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Microcontrollers unplugged! How to choose an MCU…

April 24, 2009

Last month, I’d written a post on how MCUs or microcontrollers are shaping the embedded world. Taking this discussion a little further, I got into a conversation with Upendra Patel, CTO, eInfochips, based in Ahmedabad, with the help of my good friend Princy!

MCUs shaping embedded
On being quizzed on how microcontrollers are shaping the embedded world, Patel said that devices are designed by keeping specialized application in mind and not for general purpose usage in the embedded world. As a result, embedded devices need specialized controllers/co-processors, which are designed to execute the typical functionality in real time by offloading work of main processor.

“Microcontroller fits perfectly in this role, which is self-contained and independent in execution and still supplements to the main processor for processing power. MCUs are the integral part of embedded world and depending on the application, they assume the role of a main processor or a co-processor.

Today’s MCU possesses rich features like, ADCs, PWM generator, communication ports like I2C, SPI, UART, USB, CAN etc. reducing the gap between Microprocessors but still maintaining their unique identity by only focusing on controlling and not on number crunching (for which micro-processors are more suitable). The MCUs allow the designers to create head room for future expansion, as they take up some load of main processor.

Embedded devices have been penetrated in all markets where real time performance is vital. Micro-controllers have given significant contribution for this penetration. For example: communication market, surveillance market, biometric devices, micro-finance devices, medical instruments, automotive market and robotics.

Rich feature sets
Rich feature set is an imperative in the MCU market. What should one watch out for?

Even though rich feature set is imperative in MCU market, one should watch out on technical front for following features of MCU:
* Functionality coverage.
* Functionality coverage vs. Bill of Material
* Performance in terms of MIPS
* MIPS Vs. power consumption
* Simplicity in design and board layouts
* Standard compliance for low noise emission and temperature control
* robustness and consistency
* pin multiplexing of interfaces

Other non technical point to watch for is price announcements v/s feature set. The announced low price may be for least feature product and once you choose the MCU with all the features, it may be out of budget for the product

While choosing an MCU
Let us study the key factors that influence choice of MCUs today.

According to Patel, customers looks for following factors of MCUs which influence the choice of MCU.

Price: In small applications, MCU price is a main contributor of BOM and reduced price of the same reduces BOM to a large extent in mass production. For example Built-in peripherals – significantly reduce cost and space. Also, prices of IDE licenses may also affect the decision making of customer.

Performance: Cache memory or internal memory size – significantly impacts performance, similarly Memory width and speed

Availability: Availability is one of the main factor which board designers take care of because slight delay in product launch can result in significant market share erosion for customer. Also Availability in industrial and commercial temperature grades and through distribution channels.

Life span: The life span of the MCU depends on number of years a MCU manufacturer determines to continue support for it as a main stream product. More the number of years, higher the chances of selection. It also includes roadmap for future upgrades

Power consumption: This applies to battery operated devices. Lower the power consumption, longer the recharging/replacement time. There is a trade off between speed vs. power consumption

IDE support: These days products have become more sophisticated, while at the same time the life cycle has become shorter. It is very important to have a equally sophisticated IDE to speed up the development cycle and reduce time-to-market.

Technical support: History has shown that excellent products with not up to the mark technical support has never been successful. It is very essential for MCU provider to provide support at design and development stage of product development.

The other important aspect is about the track record of a semiconductor vendor in deliveries and post silicon bugs/issues reported.

Why are low-power MCUs in demand?
Now, it is time to determine the chief reasons for the demand in low-power MCUs.

According to Patel, in today’s consumer market of electronics and communications, handheld devices like the mobile phones and PDAs are mostly operated with batteries. The battery life is becoming a major concern for most of the portable devices because they run several different types of applications hence a demand for low power MCUs.

Also, designs are becoming smaller, and the weight and size of power supply/batteries is impacting the MCU choice and power consumption. Another trend is toward fanless designs to reduce noise and cost, and at the same manage thermals in smaller form factors.

For example, the MSP430 from Texas Instruments is an ultra-low-power MCU, which is suitable for devices where power saving is crucial. Those MCUs which has capability to handle data with CPU intervention participate in family of low-power MCUs.

In order to support low power functionality, MCUs support several power down modes for battery consumption. For example,
* Turning off CPU leaving every thing else functional.
* Running only low frequency clock oscillator and the peripherals running on it.
* To individually and automatically turning on and off peripherals when needed thus saving power being consumed by peripheral.

Applications
MSP430 ultra-low power MCU from Texas Instruments: used in wireless, medical and low power industrial applications.
Atmel’s ultra-low per MCU: used in Zigbee, security and keyless entry applications which spend significant amount of their time in sleep mode.

8- vs. 16-bit MCUs
Affordable prices are among the reasons that the Asian region is witnessing a migration to 16-bit architectures. Then, why still have 8-bit MCUs?

Patel advised that although 16 bit MCUs are available at low prices in Asian market, the primary reason for selecting 8-bit MCUs are as follows:
* Design simplicity
* Easy up-gradation to 16 or 32 bit architecture
* Cost effectiveness
* Development tools
* Easy to adopt

Finally, let us have a look at eInfochips’ roadmap in the embedded domain. Patel said that eInfochips is more focused on RISC based higher end MCU based products in streaming media, industrial, avionics, security and surveillance domains.

“The primary focus is on 32-bit MCUs, but 8- and 16-bit MCUs are used for specific applications like medical, automotive, micro finance handheld devices, health monitors, and home automation functions in the overall product. With more than 250 engineers in eInfochips embedded divisions, we work on most semiconductor vendor’s MCUs.”

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