Large appliances typically utilize 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. Capacitive touch sensing is commonly used for home appliance UI given that their robustness and “look and feel” 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. This article describes a different approach that integrates multiple home appliance functions into a single programmable system-on-chip (PSoC) controller that is flexible, low cost, and enables a broad range of differentiating features for an appliance.
Integrating many functions of a complex system such as a large appliance into a single IC requires a different approach to design. Specifically, programmable system-on-chip controllers have analog and digital peripherals that are interconnected with a highly configurable matrix of signal and data bus meshing that allows the creation of custom designs.
Figure 2 shows the block diagram of a PSoC configured with the multiple functions of a home appliance.
User Interface – Touch Sensing
The UI is one of the most important features that can be integrated onto a programmable system-on-chip controller. Capacitive touch sensors are aesthetically superior, easy-to-use, and have a long lifetime compared to their mechanical counterparts, push buttons, control knobs, etc. However, home appliances have stringent requirements when it comes to front panel design:
- The overlay (the die electric material placed on top of the PCB) needs to be thick, typically greater than 5mm.
- Sensors need to reject electrical noise generated by the appliance to avoid false touches.
To meet these requirements, the 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.
In addition, mechanical buttons and knobs provide tactile feedback which make it easy for a user to understand if 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. However, this kind of haptic feedback is impractical for large appliances. Therefore, when designing capacitive touch-based UIs, developers should 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 individual sensors; in Figure 3, there are seven sensors that make up the slider. Signals from all of the sensors are used to calculate the finger position on the slider. In this example, visual feedback is provided by a group of LEDs placed around the slider.
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 a shield electrode can be used to provide water resistance to the front panel. Capacitive sensing technology can also be used 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 the system can turn on the panel automatically. Multiple proximity sensors can also detect gestures. Display
User Interface – Display
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. A better way to implement custom LED multiplexing logic is by using the programmable digital blocks inside the programmable system-on-chip controller. The circuits created by the programmable digital blocks work independently of the CPU, similar to an external LED driver. Figure 4 shows the schematic of a custom LED driver logic implemented using programmable logic blocks.
Note that this design drives 20 LEDs using only 5 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 condition circuitry required is to make use of programmable analog blocks within the controller. The schematic in Figure 5 shows the integration of a load measuring circuitry 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 planned and careful design, developers can optimize their home appliance products by integrating multiple functions into a single programmable system-on-chip. Doing so can reduce BOM cost, increase flexibility (i.e., multiple families of large appliances can use a single device with only a slightly modified firmware), and provide market-differentiating features.
For more detailed information on using capacitive sensing in a programmable system-on-chip IC, please refer to the PSoC 4 CapSense design guide.