Engineers say | How does the MCU power saving function ensure that your system saves more power?
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This article will introduce you in detail how the power saving function of RA2E3 MCU can reduce unnecessary power consumption in applications.
Prabhath Horagodage
Senior Staff Product Management
Microcontrollers are used for many purposes. Some applications require high speed, high performance and continuous operation at full speed, while others require only partial operation during specific cycles. Renesas has been studying these use cases for years and designed the extremely energy-efficient RA2E3 to enable designers to reduce MCU power consumption through its power-saving features to meet end-user expectations for high energy efficiency and environmental benefits. RA2E3 provides four main power saving modes, which can be used individually or in combination:
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Three different low-power operating modes
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Four different power control modes
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Switch clock frequency to appropriate speed
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Stop unnecessary modules for a specific duration
Low power operation mode
RA2E3 offers the following three different low power modes:
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sleep mode
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Software standby mode
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Snooze mode
The MCU can be programmed to automatically transition between these modes when the required conditions are met. The maximum supply current in normal mode is 12 mA under certain conditions (high speed clock, all peripheral clocks enabled, etc.) and under certain conditions (all SRAM on, all peripheral modules stopped, etc.) Software The maximum supply current in standby mode is 0.25 μA. The supply current in sleep mode and snooze mode is between the supply current in normal and software standby modes, depending on the number of operating modules, clock frequency, etc. A rough comparison of power consumption in each mode can be made as shown in Figure 1. The conversion method between low-power modes is shown in Figure 2.
Figure 1 Rough comparison of power consumption in each low-power mode (conditional)
Figure 2 Low-power mode conversion method (see MCU hardware manual for details)
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sleep mode
In this mode, the CPU stops running but retains the contents of its internal registers. Other peripheral functions and oscillators are not stopped by default, but the user can set whether to stop them.
For example, if the user needs to perform A/D conversion in fast mode for a period of time, but does not require CPU operation during this period, the user can program the MCU to enter sleep mode with a high-speed conversion clock when the A/D conversion starts, and After A/D conversion is completed, it returns to normal mode. In this example, the user saves unnecessary CPU power during this period. For more details on entering, operating and canceling sleep mode, please refer to the RA2E3 Hardware Manual at the end of this article .
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Software standby mode
In this mode, the CPU, most peripheral functions, and the oscillator are stopped. However, the contents of CPU internal registers and SRAM data, the status of on-chip peripheral functions and I/O ports are retained. Software standby mode significantly reduces power consumption because most of the oscillator is stopped.
For example, if the MCU needs to wait for an external input (such as an IRQ interrupt) to initiate a specific operation, and no other operations are required during the wait, the user can program the MCU to remain in software standby mode until the input is received, thus saving most Unnecessary power consumption. After input is received, the target operation can be performed in software standby mode, or after transitioning to snooze or normal mode, if desired. After the target operation is completed, you can return to the software standby mode again and wait for the next input. For more details on entering, operating and canceling software standby mode, please refer to the RA2E3 Hardware Manual at the end of this article.
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Snooze mode
In this mode, the CPU stops running, but the contents of its internal registers are retained. Most peripheral functions and oscillator operation are optional. As shown in Figure 2, direct transition from normal mode or sleep mode to snooze mode is not allowed. Transition to Snooze mode should occur via software standby mode. However, it is possible to transition directly from "Normal" mode to "Snooze" mode.
Let's look at an example of using UART in snooze mode. Before starting UART communication, the MCU can remain in software standby mode to save power consumption. When it starts receiving UART data, the MCU can transition into snooze mode and continue receiving data without waking up the CPU, unnecessary peripheral functions, and oscillator. After data reception is completed, the MCU can return to software standby mode again and wait for the next UART data. For more details on entering, operating, ending and canceling snooze mode, please refer to the RA2E3 Hardware Manual at the end of this article.
Power control mode
There are four power control modes, mainly defined according to the maximum operating frequency and operating voltage range. Controlling the memory read speed through power mode can reduce the current consumption of memory (flash/RAM). Power control modes are available for normal, sleep and snooze modes. The power consumption of each mode is shown in Figure 3.
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High speed mode
In this mode, the MCU's maximum operating frequency and voltage range are 48 MHz and 1.8 to 5.5V respectively. In certain conditions (operating in normal mode, all peripheral clocks disabled, CoreMark code executing from flash memory) mode, the maximum supply current is 4.80 mA.
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medium speed mode
In this mode, the MCU's maximum operating frequency and voltage range are 24 MHz and 1.8 to 5.5V respectively. However, when the operating voltage is 1.6 to 1.8V, the maximum operating frequency is 4 MHz. Typical supply current under specific conditions (operating in normal mode, all peripheral clocks disabled, CoreMark code executing from flash memory) is 2.60 mA.
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low speed mode
In this mode, the MCU's maximum operating frequency and voltage range are 2 MHz and 1.6 to 5.5V respectively. Typical supply current under specific conditions (operating in normal mode, all peripheral clocks disabled, CoreMark code executing from flash memory) is 0.30 mA.
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Subosc speed mode
In this mode, the MCU's maximum operating frequency and voltage range are 37.6832 kHz and 1.6 to 5.5V respectively. Typical supply current for this mode is approximately 5 μA under certain conditions.
Figure 3 Rough comparison of power consumption in each power control mode (conditional)
clock switching
A divider ratio can be selected for the system clock (ICLK). When a high-speed clock is not needed, the user can switch to an appropriate low-speed clock and save power. Clock divider ratios are 1, 2, 4, 8, 16, 32 and 64.
The lower the frequency, the lower the current consumption. But in terms of power performance (mA/MHz), 48 MHz is the most efficient (100 μA/MHz = 4.8 mA/48 MHz). Generally speaking, for applications that require higher computing processing and CPU performance, lower power consumption can be achieved by setting a higher frequency and shortening the CPU processing time in normal mode. On the other hand, for applications such as control systems, the current consumption can be reduced by setting the frequency to a lower value in normal mode.
For example, with ICLK of 48 MHz, 32 MHz, 16 MHz, and 8 MHz, the typical supply currents are 4.80 mA, 3.45 mA, 2.05 mA, and 1.40 mA respectively with the following power-saving features. Low power mode: normal mode, power control mode: high speed mode, module stop: disable all peripheral clocks.
Figure 4 Rough comparison of power consumption under the same conditions with other power-saving functions
Peripheral clocks (PCLKB, PCLKD) also have a choice of clock divider ratios 1, 2, 4, 8, 16, 32 and 64.
Module stop function
Power can be saved by stopping inactive modules or their clocks via the following register settings:
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The operation of DTC, I2C, SPI, SCI, CAC, CRC, DOC, ELC, AGT, GPT32n, GPT16n, POEG, ADC120 modules can be stopped by setting the MSTPCRn (n: A, B, C, D) register
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The register read/write clocks of RTC, WDT, and IWDT can be stopped by setting the LSMRWDIS register.
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The running clock of MPU, debugging and BPF can be stopped by setting the LPOPT register
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In software standby mode, 8 KB of the 16 KB SRAM can be turned off by setting the PSMCR register
Each power-saving function combination
Combined with the power saving function, more power saving effects can be achieved. The table details five scenarios, which are just a few examples of the many possible combinations.
Table 1 Examples of power saving function combinations (applicable conditions)
Figure 5 Rough comparison of power consumption under various combinations
For more information about the ultra-low power RA2E3 MCU, please click at the end of the article to read the original article and visit the product page to view. You can identify the QR code below or copy the link below and open it in your browser to obtain the RA2E3 hardware manual to learn more about the RA2E3 low-power operating mode.
https://www.renesas.cn/cn/zh/document/mah/ra2e3-group-users-manual-hardware
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