ensure the sensor-controller compatibility. STMicroelectronic and Fieldscale are both well aware of this. That is why, STM has partnered with Fieldscale and provides its STM32 controller family for direct simulation via Fieldscale’s software. With SENSE , engineers can:
- • Design or import capacitive touch sensor layout as a standard DXF or Gerber file(s).
- • Evaluate the Touch Sensor by extracting results about the Sensitivity, Capacitance and Resistance.
- • Analyze the effect of any pointer touching at any location of the design, including finger, bare or gloved, and stylus.
- • Identify and correct EMI related problems. Add RF conducted noise into the system, to simulate the testing procedure as specified in the IEC61000-4-6 standard, to evaluate the noise immunity of your system.
Key takeaways from this section:
Following design guidelines cannot fully guarantee the seamless cooperation between the controller and the sensor
SENSE simplifies the process of verifying your design, or exploring alternative design choices
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