Designing the Next Generation of Automotive Dome-Light Modules

Implementing the next generation of automotive Human-Machine Interface (HMI) can be challenging, given the number of cross-functional engineering disciplines required for electrical, mechanical, industrial and software development.

Cypress’s capacitive touch technology, CapSense™, offers both flexibility and customisation, allowing designers to bridge the gap between automotive interior design and HMI systems. Designing a CapSense dome light with Cypress’s programmable PSoC chip is a simple process:

1. Set up the project by adding internal resources
2. Define behaviour and relationships between input and output
3. Tune the interface

To illustrate this process, an example is outlined below, describing the design, implementation and assembly of a dome-light module, based on both capacitive sensing, and proximity detection for illumination control.

 

 

Dome-light module

The following light-module design is based on the CY8C21534-24PVXA PSoC. This PSoC is AEC-Q100 qualified with an operating temperature range from -40°C to 85°C. With four analogue blocks, and four digital blocks, enough configurable resources are offered to implement capacitive sensing, proximity sensing, and LED control, as well as other more complex features.

A free Integrated Development Environment (IDE) allows designers to configure the PSoC to fit specific applications. For the dome-light module, this would include the following resources:

• Three capacitive touch buttons (ON, OFF, and AUTO).
• Three backlighting LEDs for button illumination.
• Two proximity-sensing electrodes to control the main cabin illumination LEDs for driver and passenger sides.
• Two high-brightness LEDs for the main cabin illumination (driver and passenger sides).
• One timer for LED-dimming control.

Figure 2 shows the block diagram, excluding the power supply, for the dome-light module.

 

 

Set up the project by adding internal resources

Setting up the project is a simple matter of adding and connecting internal resources, known as user modules. In the PSoC Designer’s Device Editor view, shown in Figure 3, designers can add and place user modules into the PSoC configuration, configure those modules, and connect them to external pins.

 

 

The CapSense Sigma-Delta (CSD) user module provides capacitive sensing using the switched capacitor technique with a sigma-delta modulator to convert the sensing switched capacitor current to digital code.

Once the CSD user module is added, it then needs to be configured. This configuration is greatly simplified by the CSD wizard, which allows CapSense elements for the buttons and proximity sensors to be implemented using a drag-and-drop user interface. Once the total number of sensors (five for the dome-light module) is entered, a blank five-element placeholder appears, which can be dragged from the PSoC pin-out matrix to the sensor-element matrix. This configuration can be modified at any time during development by changing the pin assignments or modifying the number of sensors without affecting the software development in progress.

 

 

Define behaviour and relationships between input and output

Application code can then be written to set up behaviour and relationships between the input and output. With the necessary Application Programming Interfaces (APIs) readily available in PSoC Designer, initialising the PSoC and writing application code becomes a simple task. Functions called in the code listed in Figure 5 are provided APIs to enable and initialise the required user modules.

 

 

Once initialised, specific functions can be called, as shown in Figure 6, for the CSD user module. In this example, all sensors are scanned, the baseline is updated, the state of each button is checked and finally, the LEDs are controlled based on which sensor was active. The CSD_UpdateAllBaselines() function is used to prevent any sensor activation due to measurement drifts, generally caused by environmental factors such as temperature and humidity variation.

Note that the LED_Main function is not a provided API, but written to address the domelight module’s specific LED-dimming requirements.

 

 

Tune the interface

Tuning should be performed on the final system assembly, including the system overlay. One advantage of implementing CapSense in automotive HMI designs is the wide variety of materials that can be used for the interface overlay. PSoC’s CapSense can detect through almost any non-conductive material with thicknesses varying from 0.2mm to as much as 5mm.

Once assembled, an I²C-to-USB Bridge is used to send CapSense raw data from the board to the PC client used to plot it. The information visualised in the I²C-to-USB client is used to tune the CSD user module by providing system parameters such as raw counts with no button presses, raw counts with a finger present, and system noises. By understanding the data provided by the visual tool, designers can implement the optimal parameters for a robust design.

Figure 7 shows a sample of the I²C-to-USB tool with a sample tuning window used to visualise the button behaviour during tuning.

 

 

Conclusion

The design flow presented in this article, although simplified, does provide an overview of the actual flow for a PSoC-based capacitive sense system, by highlighting the three major tasks: set up the project, define behaviour, and tune.

Cypress Semiconductor also provides a graphical development environment, PSoC Express, in which no coding is necessary to develop application such as the one described above. A CSD training kit is available to get designers started in this environment.

 

 

 

The EVT0023 evaluation board is available to members of the Future Board Club. To apply for the development board, and membership of the Board Club, go to www.my-boardclub.com/manf-offers.htm This offer is free and subject to qualification.

 

Cypress Semiconductor / Design Note