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Hello, and welcome to Introduction to MCU Power Modes, part of our TI Precision Labs Microcontroller series. In this video, we will look at the concept of power modes in an MCU and how they will influence performance and power consumption. At the end of this presentation, you will have an understanding of MCU power modes and how to adapt them for your applications.

So what is a power mode? A power mode is just a specific setting or policy which dictates what is and what is not active in your system at a given time with the focus on reducing unnecessary power consumption. For example, your home is a system that goes through different power modes throughout the day.

Inside your home, you might have several appliances, such as an air conditioner, computer, coffee maker, microwave, oven, and lights. If we turn on every appliance in a home, we will of course find that the power consumption increases or if we shut everything off, we will find power consumption is very low. Most of the time, though, we will have different appliances we need on at different times of the day. Throughout the day, the average home will see power consumption increase and decrease over time like shown in the graph on the bottom of this slide.

Let's say that during the daytime we need to have the air conditioner running, our computer on while we are working, and some of our lights on. We may also make coffee throughout the day to stay productive. During this midpoint of the day, we are consuming a modest amount of power.

In the evening time, we will turn off the computer, and we won't use the coffee machine. Since we may need to make dinner, however, we will use our electric oven and microwave to cook. These high-powered devices consume a lot of additional power. Overnight, while we are asleep, we shut off all the lights and other appliances, and all that's left on may be the air conditioning. Overnight, our power consumption will dip to its absolute lowest level.

So in a house, we can change the power consumption by controlling the power supply of the electrical appliances resulting in different power consumption levels. It is almost the same for an MCU. Our power mode depends on what peripherals or appliances we need to have running at any given time.

In an MCU, however, we could also set the power mode formally using the internal control registers. If your house worked like this, we could flick a switch to go from daytime mode to evening mode, and your PC and coffee machine would shut off on their own when you did. It's also sometimes true that certain peripherals, often watchdogs or real-time clocks, will not shut off when switching power modes unless explicitly told to do so.

In an MCU, the current power mode is controlled through the clock and power supply to the peripherals, CPU, RAM, and flash by a power management and clock unit or PMCU shown in the picture on the left. The PMCU is controlled by the CPU. In some MCUs, a PMCU may be divided into separate parts. But, normally, the functions we are going to discuss will all be present in some form.

Generally, we can divide power modes into two categories. One is an active mode. In this mode, an MCU will run at its highest performance and all or most of the peripherals will be on.

The second is a low power mode. In low power mode, the MCU will try to reduce power consumption. Power can be saved by disabling the least wanted or highest current peripherals just like what happens in a home throughout the day. As we change power modes, we may also find that the digital peripherals working speed is changed because the clock speed to different peripherals is altered.

So how do we use active mode and low power modes in a typical application? In all applications, we need the CPU to handle processing or communication with the rest of the system. When tasks requiring the CPU are finished, the CPU can go to sleep and only wake up when a peripheral, such as a toggling GPIO, a timer, or RTC interrupt or communication bus request a wake up.

When the CPU finishes all its tasks again, it can choose to go into a low power mode and wait for the next wake up trigger. As the CPU and peripherals are turned on and off, seen in the picture on the right, the current consumption decreases in low power mode and returns to high in active mode. There is a transition time between low power and active modes due to the time it takes for the starting and stopping of clocks and peripherals. By not keeping the CPU active when it's not required for processing, we can reduce the overall current consumption in our system.

Earlier, we simplified the idea of power modes into just two categories. But in truth, they are usually multiple different power mode levels in a given MCU. Usually, you do just have a single active mode. Here, we call it run mode, which uses milliamp level current and has the CPU powered on.

Here, we'll break the low power modes further though into stop mode, standby mode, and shutdown mode. Recall that the key differences between power modes are typically due to clock and power control. In run mode, all peripherals in the CPU can be running. You can fully use all the functions of the MCU. As mentioned in this mode, the current would be around the milliamp level.

In stop mode, the primary difference from run mode is that the CPU clock is gated. Stop mode is useful when the CPU is not really needed to be involved in the tasks being performed by the MCU, but you still want to use the peripherals with a faster clock. For example, let's say you want to sample an analog signal.

We could have the CPU on and just waiting to start handling new data after the ADC sampling is complete. However, you can also place the device into stop mode to save power between samples. In this mode, its current would be hundreds of microamp levels rather than milliamps.

In standby mode, the CPU is, of course, still disabled, but we reduce the clock frequency compared to stop mode. In standby, the high frequency clock shuts down, and only a low frequency clock is available. For some MCUs, certain peripherals will also be disabled. The current in standby mode would be down to a single microamp level.

Finally, in shutdown mode, typically only the GPIO peripheral is available, which can be used to wake the system up. Usually, in shutdown mode, all other peripherals and internal clocks are disabled. Only a small part of the PMCU will be maintained to support powering the GPIO. Current in this mode will usually be a single microamp or lower.

Now we know about several different kinds of power modes and their trade-offs. But how can we move between them? The picture shown on the right-hand side of the slide shows the flow between power modes.

When the MCU power is on, the default mode is run mode. In most cases, you can only go to low power modes when you are in run mode. You may recall that you need the CPU to be involved to control the PMCU. So in low power modes, when the CPU is turned off, you can't utilize the PMCU to swap to a different low power mode.

To go back to run mode, you need a trigger internally or externally. In sleep mode and standby mode, different available peripherals can be used as a trigger like a comparator, RTC, UART, I2C, and so on. In shutdown mode, however, you can usually only use a GPIO because all the clocks and most peripherals are turned off to minimize the current consumption. In shutdown mode, the RAM and run mode information are not usually preserved. So the MCU will essentially do a full reset when triggered to return to run mode.

Another factor to consider when moving between power modes is wake-up time. Actual wake-up times will be device specific and usually available in the MCU data sheet. In general, though, when going from sleep mode to run mode, it takes the least amount of time as only the CPU needs to be turned on. To get out of standby mode, it takes a little bit longer as the CPU needs to turn back on and the high speed clocks need to be re-enabled and allowed to settle. As you may expect, waking from a shutdown mode is by far the longest as the device essentially goes through a full reset.

Thank you for watching our video on MCU power modes. You should now hopefully have an understanding of the different kinds of power modes available in an MCU. To find more MCU technical resources and view devices, visit ti.com/microcontrollers.