ELE22MIC Lecture 16

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ELE22MIC Lecture 16

• Introduction to Parallel Input Output (I/O)
• 68HC11 Parallel I/O
• Ported vs Memory Mapped I/O
• IO Methods
• Polled
• Interrupt Driven
• Direct Memory Access (DMA)

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I/O

• I/O devices are peripheral devices used by the
  processor to sommunicate to the outside world,
  for example
• terminals,
• keyboards,
• displays,
• mice,
• network interface
• sound
• disk drives
• printers
• Analog to Digital Converters

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I/O Types

• I/O can be separated to two basic types
• Parallel
• Serial

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I/O Access by CPU

• Two ways of accessing I/O devices
• Ported
• I/O occurs on separate I/O address space
• Separate In Out instructions to access I/O
• Separate memory space
• Different I/O Memory timing constraints
• Memory Mapped
• I/O occurs on in memory address space
• Same instructions timing for I/O as for memory

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Ported vs Memory Mapped I/O
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I/O Ports

• A port is an 8-bit or 16 bit register where data
  en-route from the input, or to the output,
  resides.
• The microprocessor treats these ports as memory
  locations that may be read-from or written-to.

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I/O Devices

• Input devices must hold data for as long as is
  required by the processor to read the data.
• Output devices must hold data for as long as is
  required by the peripheral to output the data.
• The I/O devices must transfer data between the
  processor and the port under processor control.
• For memory mapped I/O they must operate as a
  functional equivalent of memory devices.

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Simple Parallel I/O (1)

• Simple Parallel I/O is primitive
• Simplest form of Output is latching data from the
  data bus
• Simplest form of Input is reading the state of an
  external device onto the data bus.
• Example Devices
• 74HC574 Latch for output
• 74HC245 buffer for input

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Simple Parallel I/O (2)
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Simple Parallel I/O (3)

• There is no way to asynchronously inform the
  processor when new data is arrived
• There is no way of telling when the external
  devices has read the current output data and is
  ready for the next data
• A output port is always an output port
• A input port is always an input port
• Neither versatile nor re-configurable
• Not programmer friendly if data needs frequent
  reading/writing

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Methods of I/O device access

• Three main methods of performing I/O
• Polled IO
• Interrupt-Driven I/O
• Direct Memory Access (DMA)

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Polled I/O

• Transfers take place under main program control
• Processor accepts/delivers data when it choses

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Interrupt-Driven I/O

• External interrupts trigger the beginning of I/O
  operation
• The Interrupt Service Routine (ISR) handles the
  I/O transfer
• Main program is suspended during ISR
• Sequence Interrupt is signalled.
• CPU stops current program saves state
• CPU executes ISR returns
• CPU restores previous state and continues

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Direct Memory Access (DMA)

• Processor address data bus is taken over by a
  DMA controller which performs memory lt-gt I/O
  transfer without CPU control.
• Separate DMA hardware performs Memory lt-gt I/O
  transfer
• Main program may/may not be halted during
  transfer depending on type of transfer and
  instruction in execution

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Peripheral Interface Adapter

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