Big Reasons for Small MCUs



Why microcontrollers with only a few kilobytes of memory are not part of the past.

In a world where bits, program memory, and pin count increase almost daily, here at Texas Instruments (TI) we sometimes field questions about why we continue to develop and release microcontrollers (MCUs) with only a few kilobytes of memory. The answer is really quite simple. There are hundreds of applications that can benefit by using a low-power MCU to replace standard logic gates, timers, comparators or other analog circuits. These MCU-based solutions often provide new functionality and flexibility, bringing additional value to a design. For some engineers, an MCU solution can also speed time to market and help avoid wrangling with operational amplifier (op amp) stability, analog linearization circuits, and the like.

Greenhouse-lead_in

The author explains that one type of MCU-based solution could be to control an irrigation-system-connected pump to maintain greenhouse soil moisture levels.

For instance, you could connect a temperature sensor to an analog-to-digital converter (ADC) and with a few lines of control code create a system to control a heater element to build a closed-loop temperature control system. To improve on this simple on/off control loop, you could add a few extra lines of code to create a simple proportional control term. You could then control a timer pulse-width modulation (PWM) output to vary the speed of a modulating fan or gas valve to better regulate heating and reduce system errors.

Figure 1 shows a system using the TI LMT88 temperature sensor and a potentiometer along with an MCU to create a simple closed-loop on/off control system by switching a relay to control a heating element.

By changing the temperature sensor to an ultraviolet (UV) sensor, you could build a simple UV exposure monitor that measures UV levels over a given time period. A certain exposure level could trigger an alarm warning the user to reapply suntan lotion, for example. By using a resistive moisture sensor, you could maintain soil moisture levels within a greenhouse by controlling a pump connected to an irrigation system.

Figure 1: Block diagram of a simple temperature control system.

Figure 1: Block diagram of a simple temperature control system.

Adding an Extra Dimension

You can build many of these applications with a few simple active and passive components (thermistors, op amps and comparators, or a handful of resistors and capacitors and a handy 555 timer). But with an MCU, you can easily add an extra dimension by bringing a programmable element into the feature set, enabling more advanced control features such as a proportional term controller to control a variable heating element or variable speed pump, or a dynamic hysteresis element to reduce system cycling. Including a potentiometer or keypad input is also straightforward. Such inclusions can make it possible for the designer to change operating parameters, provide light-emitting diode (LED) indicators, or even connect to a small organic LED (OLED) display. Such tasks are not always as simple to accomplish with a hard-wired analog or fixed-function integrated circuit (IC)-based solution.

Energy Savings

TI recently released two new MSP430™ MCUs with low pin count for many simple applications. With up to 4KB of embedded ferroelectric random access memory (FRAM), plus 1KB of RAM, these devices offer a compiler-friendly alternative to many 8-bit MCUs on the market today. MSP430 MCUs are a great entry point to see what FRAM is all about. Offering flexibility for programmers, FRAM technology’s unique ability to operate as both nonvolatile program and nonvolatile data memory enables you to customize the partitioning of program and data memory that was previously not possible with conventional flash and RAM combinations. Along with this flexibility, FRAM offers significant energy savings when writing to memory compared to EEPROM or flash memory; you can find out more about FRAM technology on ti.com.

As seen in Figure 2, MSP430FR2110 and MSP430FR2111 MCUs pack a significant feature set into a tiny 3mm-by-3mm package. Besides offering as much as 4KB of embedded ultra-low-power FRAM nonvolatile program storage, they also include:

  • A 10-bit 200K sample ADC with eight external input channels.
  • A low-power comparator with a 6-bit programmable threshold.
  • A real-time counter with low-power backup memory.
  • A hardware UART/SPI serial interface.

With 1K pricing under $0.50 for the MSP430FR2110 MCU, this cost-effective, feature-packed device is already finding many new applications within the home and building automation space.

To begin development, TI offers the MSP-EXP430FR2311 LaunchPad™ development kit for $13.99. LaunchPad development kits enable you to explore the device features and build and evaluate your own hardware with a build-your-own BoosterPack™ plug-in module (BYOB). The LaunchPad ecosystem opens the door to many other TI and third-party developed BoosterPack modules. There is also a 20-pin TSSOP target socket board available, the MSP-TS430PW20, which you can use with the MSP FET to fully explore the device.

Figure 2: MSP430FR2111 MCU block diagram.

Figure 2: MSP430FR2111 MCU block diagram.

To further assist you with getting started:

  • Access full product details and code examples in the product folder.
  • Order the MSP-EXP430FR2311 LaunchPad kit.
  • Order samples to try these new devices out yourself.
  • Move to the MSP430 FRAM value line with a migration guide.

If cost is absolutely king and the functionality can be relatively simple, then an oscillator built with a CD40106 inverter and RC network, or a simple one-shot timer built with an NE555D, would be hard to beat. However, if you need to add any level of programmability or integrate multiple functions, an MCU-based solution will in many cases reduce overall system cost, complexity, and time to market.


David--SmithDave Smith is the Product Marketing Manager for TI’s MSP430 FRAM-based microcontrollers (MCUs). He began his career with TI in 2001 when he joined the UK sales team. In 2008, Dave moved to Freising, Germany as an MSP430 Business Development Manager for Europe. Prior to joining TI, Dave spent three years at Arrow as a MCU FAE following five years of MCU hardware and firmware design. In his spare time, he enjoys driving (or fixing and then driving) his 1966 Mustang. d-smith@ti.com

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