CprE 211 Introduction To Microcontrollers Lecture 15

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CprE 211 Introduction To MicrocontrollersLecture
15

• Nathan VanderHorn
• Fall 2005

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Input Output Programming

• Communication with external devices
• How does the processor know an external device is
  requesting communication we will focus on two
  methods
• Polling keep checking the status of external
  devices
• Interrupt external device signals processor
  that service is requested
• Student questions example
• Polling Ask students for questions, raise
  hands, gather information
• How many questions?
• Order in which to service questions?
• Interrupt Student interrupts lecture with
  question
• Service question immediately
• What happens if a question interrupts the current
  question
• Interrupts can be more difficult

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Methods for I/O Programming

• Sampling
• Simple devices in which the device is always
  ready to accept or provide data
• The interface is extremely simple consisting of
  only a data register
• Examples are temp sensors, motor control port
• Polling
• Requires data and status/control registers
• Status is read by the microcontroller and denotes
  status of device
• Control is written by microcontroller to indicate
  the characteristics of communication baud rate,
  operation mode etc.
• Interrupt
• Device interrupts the microcontroller when
  service is needed
• Control is passed to the device if interrupts are
  enabled
• Software handles the operations to be performed

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Communication Examples

• Keyboard
• Processor ? keyboard data ready? read-data?
• Keyboard ? processor data ready
• Hard disk
• Processor ? disk seek sector, seek track,
  read/write block
• Disk ? processor data-ready, error conditions
  bad block
• Printer
• Processor ? printer data-sent, print, font,
  resolution?
• Printer ? processor printing done, error
  conditions out of paper, toner

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Dealing with I/O

• Should we include instructions for communications
  with external devices?
• Is there enough uniformity between I/O devices to
  agree on a single set of instructions to
  communicate with them?
• How about the I/O devices we have not even
  foreseen yet?
• Hard to develop an instruction set to control
  todays and tomorrows devices!
• Necessity for a generic method to communicate
  with all devices

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Memory Mapped I/O
Peripherals

• How should the Processor communicate with the I/O
  devices
• Will every device communicate in the same way?
• Need a generic method for accessing external
  devices

Keyboard
Bus
Mouse
Processor
Printer
A/D converter
Other Device
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Memory Mapped I/O

• Generic control
• Make the interface between the I/O device
  processor soft
• Develop each devices control as a unique data
  structure
• Specify a place in memory for these control and
  data registers

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Keyboard Example

• Getchar() gets an ASCII byte from the keyboard
  and puts it into r3
• Assume r13? 0xF000 0000
• Assembly code for Getchar()
• poll
• lwz r4, 0(r13)
• andi. r4, r4, 1
• beq poll
• getchar
• lbz r3, 4(r13)

Processor
Keyboard
Data register
0xF000 0004
0xF000 0000
Control Register
R
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Polling

• We have just seen how polling works with a
  keyboard example
• Is it efficient to poll?
• Sometimes yes sometimes no
• Depends upon the application

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Why Interrupt I/O

• Polling is what we have seen
• main ()
• while(1)
• PollKeyboard()
• Polling the keyboard
• Read port to see if a key is pressed
• Must poll every n ms, the minimal time period a
  key is pressed otherwise keystroke is lost

• If the program has tasks to do
• main ()
• task1()
• PollKeyboard()
• task2()
• PollKeyboard()
• Overhead polling frequency polling overhead
• Difficulty Hard to mix polling with a normal
  process

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Generic Programming Model with Interrupts
start
Interrupt
Interrupt
Interrupt
Initialize
ISR for task 1
ISR fortask 2
ISR fortask 3
Wait for events
I-return
I-return
I-return
Processing
Memory
How to support interrupt programming? How to
write interrupt-based C and Assembly code?
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Understanding Interrupts

• Normal control flow
• Exception control flow
• Exception and Interrupts
• Hardware implementation of interrupts
• Machine level interrupt programming

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Normal Control Flow

• In a processor, the program counter (PC) takes a
  sequence of values
• a0, a1, , an-2, an-1
• where each ak is the address of an instruction
  Ik.
• Control transfer each transition from ak to
  ak1
• Flow of control, or control flow sequence of
  control transfers
• Smooth control flow Ik and Ik1 are adjacent
  in memory
• Changes to smooth flow Ik1 not adjacent to Ik
• Branches
• Calls, returns

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Exceptional Control Flow (ECF)

• Abrupt changes in control flow not caused by
  program statements and variables Ik1 is
  external to the program
• Hardware timer goes off at regular intervals
• I/O request completes, e.g., A/D conversion, disk
  I/O
• Packets arrive at network adapter
• Instruction attempts divide by zero
• Virtual memory page fault occurs
• Memory access violation
• Exceptions occur at all levels in a computer
  system hardware, operating system, user

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Exceptions

• DEFINITION an abrupt change in the control flow
  in response to some change in the processors
  state
• Event change in processors state, where state
  is encoded in various bits and signals inside the
  processor
• Event might be related or unrelated to current
  instruction
• Implementation details vary from system to system