Which Power Savings Mode on Microcontrollers Should You Activate

Growing up as a teenager, I lost count of the times where I got yelled at for leaving the lights on. I would have argued that fluorescent lamps consume relatively less energy than appliances like electric kettle and heaters, but knowing that arguing back will only earn the wrath of my mom, I put a mental note to turn off the switch the next time around. Some laws can’t be designed around.

In electronics, I was intrigued by the power savings mode in microcontrollers. But I never dived into the various modes until I was in desperate need to minimize current in an outdoor solar-based application. That was when I realized the difference between the various modes of power saving mode and how they affect the entire PCB design.

Why Do You Need Microcontroller Power Savings Mode?

Microcontrollers had gone through multiple phases of evolutions since the days when Zilog was still a popular option. The demand for faster and peripheral-rich microcontrollers means smaller, powerful but power-consuming silicon. For instance, the LPC4300 series of Cortex M4 microcontroller consumes 81.5 mA at 204 MHz, with all peripherals disabled.

If you’re testing out the microcontroller in the comfort of the lab, you’ll have no problem pushing it to its limit. But when actual applications are concerned, power consumption matters as greener devices are more appealing. A microcontroller-based hardware that consumes less current is preferable to its power-hungry counterpart.

Environmental perception aside, you’ll realize the virtue of power saving modes when you’re designing a complex microcontroller based PCB for solar-dependent applications. Your challenge will multiply ten folds when the hardware is required to function for a stipulated time in the event of insufficient sun exposure.

Placing the microcontroller in power saving mode minimizes power consumption. In power-sensitive applications, every single milliamp saved can translate into hours of continuous operation.

Types of Power Savings Mode

Microcontroller manufacturers realized the importance of power savings mode. In recent years, power savings in microcontrollers have become more sophisticated. While each manufacturer may name the power saving modes differently, they share similar characteristics.

Here are some power saving modes that you may find in a microcontroller datasheet:

1. Sleep

Sleep mode is where the clocking signal of the CPU core is disabled while peripherals clocks are kept active. This enables the CPU to wake up within the shortest time possible.

In sleep mode, the CPU clock is turned off.

2. Deep Sleep

A microcontroller that is put into deep sleep mode will have the clocking signal deactivated for the CPU and other peripherals. The values of registers within the RAM are retained.

3. Power Down

The power down mode involves more than deactivating the oscillator to the CPU and peripherals. SRAM memories are usually powered down as well.

4. Deep Power Down

The deep power down mode is akin to almost turning off the power to the microcontroller. All clocking signals are shut down. Memories and registers are powered down. Input pins typically lose its value at this state. Only RTC circuitry retains its power circuitry.

Which Power Down Mode Should You Choose?

There are various factors that you should take into consideration before deciding which power saving mode is the best choice. The lower current consumption of the power saving mode will result in a more complicated hardware and firmware design approach. This is because peripherals and memories lose their values and designers need to mitigate the effects in firmware and hardware.

Deep power down mode may be needed for solar powered system.

In cases where you need a quicker response time and an adequate decrease in power consumption to prevent heat dissipation issue, a regular sleep mode will suffice. When deep power down mode is chosen, the methods of waking up the microcontroller are usually limited. You’ll need to design the hardware to wake up the microcontroller reliably. This is often through applying a voltage signal on a designated WAKEUP interrupt pin.

Throughout the process of determining power supply necessities, voltage and current flows, as well as any heat dissipation demands, accurate component model libraries will be paramount. Being able to take component parameters and utilize that data to accurately assess power supply needs and heat interactions with other components in your design’s layout will minimize the strain placed on determining the power savings mode on microcontrollers.

If you’re looking EDA software that enables your design analysis, consider the design tools that work for you. Furthermore, feel comfortable relying on a layout tool like Cadence's OrCAD to ensure that your design vulnerabilities are modeled accurately.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.