1. Embedded System Design Techniques™:
Mastering the ARM© Cortex-M© Processor
Class 5: Low Power Design Techniques
May 22, 2015 Jacob Beningo, CSDP

2. Course Overview Overview of STM32 ARM Cortex-M0+
Project Setup and Compilation
ARM Cortex-M Architecture
Writing Embedded Software with CMSIS
Low Power Design Techniques

3. Session Overview Energy Profiling Wait for Interrupt Sleep-on-exit
Low Power Modes
Low Power Example

4. Introduction So what is the problem? Consumer Level
Demand for longer battery lives
Devices are used more often for longer periods of time
Engineering Level
Short design cycles
Management Decisions
Lack of low power experience
Use of Proper tools

5. Energy Profiling 3.7 V Energy Profile Key Points
Power profile can be calculated from A x V
Energy Profile can be approximated by current only!
Integrating the energy profile will provide mAh used
Can be used to identify heavy energy usage periods
Can be used to estimate battery discharge time

6. Energy Profile – STM32CubeMx

7. Low Power Modes – STM32Lx

8. Low Power Mode Summary

9. Power Mode Registers

10. WFI vs WFE
if(WakeupType == POWER_WAKE_WFI) { /* Wait For Interrupt */ __WFI(); } else /* Wait For Event */ __SEV(); __WFE();

11. Sleep on Exit

12. Wakeup Considerations

13. STM32 Sleep Mode Example DEBUGGER WARNING!
void Power_ModeStop(PowerWake_t WakeupType) { uint32_t temp = 0; temp = PWR->CR; temp &= (uint32_t)~(PWR_CR_PDDS | PWR_CR_LPSDSR); PWR->CR = temp; SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk; if(WakeupType == POWER_WAKE_WFI) __WFI(); }
WARNING!
DEBUGGER
Sleep

14. STM32 Sleep Mode Example
// Enable the SYSCFG block RCC->APB2ENR |= RCC_APB2ENR_SYSCFGEN; // Setup Interrupts on GPIO C13 SYSCFG->EXTICR[3] |= SYSCFG_EXTICR4_EXTI13_PC; // set pin to use // Configure GPIO C13 as an external interrupt for falling edge EXTI->IMR |= EXTI_IMR_IM13; // unmask interrupt EXTI->EMR |= EXTI_EMR_EM13; // unmask event EXTI->RTSR &= ~EXTI_RTSR_TR13; // set rising edge EXTI->FTSR |= EXTI_FTSR_TR13; // set falling edge // Interrupt to enable for button NVIC_EnableIRQ(EXTI4_15_IRQn);

15. STM32 Sleep Mode Example
void EXTI4_15_IRQHandler(void) { if (EXTI->PR & (1<<13)) // clear pending interrupt EXTI->PR |= (1<<13); } void Tsk_1000ms(void) Power_ModeStop(POWER_WAKE_WFI);

16. Tips for Energy Conservation
Tip #1 – Turn Off Unused Module and Clocks
If it is not being used then don’t let it consume energy!
Clock gating stops the peripheral from causing logic to switch which uses energy
P = C * F * Vcc2 + IL * Vcc

17. Tips for Energy Conservation
Tip #2 – Use a LPT or RTC
Lower Energy Usage due to slower OSC
Used to interrupt system and wake up from extremely low power state

18. Tips for Energy Conservation
Tip #3 – Beware Communication Channels
Can be power hungry peripherals
May require periodic waking
Use DMA to lighten the Load
Can keep the CPU asleep
Transfer data from peripheral to memory
Uart Serial Wake-Up
Peripheral Sleeps until data is received
Wakes up and then performs functions

19. Tips for Energy Conservation
Tip #4 – Use Energy Profiling Tools

20. Tips for Energy Conservation
Tip #5 - Think Outside the Box
USB
Device
USB
Host D+ D-
USB
Device
USB
Host Tx Rx

21. Additional Resources Download Course Material for
Updated C Doxygen Templates (May 2015)
Example source code
Templates
Microcontroller API Standard
EDN Embedded Basics Articles
Embedded Bytes Newsletter
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