Features of Renesas 8- and 16-bit microcontrollers allow the design of functions such as induction heating control and audio playback for rice cookers

There is a well-known movement to add features to increase the value provided by air conditioners, washing machines, and other whiteware products. Similarly, our customers in the small cooking appliances sector have been working hard to produce a steady stream of fresh products with added-value functions that have high consumer appeal. Examples include ovens that can cook using steam, rice cookers that use ingenious cooking vessel materials, and system kitchens that include induction hobs.

Likely areas for the future product enhancements include improved user interfaces with features such as audio guidance and dot-matrix LCD panels, better safety-management features, and centralized control of home appliances via an in-home network. Solutions for implementing such convenience and utility features and functions are readily available in the wide range of microcontrollers Renesas has offers, particularly our 8-bit and 16-bit models (see Figure 1).

To illustrate the system design requirements for microcontrollers in small cooking appliances, this article uses as an example the block diagram of an induction heating (IH) rice cooker (see Figure 2). Although mask-ROM microcontrollers have typically been used in the past for such mass-produced products, there now is growing demand for flash microcontrollers that support on-board program updating and are sufficiently economical to be used in full-scale production. Flash microcontrollers eliminate the repeat testing of OTP and mask-ROM versions and avoid the need to insert evaluation chips to perform on-board debugging. They also prevent buildups of excess stock by allowing user programming to be deferred until the production phase.

Renesas has an extensive range of flash microcontrollers from which customers can select the optimum devices for their designs. Choices can be made from the 740 family (QzROM devices), R8C/Tiny series (flash devices), and M16C family (flash devices), among others. (The QzROM devices can be programmed only once using standard flash memory programmers, while the flash chips can be written and rewritten many times using such programmers, as well as on-board, in-system programming methods.)

Most microcontrollers for small cooking appliances operate on 5V. However, products such as rice cookers must maintain the clock count and stored timer settings, even when unplugged. Therefore, they have to be able to operate using only a button battery as a power supply. This requirement is met by the QzROM devices in the M38D5 group in the 740 family, which can operate on voltages between 1.8 and 5.5V. Only a few microamperes are needed to keep the time clock running, so the microcontroller can operate on battery power alone for an extended period of time.

Portable home appliances use low-power LCD panels. Our M38D5 devices in an 80-pin package have an internal LCD driver with a maximum of 36 segment outputs and 8 common outputs. This means they can be matched with LCD displays of up to 256 pixels in the 100-pin class. Also, any segment outputs not required for the display can be reassigned as general-purpose I/O pins. The microcontrollers also have six ports capable of driving LEDs directly, providing additional opportunities to reduce component count and cost.

Induction is becoming the standard heating method for rice cookers in Japan. Chefs like this feature because it heats the cooking vessel efficiently and produces well-cooked rice. The "Timer X" 16-bit timer on devices in the M38D5 group has been designed with an IGBT output mode. This enables IH control to be implemented with a smaller circuit. Application notes that aid software development are available on the Renesas web site.

Small cooking appliances are becoming more sophisticated. Thus, demand is growing to increase the resolution of the A/D converters used to monitor these new functions and to add more channels. Microcontrollers in the M38D5 group have a high-resolution 10-bit internal A/D converter with 8 channels. This A/D can be used for functions such as controlling safety features or monitoring aspects of the cooking process, such as the temperature of the cooking vessel or lid.

Another function that is adding value to small cooking products and other home appliances is audio playback (see Figure 3). Using a microcontroller to implement this function has the advantages of utilizing unused CPU resources and allowing I/O expander functions to be added using instructions from the microcontroller. Renesas offers an audio middleware kit for use with our Tiny series microcontrollers. A demonstration set built with a 16-bit R8C/25 Tiny-series flash device is also available (see Figure 4).

The fact that small cooking appliances must be compact puts a premium on devices that have small packages that suit different types of production equipment. Renesas meets this requirement, too. Microcontrollers in the M38D5 group, for example, are available in two types of 80-pin LQFP packages: 20mm x 14mm with 0.8mm pitch, and 12mm x 12mm with 0.5mm pitch. Those in the M38D2 group are available in two types of 64-pin LQFPs: 14mm x 14mm with 0.8mm pitch, and 10mm x 10mm with 0.5mm pitch. We can supply the devices as bare chips, too.

Providing solid proposals and other support for microcontrollers necessitates a good understanding of the background to customer requests. The IH rice cooker application previously described is a system developed by our Japanese customers based on the Japanese culture. Requirements for home appliances and the microcontrollers that implement them often differ in other parts of the world. Renesas recognizes this diversity and meets the different requirements by analyzing a vast pool of knowledge gained from working closely with customers in local and regional markets around the globe.

Like the inverter-control microcontrollers covered in Vol. 16 of the EDGE magazine, the microcontrollers used for system control in home appliances are riding a wave of demand for high-end home appliances. Requirements are growing for more sophisticated functions and greater on-chip memory capacity. Our technology roadmap takes these and other trends into account. For example, look for an extension of the range of Renesas flash microcontrollers for mid-range systems based on products like the Tiny series of devices and the R8C/Tiny chips that provide built-in LCD drivers.