Low power embedded design

When considering low power, you might think different areas of applications. I would say that low power can be divided to 2-application area. First are low power devices which are still powered from mains or via system power example from vehicle. The other low power is devices which are battery powered without any other power source. I would call this sector as ultra-low power. What usually is driving low power is heat generation causing too much heat in the system. You might as well need passive cooled device without any moving fans so you will low power. You might want to have as light and device without moving parts due to ruggedness needs. Here as well you need to have low power as the power is affecting to cooling and weight of the system. You might want to have closed enclosure due to dust and moisture and therefore need a low power to enable the cooling via chassis. Today as well the energy prices and cost are drivers for low power. Ultra-low power is usually needed in handheld devices and devices which are stand alone and need to survive even years with just battery operation. For sure in these devices, you might add solar panel to create some additional power.

Ultra-low power and Low power

The base choice is between performance & power of the device. For ultra-low power devices with very limited power envelope like sport watches you most likely are selecting an ultra-low power MCU (Micro Controller Unit)). These small MCU’s with like ARM Cortex M33 -cores are running with uW power levels and having very many power saving modes. Running them in sleep mode is the consuming least power but waking up them is adding power consumptions a lot. So, the power consumption is defined how the system and system software is running in the device and how much resources the device needs in each time. Example GPS in these devices is eating a lot of power and is consuming the battery quickly. In cases where you have display it is other major power consumer. Selecting the display to this kind of device is therefore important. In low power devices the reasons for consumptions are similar but usually the power envelope is larger and usually we are talking about several from few watts to 10+ watt. In this kind of devices, the performance is leaning towards performance than to pure energy saving. These systems are usually running with CPU -level products like ARM Cortex A53 with low power M -cores as well Intel Atom and AMD Ryzen -based products. Some CPUs are offering a lot of power saving modes as well to minimize the power consumption.

Building low power system

First step is to figure out your overall system environmental conditions. Then think about the power budget and how to dissipate the power from system. In case you will use display, it is important to look at the display technologies. The mainstream industrial displays today are TFT’s where the backlight is the key source for power consumption. By adjusting the backlight according to light conditions, you can reach lower power consumption. The more brightness you have outdoor the more backlight power you need. There are ways to improve the visibility in outdoor conditions like optical bonding and certain films which can reduce the need to consume power. There are also other display technologies available offering very low power like e-paper, memory in pixel and green displays. With these technologies you do not bright backlight as they are reflective technologies. That will save a lot of power consumption and will give very good visibility in bright sunlight. Therefore, thinking how the product is going to be used is playing important part in the power consumption. ?

What to choose for low power CPU product?

From the smarc -family options could be cost efficient SM2S-G2UL Smarc equipped with Renesas RZ/2GUL with Cortex-A55 cores and Cortex-M33 cores. The total power envelope is 3-4 Watt and can be reduced to very low level by utilizing Cortex -M33 low power functionality with different sleeping modes. SM2S-IMX93 Smarc is offering similar kind of cores with power levels 2-4W. NXP’s power flex architecture is enabling to create 20 -different power stages and reach even30uW -power consumption levels. The most power efficient product in Smarc -lineup is SM2S-IMX8ULP?equipped with NXP’s ULP CPU which has power efficient Cortex-A35 and Cortex-M33 -cores. Typical power consumption is 1-3W and with this one you can reach uW -level power consumption by utilizing 20 -different power saving modes. With Smarc ULP we are talking about very close to ultra-low power class device.

Looking for low power display?

Paper display are offering excellent choice for showing static pictures and information. Their power consumption is uW to mW -level. Downside is they are not always having abilities for very harsh conditions. The other issue can be the speed of the display as they are not suitable for video content. Today you can get paper display in various sizes, and they are available in various sizes. Memory in pixel -displays are usually very small one’s and focusing into handheld devices. They have similar features to e-paper but are based on TFT -technology which makes them good even for video. The amount of color display options is limited. One new technology is green displays where you have the benefits of paper display and tft in same package. Power consumption is uW to mW -level and options are from small size up to 32” displays with transreflective technology. With all these technologies there is no need for B/L in conditions where you have some light. But in dark conditions you need to add backlight to get visibility. With TFT you can reach lower power as well but then it is important to control the backlight in the way that it does not consume too much power.

Low power conclusion

The overall system design and utilization of the CPU -resources is as important as selecting the right CPU and display to reach the target power consumption. If the use conditions, cooling and power envelope are not considered in the first place it is hard to realize the targeted power envelope. Top of these things you need to be able to utilize system resources correctly in different use conditions to keep power consumption low. There is good co-operation with hardware and software teams needed as the power consumption is strongly related how you drive your system and resources. Designing low power device is affecting to the customer experience as they feel the power consumption in use as well in their pockets with the rising energy prices. Low power is part of good design.