- Featuring analog and digital peripherals interconnected with a highly configurable matrix of signal and data bus meshing, programmable system-on-chip (PSoC) controllers provide the flexibility to get better products to market faster.
- Custom digital blocks can be used to multiplex LEDs, offloading processing tasks from system CPU.
- Programmable analog blocks can reduce the amount of external signal-conditioning circuitry required to integrate multiple analog sensors.
PSoC controllers speed design of smart home appliances
Editor’s Note: This article appears courtesy of EDN, which hosted the original version.
Large appliances typically leverage several integrated circuits (IC) to enable different functions, including the user interface (UI), sensing and process control. An aesthetically pleasing UI is a major differentiating feature for home appliances such as ovens, washing machines and refrigerators. Home appliance UIs commonly use capacitive touch sensing, given that the robustness and “look and feel” of the interface are unmatched by mechanical buttons. In addition to touch sensing, a UI has to provide audio and visual feedback. Large appliances also require additional ICs for sensing/measuring physical quantities, process and feature selection and driving the final control elements. Instead of using multiple ICs, makers can choose an approach that integrates multiple home appliance functions into a single programmable system-on-chip (PSoC) controller. The solution is flexible, low cost and enables white goods manufacturers to easily add a broad range of differentiating features to their products.
Figure 1: Programmable system-on-chip (PSoC) controllers (right) offer a higher level of integration and functionality than MCU-based systems (left), cutting costs and simplifying product differentiation.
Integrating many functions of a complex system such as a large appliance into a single IC requires a different approach to design. Specifically, PSoC controllers have analog and digital peripherals that are interconnected with a highly configurable matrix of signal and data bus meshing that enables the creation of custom designs.
Figure 2: Block diagram of PSoC configured for home appliances shows how the approach simplifies the addition of functionality like displays and touch sensing to speed the development of custom designs.
User interface–touch sensing
The UI is one of the most important features that can be integrated onto a PSoC controller. Capacitive touch sensors are aesthetically superior, easy-to-use and have a long lifetime compared to their mechanical counterparts of push buttons, control knobs, etc. Home appliances, however, have stringent requirements when it comes to front panel design:
- The overlay (the dielectric material placed on top of the PCB) needs to be thick, typically greater than 5 mm.
- Sensors need to reject electrical noise generated by the appliance to avoid false touches.
To meet these requirements, capacitive sensors need to have a high signal-to-noise ratio (SNR). Moreover, appliances that are used with or around liquids need a touch panel that is also water resistant. This is because droplets of water or a pool of water on the overlay must not cause false touches.
Mechanical buttons and knobs provide tactile feedback that makes it easy for a user to understand whether a button has been pressed properly or how much a knob has been turned. Appliances with touch sensing can have haptic feedback as well, using small motors for creating vibrations in response to a touch; this kind of haptic feedback is impractical for large appliances, however. When designing capacitive touch-based UIs, developers should thus make sure that adequate visual and audible feedback is provided for the capacitive sensors used in the design.
Consider the following example of a radial slider. A radial slider is a rotary control, similar to a mechanical knob, commonly used to control a continuously varying quantity such as the heating level (temperature) of an oven. The slider detects finger movement and the degree of rotation is read as the desired input. The slider layout on the PCB is actually made up of multiple individual sensors (see figure 3). Signals from all of the sensors are used to calculate the finger position on the slider.
Figure 3: In this radial slider, seven sensors provide signals used to calculate finger position. LEDs located around the slider provide visual feedback to track finger position; a piezo speaker, driven by a PWM integrated on the controller, can provide additional audible feedback.
In the example shown, a group of LEDs placed around the slider provide visual feedback. These LEDs are turned on by the controller in such a way that it tracks the user’s finger position. In addition, audible feedback can be provided by a piezo speaker, driven by a PWM integrated on the controller.
Additional PCB elements such as shield electrodes can be used to provide water resistance to the front panel. Capacitive sensing technology can also be applied to add other differentiating features such as proximity sensing to give a more intuitive feel to the UI. Proximity sensing allows the front panel to detect the presence of a user’s hand as it approaches so that the system can turn on the panel automatically. Multiple proximity sensors can also detect gestures.
Segment LCDs and LEDs are commonly used in UIs to display alphanumeric data. Segment LCDs are relatively inexpensive, consume very low power, and can be directly driven by the system controller. Segment LEDs offer good viewing angles and require no backlighting compared to LCDs.
Segment LEDs are also multiplexed to reduce the number of pins required. Typically, this multiplexing is done in the firmware. Firmware-based LED driving consumes valuable CPU cycles and the display refresh can be uneven or unreliable, depending on the firmware. Programmable digital blocks inside the PSoC controller offer a more effective way to implement custom LED multiplexing logic (see figure 4). The circuits created by the programmable digital blocks work independently of the CPU, similar to an external LED driver.
Figure 4: Custom LED driver using programmable digital blocks delivers more effective display performance in a more efficient design; note that this version drives 20 LEDs using only five pins.
Large appliances contain multiple analog sensors that measure quantities such as temperature, liquid level, etc. An efficient way to reduce the amount of external signal-conditioning circuitry required is to make use of programmable analog blocks within the controller. This technique can be used, for example, to integrate load-measuring circuitry with temperature compensation (see figure 5).
Figure 5: PSoC controllers feature programmable analog blocks that reduce the amount of external signal-conditioning circuitry required, as in this example of load-measuring circuitry integrated with temperature compensation.
Programmable analog and digital blocks can also simplify driving the final control element in the appliance, such as a heating coil or motor.
Through careful planning and design, developers can optimize their home appliance products by integrating multiple functions into a single PSoC. Doing so can reduce BOM cost, increase flexibility (multiple families of large appliances can use a single device with only a slightly modified firmware) and provide market-differentiating features.
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