Understanding the Six Power Modes for Maximum Efficiency on NXP’s LPC55S69 MCUSeptember 24, 2020 by Mark Dunnett, on Behalf of NXP
This article explores the power rails and power modes of the LPC55S69 and investigates how the modes affect the MCU in various applications.
When looking for the right microcontroller, it’s important to know all of its key features to ensure the proper execution during any project. Fortunately, there are many microcontrollers on the market that offer a variety of features to meet the needs of demanding applications. NXP’s LPC5500 series of microcontrollers can be applied to many real-world applications, including industrial and commercial, offering power consumption benefits, among many other things, which will be discussed in this article.
A previous article, Leveraging the LPC55S16-EVK for Industrial Applications, investigated the power consumption and clock speed of another member of the LPC5500 series of microcontrollers, the LPC55S16 MCU, a useful board for industrial applications due to its variety of interfacing options. This article explores the power rails and power modes of the LPC55S69 and investigates how the modes affect the MCU in various applications.
Figure 1. The NXP LPC55S69-EVK
How Much Current Does the LPC55S69 MCU Draw?
The ultra-efficient 40nm flash technology, combined with a unique architecture, makes it possible for the LPC55S69 device to only consume around 32µA/MHz. Therefore, the chip should theoretically only consume around 4.8mA when it operates with its maximum frequency of 150 MHz. However, that’s under the assumption that the code is running in RAM and that the application is highly optimized. Realistically, in many cases, the current consumption will be around 10mA.
Before determining the power consumption of the MCU, it's important to take a look at its power supply. On the LPC55S69 device, it's organized as shown in Figure 2.
Figure 2. The organization of the LPC55S69 device power supply.
The two power rails of interest for this experiment are marked with red circles in the figure above. VDD is the power supply that drives all the GPIO pins, whereas VBAT_PMU and the VBAT_DCDC rails form the main supply that powers the logic of the chip. So, the combined current consumption of all these rails must be taken into consideration to find the overall current that the MCU draws in a realistic setting.
Running with the maximum clock speed of 150MHz, the combined current draw is around 7.54mA. Considering the MCU’s clock, the chip has a current consumption of around 50µA/MHz.
Even with a slightly higher current consumption than theoretically possible, the MCU requires very little power to operate, especially when considering that it runs at 150MHz. The results could also vary dramatically when the code is optimized, executed from RAM, and when more precise measuring equipment is used.
These properties make the LPC55S69 MCU ideal in all scenarios that require significant processing power and efficiency.
Power Modes of the LPC55S69 MCU
To further mitigate the power consumption of the MCU, the LPC55S69 device supports six power modes that disable certain parts of the chip to save power. To demonstrate how the different power modes affect an Okdo E1 development board, which uses an LPC55S69 MCU, the setup shown in Figure 3 is used.
Figure 3. The setup used to demonstrate how the different power modes affect an Okdo E1 development board.
A 200mAH CR2032 coin-cell battery is directly connected to the VDD_TARGET pin of the development board to power it. Besides that, it’s also hooked up to a small e-ink display, which will output the clock speed of the microcontroller. A demo application manages the power mode that the LPC55S69 device is in. The mode can be changed using the wake button on the right-hand side of the Okdo E1 board.
Using this setup, the microcontroller was put into different power modes, and the following table highlights the measured current in each mode:
Note that the measured current includes all the peripherals connected to the Okdo E1 board.
In full-speed mode, the MCU operates with a frequency of up to 150 MHz. Therefore, this mode is best suited for applications that require fast processing, while power-efficiency is not of utmost importance.
The run mode will lower the MCU’s clock frequency to 12 MHz, which helps reduce power consumption while still ensuring fast response times when performing complex calculations. This mode represents a good mix of processing power and power efficiency.
The first low-power mode is the sleep mode — imagine it as a nap mode for the microcontroller where the core clock is stopped, but all the peripherals remain operational. Since the setup in this experiment consists mainly of peripherals, the improvement in current consumption is rather small when switching from run mode into the first sleep mode. In this demo, the MCU can be woken up by using one of the user buttons on the Okdo E1 board.
In deep-sleep mode, the core is stopped like in sleep mode. However, here, many of the peripherals also shut down to reduce the current consumption of the system. Pressing a user-button will, again, wake the MCU up.
The power-down mode further reduces the current consumption by disabling the DCDC converters, turning off the digital logic, and stopping the clock speed. There is an option to retain the contents of the RAM and keep one or two asynchronous peripherals running. Pressing the ISP button on the Okdo E1 board will wake up the core.
In deep-power-down mode, the digital logic, DCDC converters, and most of the chip are turned off. Only the real-time-clock remains active, which will wake the MCU after a 10-second delay and reset the MCU to the run mode in this application.
Note that the table is not representative of the current consumption of the microcontroller itself due to the peripherals and the added current draw of the Okdo E1 development board which requires around 5.7mA. Therefore, the MCU only draws a few micro Amperes of current in the deep-power-down mode. However, in practice, it’s unlikely that the MCU will be the only component in a circuit. Therefore, this experiment illustrates how the MCU behaves in its different power modes in a realistic setting.
The LPC55S69: A Low-Cost, Highly-Efficient MCU
The LPC55S69 MCU requires very little power to run, especially considering that it contains various co-processors, DSPs, and security-features. These characteristics enable the LPC55S69 to be used in a wide variety of different applications ranging from hobbyists projects to processing on-the-edge to commercial and industrial products. This article introduced the six power modes of the MCU and demonstrated how they affect the operation and the current consumption of the overall system.
In an average application, running at 150MHz, the MCU will consume around 10mA in most cases. In its deep-power-down mode, the chip’s current consumption is in the micro Ampere range. NXP's community page offers a list of application notes, tutorials, and videos based on the available MCUs, which can be helpful to better understand their capabilities.
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