This interview discusses the best takeaways of using low power MCUs, the benefits of using modern reduced power modes, utilising power budgeting techniques, and the benefits of selecting an MCU based on target application or on development support.
Abhishek Mutha of EFY got in touch with industry experts from Microchip, Freescale, Texas Instruments and Renesas who shared their thoughts on today’s microcontrollers, how they have evolved and how they can benefit design engineers incorporating microcontrollers in their projects. Combining all their responses, we bring to you the broader perspective.
Q. What are the greatest benefits for designers who use MCUs developed specifically for low power?
Jason Tollefson (JT): There are a variety of reasons that designers are looking for lower power MCUs. The primary reason is to extend battery life. Applications such as wireless sensors consume a lot of power processing and transmitting data that they collect. The value of the sensor increases the longer it operates, so extending the battery life becomes important.
Another reason is that designers want to do more with their MCUs, but still stay within their energy budget. In the past, when dealing with a tight budget, the MCU would have to sleep more often.
Advances in MCUs have dramatically dropped their RUN currents, allowing the MCU to RUN more often, thereby allowing more features to be designed into the application software.
Vivek Tyagi (VT): The low power MCU gives designer an edge to work over various applications and get a wide range of advantages like low power libraries, gate level optimization and power gating. Performance-per-watt levels have evolved in many embedded applications to enable power efficient designs which help the end user to conserve energy. In fact, considering the looming power crisis, regulatory authorities have made it mandatory for consumer appliance companies to upgrade their products and display the energy efficiency ratings enabling the consumer to make smart choices. Energy efficiency is the need of the hour and low power MCUs are the foundation for efficient devices/appliances.
Shailesh Thakurdesai (ST): Energy efficiency and power management are the key technology trends that designers are looking for. Across all electronics devices, from consumer electronics, household appliances, manufacturing equipment and metering, low power MCUs help in increasing power efficiency of that product, by harvesting and effectively utilising the power, thus making the systems more cost effective. Low power MCUs are also important for designers to help them develop environment friendly products, as low power helps in reducing radiated emission effects.
Boon HUI Ong (BHO): Consumers and industrial companies alike are increasingly looking towards wireless and battery-backed instruments. It is then important for each and every component in a battery operated system to consume as little power as possible, as this will translate into a smaller power source and indirectly lower component cost and PCB size. From the MCU design point of view, recent developments in advanced low power Microcontroller technology has enabled designers to attain high MCU processing performance, and yet low power consumption in standby modes to maximize the battery life in those portable battery-operated and battery backed applications.
Q. In many applications, the controller does not run continuously and may be ‘sleeping’ much of the time. How have sleep modes evolved during the years?
JT: Rather than having two modes, i.e. RUN and SLEEP, modern MCUs have a spectrum of modes in-between that offer varying degrees of functionality.
On the PIC® MCUs with eXtreme Low Power, the modes are named Doze, Idle, LVSleep, Sleep, and Deep Sleep. Modes like Doze allow the peripherals to be clocked at a higher rate to meet the needs of communications or timer sequences, but allow the core to run at much lower speeds, reducing the current consumption by up to 75%. Modes such as Idle turn the core off completely. Modes like LVSleep, Sleep and Deep Sleep turn off all peripherals but preserve select features like LCD, ADC, Timers and Interrupts. So a lot can be accomplished even in these modes which save 97-99.9% of the current than RUN mode.
VT: Real-time low-energy environments pose special challenges for the designer and programmer because the same sleep states that reduce an MCU’s energy consumption often also reduce its ability to quickly respond to an event. Low-power modes typically range from a light sleep or standby mode, through deep-sleep, generally, the deeper the sleep, the less power is consumed by the MCU.
A number of strategies are used in projects to determine how these variables can be set to take best advantage of an MCU’s sleep modes.
One is to identify the routine low-level functions required by the application and identify MCU architectures that can handle as many of them as possible using on-chip hardware that does not require CPU intervention. Other methods are clock gating, gate level optimization, voltage scaling and digital architecture techniques.
ST: Sleep modes are available on all the MCUs now days, but it depends on the architecture of the MCU and the overall design implementation (both hardware & software). Basically, if the system is designed intelligently and if the MCU Architecture supports this, power consumption can be reduced significantly. For example most MCUs have DMA now and if a data transfer function has to be done, the core can actually be kept in “OFF” Mode which saves a substantial power. Also, the time which the internal PLL or FLL takes to stabilise from low power to active mode also determines the average power consumption.
There are standard techniques of reducing power by optimising the voltage levels and clock frequency. Additionally, attention needs to be paid on the IO lines leakage (including communication interfaces) which can help in substantial power savings.
BHO: “Sleep” mode or “standby” mode has certainly evolved over the years. In Renesas’s RL78 series, we certainly showcased such evolution. In this product, we implemented 3 types of ‘sleep’ mode. They are –
1. HALT mode
2. STOP mode
3. SNOOZE mode.
SNOOZE mode is especially interesting because it allows some peripherals like ADC and UART operation while in standby modes. Besides taking advantage of the various MCU “sleep” modes available, designers can moderate the CPU performance by switching off or scaling back the clock speed whenever possible.
They can also further reduce the MCU power consumption by turning off any clocks and peripheral blocks that are not being used.