Key Points for Selecting MCU in Home Appliance Development

Today, microcontroller units (MCUs) are embedded in a wide range of systems, including home appliances, mobile devices, vehicles, industrial equipment and infrastructure. Many home appliances, such as refrigerators, rice cookers and air conditioners, are now controlled by MCUs. This article takes a home bakery appliance as an example and introduces the role of the MCU in the operation of the appliance, as well as the requirements for microcontroller integrated circuits (ICs) in other home appliances. ROHM offers a line-up of MCUs that utilize unique, low-power technology. This article introduces a usage example of the ML62Q2700, a 16-bit general-purpose MCU, as a solution for home appliances. A video ‘Microcontroller Tips’ is also available to help you learn the basics of these MCUs.

The Problem with Entering the Cooking Appliance Market

Recently, an equipment manufacturer asked how microcontroller ICs should be selected for the development of home appliances. This company manufactures commercial kitchen appliances for use in food factories, but was now considering entering the market for general consumer cooking appliances. An in-house team was formed to plan and develop MCU-controlled cooking appliances, and was tasked with researching the MCUs embedded into the devices, as well as designing and implementing MCU software. However, although the development team members had experience with industrial processors, they had little experience with MCUs for home appliances, and there were few engineers familiar with microcontroller design.

What aspects should be considered in selecting a microcontroller IC for developing home appliances?

Four Challenges in Developing Home Appliances

In developing home appliances, the common requirements for many products are a user-friendly interface, electricity and energy savings, reduction of component and manufacturing costs, and a development environment that reduces the complexity of microcontroller design.

Here, using a home bakery as an example, these challenges and the key points to consider when evaluating microcontroller ICs to meet these issues are explained.

The Cooking Appliance is Controlled by the MCU

A standard home bakery appliance has the structure shown below. The housing is equipped with a temperature sensor, an electric heater and a motor, and contains an inner pot that holds the bread ingredients.

Figure 1 Structure of a Home Bakery

A home bakery performs the four processes “kneading,” “resting,” “fermentation” and “baking” on the bread ingredients in the inner pot. The processing time and number of times for each process, as well as the timing for adding dry yeast and other ingredients, vary depending on the type of bread to be made (e.g., loaf, quick bread, French bread, whole grain bread, etc.), color, and room temperature. An example of home bakery cooking processes is shown in Figure 2.

Figure 2 Example of Home Bakery Cooking Processes

In addition to managing the cooking processes, the microcontroller measures the temperature, controls the temperature of the inner pot, controls the motor for bread ingredient mixing, and processes user interface using sensors. Table 1 shows an example of the requirements for a home bakery.

Table 1 Requirements for a Home Bakery

Manufacturers’ expertise is incorporated into the time for each cooking process, mixing control for kneading, and heating control for fermentation and baking. For example, some products are equipped with sensors that directly measure the temperature of the dough, while others are equipped with a high thermal power IH heater (induction heating coils).

MCU Selection Points

When selecting a MCU for home appliances, the following should be considered.

1) User Interface – User-Friendly with Voice Guidance

Many home appliances are equipped with buttons for information input and LEDs to indicate the status of the machine. Some have LCD panels that convey richer information using both text and graphics.

The home bakery appliance used in this example has several buttons and one LCD panel. The buttons are connected to the microcontroller’s GPIOs (general-purpose input/output) to determine whether they are on or off. The LCD panel is controlled by connecting to the microcontroller’s built-in LCD driver.

An increasing number of products nowadays use a combination of on-screen display and audio to guide the user. The voice guidance function is useful for addressing user issues, such as ‘the text on the display is too small to read’ or ‘the operation is difficult to understand.’

To play audio on a microcontroller, it is common to use audio decoding middleware. Compressed audio data is decoded by the CPU, and audio signals are generated by a D-A converter or PWM and then output to an external amplifier or speakers. Some microcontrollers also have a built-in audio decoder circuit. With this type of microcontroller, audio playback can be performed without consuming CPU resources.

2) Power and Energy Savings – Energy-Efficient Designs at Both System and Component Levels

Some home appliances, such as heat-exchange air-conditioning equipment, high-power kitchen appliances, washer/dryers, and irons, consume more than 1000 W of power. Designing for low power consumption is essential in the development of such appliances.

A home bakery appliance uses high power of about 350 to 700 W during the baking process, in which the heater is in continuous operation. Therefore, lower power consumption in the heater control system and motor control system, as well as higher efficiency of the power supply system should be considered at the system design stage.

Low-power design is also important for battery-powered devices. To prolong the continuous use of these devices, the power consumed by the MCU and other components must be reduced.

From a user experience (UX) design perspective, some kitchen appliances, including home bakeries, use a built-in battery to continue display operation of the LCD panel and retain the previous settings even when the power cord is unplugged. In this case, energy-efficient design should be kept in mind, as in the case of battery-powered devices.

To reduce the power consumption of the MCU itself, a power management function is typically built into the MCU. Many MCUs have standby modes. By operating the MCU intermittently, such as in a cycle of “run → standby → run → standby,” power consumption can be reduced. Table 2 shows examples of MCU standby modes.

Table 2 Microcontroller Standby Modes (Rohm’s ML62Q2700)

Notes: In all modes, data in RAM and SFR (special function register) are retained.
Clock run/stop and reset can be controlled for each peripheral function circuit block.

3) Parts and Manufacturing Costs – Fewer External Components

Compared to industrial equipment, the market for home appliances is highly competitive and the prices tend to be set lower. Therefore, there is a strong need to reduce material and manufacturing costs, and as a result, inexpensive 8-bit and 16-bit MCUs are often used in home appliances.

By choosing a MCU with peripheral functions that meet the function and performance requirements, the number of external components and their costs can also be reduced. A reduced number of components also means a smaller board area and lower board costs. However, it should be noted that the peripheral functions built into the MCU may have inferior performance, accuracy, and resolution compared to external components. Examples of peripheral functions used in home appliances are shown below.

Figure 3 Peripheral Functions Used in Home Appliances

External components can be used for clock generation. Using an external resonator or an oscillator increases the component cost but improves the clock quality. On the other hand, using the microcontroller’s on-chip oscillator reduces the component cost but also the quality of the clock.

The Rohm’s 16-bit microcontrollers can limit the clock frequency error to ±1.5% through an internal clock correction function. If the system requires clock functions, accurate time measurement, or high frequency precision, an external crystal resonator is recommended; if not, the on-chip oscillator is an option.

Figure 4 shows the connection between the microcontroller and peripheral components for a home bakery appliance, where the Rohm’s 16-bit microcontroller ML62Q2700 is used. ROHM’s 16-bit microcontrollers are currently used in cooking appliances such as rice cookers, microwave ovens, and coffee makers, as well as in refrigerators, washing machines, vacuum cleaners, air conditioners, and LED lighting.

Figure 4 Connection between the MCU and Peripheral Components

As shown in Figure 4, the ML62Q2700 incorporates an audio decoder (ADPCM/PCM method), segment LCD driver (up to 480 pixels, with the driving voltage generated by a capacitive divider), a 12-bit A-D converter and a PWM circuit, thus reducing the number of external components. However, an external crystal resonator is used for clock generation. This is because the clock function is required to set the baking time according to user specifications.

4) Development Environment – Easy to Use Utility Tools

Microcontroller vendors provide integrated development environments (IDEs), debug probes (emulators), evaluation boards, and utility tools for their microcontrollers. In many cases, the IDEs they provide are built using open-source software (e.g., Eclipse) and are available free of charge or for a relatively low cost.

Utility tools include tools for generating microcontroller configuration files and initialization code, tools for assisting in the creation of voice and image data for use with the microcontrollers, and tools for assisting in motor control design. Utility tools tend to be unique to each vendor. Easy to use tools reduce the burden on the developer. Figure 5 shows examples of utility tools.

(a) Tools for Creating Voice Data (ADPCM/PCM) (Rohm’s Speech LSI Tool)

(b) Tools for Viewing LCD Panel Display (Rohm’s LCD Imaging Tool)
Figure 5 Utility Tool Examples

It would also be useful if application notes and videos explaining the use of microcontroller ICs were available.