Large appliances usually use some integrated circuits (IC) to achieve different functions, including user interface (UI), sensing and process control. Beautiful user interface is an important distinguishing feature of household appliances such as oven, washing machine and refrigerator. Capacitive touch sensing is a common function of the user interface of household appliances, because they are very robust and reliable, and their “appearance and feel” are unmatched by mechanical buttons. In addition to touch sensing, the user interface provides sound and visual feedback. Large appliances also need additional integrated circuits to sense / measure physical quantities, complete process and function selection, and drive final control components. This paper introduces another method to integrate many functions of household appliances into a system on a programmable chip (PSoC) controller. This method is very flexible, low cost, and can make household appliances have a lot of different characteristics.
Different design methods are needed to integrate many functions of complex systems (such as large household appliances) into a single integrated circuit. Specifically, the PLC has analog and digital peripherals, which are connected through a highly configurable signal matrix and data bus grid to support the creation of custom designs.
User interface is one of the most important functions that can be integrated into the system controller on programmable chip. The capacitive touch sensor has beautiful appearance and is easy to use. Compared with mechanical sensor, button and control knob, it has longer service life. However, the requirements of front panel design are very strict. In order to meet these requirements, capacitive sensors need to have high signal-to-noise ratio (SNR). In addition, for household appliances that need to use liquid or are surrounded by liquid, the touch pad must also be waterproof. This is because even if there are water drops or accumulated water in the covering layer, it can not cause false touch.
In addition, the mechanical buttons and knobs provide tactile feedback, which makes it easy for users to know whether the buttons have been pressed correctly or how much the knobs have been turned. Electrical appliances with touch sensing function can be set with tactile feedback, and small motors can be used to form vibration in response to touch. However, this kind of tactile feedback is impractical for large appliances. Therefore, when designing the user interface based on capacitive touch, designers should ensure that the capacitive sensor used in the design can provide sufficient visual and auditory feedback.
A radial slider is a rotary control device – similar to a mechanical knob – that is usually used to control continuously varying amounts, such as the heating level (temperature) of an oven. The radial slider can detect the finger movement and read the rotation as the required input value. The slider layout in PCB is composed of several sensors. The signals generated by all the sensors are used to calculate the position of the fingers on the slider. In the above example, a set of LEDs placed around the slider is responsible for providing visual feedback. These LEDs can only be turned by the controller, and after turning, they can track the position of the user’s fingers. In addition, auditory feedback is provided by a piezoelectric speaker, which is driven by a PWM integrated on the controller.
Other components of the PCB, such as shielding electrodes, can be used to provide waterproofing to the front panel. Capacitive sensing technology can also be used to add other differentiation features, such as proximity sensing, so as to make the user interface more intuitive. Through proximity sensing, the front panel can detect the presence of the user’s hand, because as the hand approaches, the system will automatically open the panel. Multiple proximity sensors can also complete pose detection. For more details on the use of capacitive sensing technology in programmable system on chip ICs.
Segment LCD and LED are usually used in user interface to display alphanumeric data. Segment LCD has low cost and power consumption, and can be directly driven by the system controller. The viewing angle of Segment LED is very good. Compared with LCD, it does not need backlight.
Segmented LEDs can also be multiplexed to reduce the number of pins required. In general, this multiplexing is done in firmware. Firmware based LED drivers consume valuable CPU cycles, and display refresh may not be uniform or reliable, depending on the firmware. A better way to implement custom led multiplexing logic is to use programmable digital module in the system on chip controller. The circuit created by the programmable digital module can work independently of the CPU, similar to the external LED driver.
Large appliances contain many analog sensors, which can be used to measure temperature, liquid level, etc. An effective way to reduce the number of external signal conditioning circuits is to use the programmable analog module in the controller.
Programmable analog and digital modules can also simplify the final control elements in appliances, such as heating coils or motors. Through careful design of detailed plan, designers can integrate multiple functions into a single programmable system on chip, so as to optimize household appliances. This can not only reduce the cost of BOM and improve flexibility (that is, multiple series of large appliances can use the same device, only need to modify the firmware slightly), but also provide differentiated marketing features.