Evolution of Microcontroller Firmware Development

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Evolution of Microcontroller Firmware Development

• David Benjamin

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Overview

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

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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.

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Polling Example

• void main ( void )
• while(1)
• while (!button1_pressed)
• turn_on_led
• while (!button1_pressed)
• turn_off_led

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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.

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Interrupts

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

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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.

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What happens when an interrupt occurs?
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Interrupts
XXX XXX
0x08A3 0xFE
PC 0x08A2 SR 0xFE
PC 0x0FE0 SR 0x7E
PC 0x8A3 SR 0xFE
STACK

• 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

• 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.

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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.

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Real-time Operating Systems
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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.

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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.

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Real-time Operating System
Collection of Tasks
Single Task
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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.

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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

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Real-time Operating System

• 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.

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Real-time Operating Systems
void main ( void ) initSystem() while
(1) work() sleep() void work
(void) doTask(RecieveMsg) doTask(Process) d
oTask(TransmittResponse) __interrupt void
Timer_A (void) wakeUp()
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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

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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.

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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

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Demonstration
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Questions?