Low-power MCUs are becoming increasingly popular, where budding applications like IoT and AR/VR require standalone operation without sacrificing performance. In the wake of this market boom, designers may find it difficult to compare the vast pool of available low-power MCUs.

Renesas has taken note of this trend and is expanding its RA4 MCU family with 12 new MCUs, called the RA4M2 Group. Interestingly, Renesas is toting fast wake-up time as one of the dominant features contributing to the MCU’s low power consumption.


Renesas released 12 new RA4M2 MCUs this week. Image used courtesy of Renesas

Renesas says these new MCUs have been designed specifically for IoT and industrial applications. This article will go over some of the new device specs and deep dive into how certain specs, namely wake-up time, can affect power consumption.

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Key Features of the RA4M2 Group

Three centerpiece features of the new MCUs are high performance, low power, and enhanced security, according to Renesas.

The datasheet points to a 100-MHz maximum clock speed as one of the sources of high performance. As far as memory goes, the family includes integrated flash memory ranging from 512 KB to 256 KB and a 128 KB SRAM.

Based on the Armv8-M architecture, the new family of MCUs uses an Arm Cortex-M33 core. This Armv8-M-based architecture is said to provide a strong blend of performance, power consumption, and security.

Block diagram of the RA4M2 Group

Block diagram of the RA4M2 Group. Image used courtesy of Renesas

Low power consumption comes in many forms in this MCU family. The device can operate in multiple power modes, including Active Mode, Software Standby Mode, and Deep Software Standby Mode.

When the device is operating at a supply voltage between 2.7 V and 3.6 V in Active Mode (at an unspecified VCC), it consumes 80 uA/MHz, which comes out to 80 mA at its maximum 100 MHz. When the device is in Software Standby Mode (allowing for extra power supply recovery time), it consumes 0.7 mA. Finally, when the MCU is in Deep Software Standby Mode while keeping SRAM and USB powered, it can consume as low as 16 uA—even less if memory isn’t maintained.

Wake-Up Time and Power Consumption

Among the many specs given in the datasheet, Renesas explicitly advertised the MCUs’ wake-up time specs. According to the press release, “the new devices have an extremely fast wake-up time from standby of 30 µs.”

However, this specification does not show up in the datasheet. The closest semblance to this spec is a wake-up time of 35 µs from Software Standby Mode (not the lowest power mode) if the system clock source is the middle-speed on-chip oscillator (MOCO), which operates at a nominal 8 MHz.

The wake-up time specs for the RA4M2 family

The wake-up time specs for the RA4M2 family. Image used courtesy of Renesas

Different power modes generally operate by turning off different circuit blocks inside the MCU. The more blocks turned off (oscillators, memory, timers, etc.), the less power consumption. On the flip side, the more blocks you turn off, the longer it takes to get the MCU back into a usable active mode. This delay is known as “wake-up time,” which is the amount of time it takes an MCU to return from a low power mode.

Wake-up time is an important specification when considering low-power MCUs because this specification can limit the MCU’s usability in certain applications. According to Digi-Key, “Wake-up time will usually be the limiting factor that determines which power-down mode a designer can use at any given point in an application.”

You can use wake-up time to understand whether or not a certain power mode is feasible for your application. A certain low power mode may sound great, but if its wake-up time is too long for an application, then the design cannot use that MCU. Knowing and understanding wake-up times will then make it easier for designers to compare MCUs for a design.

Choosing a Low-Power MCU

Renesas is the newest in the long list of companies to add low-power MCUs for IoT to the market. The datasheet gives insights into the MCU’s merits by highlighting important specs, including uA/MHz, power consumption in low power modes, and wake-up times.

When designing for low power, it’s important to consider these kinds of specs to be able to compare MCUs and ultimately choose the right one for a design.

This post was first published on: All About Circuits

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