Embedded Systems: Interrupts and Power Management

MECHTRON 2TA4: Embedded Systems Design I

• Interrupts and Power Management are the topics.

Introduction to Interrupts

• Embedded systems respond to asynchronous requests or perform operations periodically.
• Polling involves the CPU continuously checking for events in an infinite loop until a request or period is reached.
• Interrupt usage serves as a better approach to handling such operations.
• The CPU can perform its usual tasks while handling asynchronous or periodic operations when requested through interrupts.
• A recent trend in embedded systems focuses on consuming minimal power.
• Systems deployed in remote locations can be powered by batteries or energy harvesting modules.
• Power management and interrupt concepts are related to each other.

Interrupt Concept in Embedded Systems

• Polling involves software periodically checking for events which wastes CPU cycles, is triggered by software, and occurs periodically.
• Interrupts entail the CPU taking action only when an event occurs which does not waste CPU cycles, can be triggered by hardware or software, and can occur at any time.

Interrupt Concept in Embedded Systems: Implementation Example

• Microcontroller (MCU) controls a DC motor by sending it a signal periodically.
• A user can start/stop the motor with a button or change the rotation direction with another button.
• Button presses are asynchronous inputs that the CPU must handle while feeding the signal to the motor.
• Interrupts are generated when buttons connected to MCU pins are pressed (GPIO interrupts).
• The MCU is set up so that the GPIO pins generate interrupt requests.
• When a button controlling direction is pressed, an edge detector circuit senses the voltage difference to the pin of the microcontroller
• A flag is set indicating that the pin had generated an interrupt request
• A special module handles the interrupt request and checks whether it is allowed, and the priority of the incoming interrupt (by default all interrupts are first-come-first-served).
• The CPU is diverted to the interrupt service routine (ISR) function, and a related flag is set.
• It knows the interrupt is being handled.
• The CPU generates a hardware signal associated with pressing the button to call each function
• The motor start/stop and direction change operations should be done within specific ISR functions.
• Once the ISR function is completed, the CPU returns to its previous operation.

Interrupts in the STM32F4 Microcontroller

• The STM32F4 Microcontroller has GPIOx pins which connect to the EXTI then NVIV then Processor and Peripherals
• Block diagram of the EXTI module has 16 interrupt lines, EXTIO to EXTI15 which represent pin numbers
• PAO is connected to EXTIO, and PA13 is connected to EXTI13
• Pins with the same number are connected to a multiplexed line
  • Cannot use two pins on one line simultaneously
  • PAO, PBO, PCO are connected to EXTIO, so only one pin at one time can be used to handle the interrupt
  • PAO and PA5 can be used at the same time because they are connected to different lines

Power Management in Embedded Systems

• Embedded systems in standalone applications often depend on batteries.
• To extend working time:
  • Select a battery with larger capacity.
  • Design a system with battery recharging capability.
  • Decrease power consumption.
• Recent microcontrollers provide power management options, such as the Arm® Cortex™-M architecture.
• Power management starts by decreasing the power consumption of the device.
• Microcontrollers operate in active (run) mode which consumes the most power.
• Operating the CPU in low power (sleep) mode is an important use of interrupts.
• An external event triggers an interrupt, the CPU wakes up, performs an operation related to the event in active mode, and returns to low power mode.
• Systems stay in active mode only when needed and in low power mode the rest of the time to minimize power dissipation.
• Embedded systems can be powered by batteries in stand-alone applications.
  • Rechargeable batteries can be used more than once, but require a charging circuit increasing the overall cost and are more expensive
    • A rechargeable battery typically has a lifetime of 1000 to 2000 charging cycles
  • Disposable batteries can be used without any charging circuitry and are inexpensive
    • Disposable batteries can only be used once and need to be replaced when depleted
• The capacity of a battery is provided in milliampere hour (mAh) by the manufacture.
• STM32F4 microcontroller power supply options
  • VDD: Main power supply (1.7 - 3.6V)
  • VDDA: Analog power supply (1.7 - 3.6V)
  • VBAT: Backup power supply (1.65 - 3.6V)
  • VREF: ADC/DAC reference voltage (1.7 - 3.6V)
• STM32F4 microcontroller power modes
  • Run: All active (CPU/SRAM/Peripherals), Max Clock Speed (120 - 180 Mhz)
  • Sleep: SRAM/Peripherals Active, Max Clock Speed (120 - 180 Mhz)
  • Stop: SRAM Active, Max Clock Speed (-) Mhz
  • Standby: Power-down mode, Max Clock Speed (-) Mhz