Evolution of Microcontroller Firmware Development

1. Evolution of Microcontroller Firmware Development David Benjamin

2. Overview • Traditional microcontroller firmware • Polling • Interrupts • Real-time Operating Systems • Demonstration

3. Polling • Polling is when a process continually evaluates the status of a register in an effort to synchronize program execution. • Polling is not widely used because it is an inefficient use of microcontroller resources.

4. Polling Example void main ( void ) { while(1) { while (!button1_pressed); turn_on_led; while (!button1_pressed); turn_off_led; } }

5. Interrupts • Interrupt as defined by Merriam Webster • to break in upon an action • Interrupt • An event that causes the Program Counter(PC) to change. These events can be internal or external. • Interrupt Service Routine (ISR) • Set of instructions that is executed when an interrupt occurs • Interrupt Vector • Memory address of the first instruction in the ISR.

6. Interrupts • Why should one use interrupts? • Provides more efficient use of microcontroller resources. • Provides a means to create firmware that can “multi-task”.

7. Interrupts • Interrupts are often prioritized within the system and are associated with a variety of on-chip and off-chip peripherals • Timers, A/D D/A converters, UART, GP I/O • A majority of the mC interrupts can be enabled or disabled. • Globally • Individually • Those interrupts that cannot be disabled are called Non-Maskable Interrupts (NMI). • Interrupts can be nested. • Interrupts can occur at anyplace, at anytime. • ISRs should be kept short and fast.

9. What happens when an interrupt occurs?

10. The current program instruction completes execution. Main() { … some code turn_on_led1; 0x08A2 turn_off_led1; 0x08A3 … } __interrupt void port1_ISR(void) { disable_interrupts 0x0FE0 ... reti } Interrupts XXX XXX 0x08A3 0xFE PC = 0x08A2 SR = 0xFE PC = 0x0FE0 SR = 0x7E PC = 0x8A3 SR = 0xFE STACK • The PC and status register values are placed on the stack. • The interrupt vector is loaded into the PC and SR is updated. • Program execution resumes with the first step of the ISR. • When the ISR has completed execution, the values of the PC and status registers are restored. • Program execution resumes with next instruction that would have occurred had the interrupt not taken place.

11. Interrupts vs. Polling • Allowed for more efficient use of the microcontroller. • Faster program execution • Multi-tasking • Facilitated the development of complex firmware. • Supports a modular approach to firmware design.

12. Real-time Operating Systems

13. Real-time Operating System • A real-time operating system (RTOS) is a multi-tasking operating system intended for real-time applications. • Mainly used in embedded applications. • Facilitates the creation of a real-time system. • Tool for the real-time software developer. • Provides a layer abstraction between the hardware and software.

14. Real-time Operating System • State • A unique operating condition of the system. • Task • A single thread of execution through a group of related states. • Task Manager • Responsible for maintaining the current state of each task. • Responsible for providing each task with execution time.

15. Real-time Operating System Collection of Tasks… Single Task

16. Real-time Operating System • A more detailed explanation state • A function void idleState ( void ); • Should be kept short and fast • Should represent a logical step in the task • i.e. Evaluating different parts of an incoming message.

17. Real-time Operating System void idleState( void ) { if (rx_buffer_full) { read_rxBuffer; if (syncByte_received) { transition_to_workState1; } else { stay_in_idleState; } } else { stay_in_idleState; } }

18. Event-driven Tasks are granted execution time, based on an event (interrupt). Tasks of higher priority are executed first (interrupt priorities). Time sharing Each task is granted a given amount of time to execute. Tasks may also be granted execution time based on events. “Round-robin” approach Creates a more deterministic multi-tasking system. Real-time Operating System

19. Real-time Operating Systems void main ( void ) { initSystem(); while (1) { work(); sleep(); } } void work (void) { doTask(RecieveMsg); doTask(Process); doTask(TransmittResponse); } __interrupt void Timer_A (void) { wakeUp(); }

20. Real-time Operating System • Available commercially • TinyOS -an open source component-based operating system and platform targeting wireless sensor networks (WSNs). • Salvo - an RTOS developed by Pumpkin Inc. • FreeRTOS - free RTOS that runs on several architectures. • DrRTOS - works with ARM 7 • Implement a custom RTOS • Can be highly optimized to suit your application

21. Real-time Operating Systems • A tool for real-time software developers. • Allows increasingly complex systems to be developed in less time. • Provides a level abstraction between software and hardware. • Continues the evolution microcontroller system design.

22. Example Implementation • Example of RTOS application that requires wireless communication. • Hardware abstraction layer for the radio. Performs low-level interaction between mC and radio. Application Application • Groups low-level interactions from the HAL into higher-level functions. • Sending a packet Protocol Radio Operation Layer • Middle-ware that provides a gateway between the application and RTOS. CC1100 HAL • Application

23. Demonstration

24. Questions?