Microprocessor-based Systems

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Microprocessor-based Systems

• Course 9 Design of the input/output interfaces
  (continue)

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Transfer through direct memory access (DMA-
Direct memory access)

• Why is it used?
• To increase the transfer speed
• In order to release the main processor from the
  transfer task
• Implementation mode
• Specialized circuit for handling the transfer
  the DMA controller
• The transfer is programmed by the processor and
  than it is performed by the DMA controller the
  transfer is made between the memory and an I/O
  interface without the direct implication of the
  main processor
• The transfer phases
• Initialization - the main processor programs the
  parameters of the transfer (the DMA controller is
  behaving as a slave)
• The address of the memory zone
• The direction of the transfer
• The number of transferred data
• The transfer phase the controller performs the
  transfer and solve the synchronization with the
  peripheral device (the DMA controller is a
  master)
• End of the transfer the processor test the
  correctness of the transfer by reading the status
  of the DMA controller (the DMA controller is a
  slave)

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The scheme of connecting a DMA controller into a
microprocessor system
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The sequence for handling a DMA request

1. An interface requires a transfer by activating
   the DRQi signal to the DMA controller
2. The DMA controller tests if the request is
   enabled and if there are no other higher priority
   transfers in progress
3. If the request is enabled and there is no other
   request than it requests the control of the bus
   by generating a HOLD signal to the processor
4. The processor finishes the last cycle on the bus
   and than disables its bus amplifiers (leaving the
   control of the bus to the DMA controller) and
   grants the HOLD request with the signal HOLDA
   (Hold Acknowledge) to the controller
5. The DMA controller take over the control of the
   bus by activating its bus amplifiers it
   generates an address for the memory and DACKi
   (DMA Acknowledge) signal to the interface
6. The DMA controller generates a memory read signal
   (MRDC/) or an interface read signal (IORC/)
7. The addressed unit (memory or interface)
   generates the required data
8. The DMA controller generates an interface write
   signal (IOWC/) or a memory write signal (MWTC/)
   as result the data is transferred from the memory
   to the interface or vice-versa
9. The interface disable the DRQi signal and as a
   result the DMA controller disables its bus
   amplifiers
10. The DMA controller disables the HOLD signal,
    giving up the control of the bus in favor of the
    processor
11. The processor disables the HLDA signal, it
    activates its bus amplifiers and continues with
    the execution of instructions

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Time diagram for a DMA transfer
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Internal structure of the I8237 DMA controller
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Features of the I8237 DMA controller

• can handle up to 4 peripheral devices (it has 4
  independent DMA channels)
• transfer speed 1,6Mocteti/s
• more controllers may be connected in a cascading
  mode in order to increase the handled channels
• the maximum dimension of a transferred block
  64KB
• it can handle memory to memory transfers
• More transfer modes
• Single Transfer
• Block Transfer (burst)
• Auto-initialized Transfer
• Memory to memory transfer

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Transfer through I/O processor

• When is it used
• a higher speed is required
• the peripheral device is complex (it has a
  complex behavior)
• we want to discharge the main processor form I/O
  tasks
• Implementation
• Input/Output processor
• Transfer program
• we use DMA-like mechanisms and interrupts to
  synchronize the main and the I/O processors
• Types of processors used for this purpose
• Specialized I/O processors
• General purpose processors integrated into an
  interface (ex Z80)
• Microcontrollers and signal processors (ex 8048,
  8032, etc.)
• Processors specialized for a specific peripheral
  device (e.g. graphical processor)

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Advantages and drawbacks of the I/O
processor-based transfer

• Advantages
• Higher speed
• Can be adapted to the behavior of a complex
  peripheral device (through a program)
• Drawbacks
• Hard to implement and test
• Higher costs
• Concurrency problems on the system bus and memory
  inconsistency

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Comparison between transfer modes
Transfer Complexity Cost Speed Processor Involvement
Through program Small Small Small total
Through interrupts Average Average Average Significant
Through DMA High High High Small
Through I/O processor Very high Very high High Very small
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The serial interface

• Interface types
• Serial reduced number of signals (2-3) and a
  bit-by-bit transfer
• Parallel a number of parallel signals (usually
  8 for data and some for control) transfer of
  data is made in parallel
• Serial transmission
• A limited number of signals used for the
  transmission (usually 2 or 3 for a transmission
  direction)
• the transmission is made sequentially bit-by-bit
• used for long distance transfers
• used to interconnect a computer with a peripheral
  device or two or mode computers
• Serial transmission types
• Based on the synchronization mode
• synchronous serial transmission the transfer is
  controlled with a clock signal
• Asynchronous serial transmission no clock
  signal is used
• based on the interconnection mode
• point-to-point connection between two
  equipments
• Multi-point more equipments connected on the
  same link
• based on the direction of the transfer
• Unidirectional transfer only one direction
• Bidirectional transfer
• Full duplex 2 independent directions

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Synchronous serial transfer

• Issue synchronization between the transmitter
  and the receiver when is the optimal time for
  reading the next bit
• Synchronous transfer the speed of the transfer
  is controlled with a clock signal
• Examples
• I2C serial bus for microcontrollers
• the keyboard interface of PCs
• Advantages easy implementation, reliable on
  short distances
• Drawbacks extra wire for clock signal, limited
  transmission distances

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Asynchronous serial transfer

• no clock signal is used synchronization is made
  through the specific structure of the transmitted
  data
• Types
• Character-based asynchronous transmission
• Message-based asynchronous transmission
• The communication rules established through a
  standard protocol
• The protocol specifies
• Codification of the binary information (e.g.
  voltage levels, current levels, light impulses,
  etc.)
• The communication environment (twisted or coaxial
  wires, optical fibers, radio waves, etc.)
  topology
• The structure of the transmitted data
• The method used for synchronization
• The method used of error detection
• The method used for the flow control

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The RS232 (V24) standard

• RS232 the best known and most used serial
  asynchronous transfer standard
• Information coding voltage levels
• 0 (3..15V) usually 12V
• 1 (-3..-15V) usually -12V
• Communication environment electric wires
  (including phone wires)
• The structure of the transmitted data
• synchronization through the first bit, stop bits
  and frequencies 300,600, 1200, 2400, 19200
  Bauds
• Error detection parity bit

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The RS232 (V24) standard

• The data-flow control
• Hardware through pairs of signals
• DSR-DTR Dataset ready, Data Terminal Ready
• RTS-CTS Request to Send Clear To Send
• Software through special ASCII codes
• XON-XOFF Start/stop transmission
• Transmission distance max. 100 m

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The RS232 (V24) standard

• Long range transmission through MODEM and phone
  networks
• Signals
• TXD transmission
• RXD reception
• GND ground
• DSR Data Set Ready
• DTR Data Terminal ready
• RTS Request To send
• CTS Clear To sent
• RI - ring

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The RS232 (V24) standard

• Transmission modes
• One direction,
• 2 wires/signals
• Bidirectional with software control of data flow
  (XON XOFF)
• 3 wires/signals
• Bidirectional with hardware control of data flow
  (DTR-DSR or CTS-RST)
• 5-7 wires/signals

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Universal Serial Asynchronous Controller (USART)
I8251
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Other serial transmission standards

• RS485
• Features
• digital information coding through differential
  voltages (not through voltage levels)
• twisted pair of wires for bidirectional
  transmission
• more devices connected on the same cable
  multi-point transmission
• used as low level protocol for industrial
  networks (ex Profibus, CAN)
• Advantages
• longer transmission range (till 1 km)
• higher noise immunity (because of twisted wires
  and differential coding)
• used mainly in industrial environments

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The RS485 standard
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Other serial transmission standards

• HDLC/SDLC
• Serial asynchronous transmission on messages
• used for network transmission
• More efficient than the character-based
  transmission (useful data/total no. of bits
  ration is better)
• Very good error detection mechanism (CRC)
• Synchronization through PLL (Phase Lock Loop)
• The structure of the transmitted data message