Serial Input and Output

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Serial Input and Output
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• ASYNCHROUS COMMUNICATIONS INTERFACE ADAPTER (ACIA)

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Lecture Outline
1. Data communication
2. ACIA 6850
3. EIA-232
4. Programming and interface
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ACIA LEARNING OBJECTIVES

• After completing this course, you should be
• able to
• Make money?????
• Teach other people about microprocessor
• Design your own system by using ACIA with serial
  link to pc
• Write a program to configure the ACIA
• List the internal registers of the ACIA and state
  their functions
• Or get extra tensions

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ACIA

• ACIA is a special chip that provides the data
  formatting and control to interface serial
  asynchronous data communication infornation to
  bus organized systems.
• Its allows two or more microcontroller to
  interface together via 3 wire with serially
  transmitted or received
• Normally used as modem peripheral

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• The 6805 is an adaptor which provides controlled
  connection to and from the 6802 and peripherals
  that receive and receive data.
• The 8-bit parallel data from the microprocessor
  is stored in a data register within the ACIA, it
  is formatted then outputted one bit (bit by bit)
  at a time to serial port.
• The speed at which bits are sent, the number of
  bits persecond is called BAUD RATE. (units bps)
• Typical baud-rate values are 1200bps, 2400bps,
  4800bps,9600bps and 19200bps.
• 1 start bit-the signal line is normally high so
  the beginning of character is indicated by a
  start bit going low.

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• 7 or 8 bits data-normally set to 7 bits (D0-D6)
  for ASCII characters
• 1 Parity bit-optional if transmit 7 bits of data.
  Used for error detection
• 2 stop bits-the end of character is indicated by
  2 stop bits, both high states.
• Two type of parity bit-even and odd.
• If even parity bit is used, this bits will set to
  0 or 1 to make the sum of bits D0 to D7 an even
  number. Errors during transmission will indicated
  by a logic 1 in bit 6 of ACIA status register at
  receive side.

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REGISTERS OF THR ACI

• At the bus interface, the ACIA appears as two
  addressable memory locations. Internally, there
  are 4 registers, two deal with data and two, with
  status and control.
• Two read-only registers (status and Receive Data
  registers).
• Two write-only registers (Control and Transmits
  Data registers)
• Receive Data and Transmit Data registers used
  same address.
• Status and Control registers also used same
  address.

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MASTER RESET

• Done automatically after power-up to ensure the
  reset condition and prepare for programming the
  ACIA functional configuration when the
  communications channel is required.
• For interfacing with MODEM, three signal, which
  permit handshake facility are provided

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• CTS-clear to send
• DCD-data carrier detect
• RTS-ready to send
• When a MODEM is not used, CTS and DCD
• will connect to logic LOW. RTS can be left
• no connection.

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• The ACIA is memory-mapped using CS0, CS1, CS2 and
  RS it appears as only 2 addressable memory
  locations.
• The register select pin together with R/W is used
  to select each register.
• Transmit Data Register (TDR)
• The byte of data to be transmitted is written to
  the TDR by microprocessor. It is transferred to
  the transmit shift register, if its is empty, the
  byte is then shifted out serially.
• Data will written into TDR during negative
  transition of the enable, pin E when the ACIA has
  been addressed with RS high and R/W low.

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• Receive Data Register (RDR)
• Information from external devices enters the ACIA
  serially via the receive shift register, which
  strips off the start and Stop bits and sends the
  byte in a parallel transfer to the RDR.
• The processor must read the RDR while the second
  byte is being received.
• Control Register (CR)
• This register controls the format of both the
  transmitted and received data.
• Status Register (SR)
• Monitors the progress of data transmission and
  reception and sends it to the microprocessor.

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PROGRAMMING THE ACIA

• Programming is accomplished by sending an 8 bit
  number out to the ACIA control register (CR).
• Bit CR0 and CR1 select the appropriate baud-rate
  multiplier and also can reset the chip.
• Example if the input clock frequency is
  19,200Hz, divide by 1 would cause operation at
  19,200 bps. If divide by 16 would cause operation
  at 1,200 bps.
• This clock signal comes from baud-rate generator
  (MC14411 or MC145411).
• The ACIA is reset to clear any error indications
  in the status register (SR).

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• Bit CR2, CR3, CR4 determine the format of the
  character that will be transmitted and received
  by the ACIA.
• Example If all 3 bits are programmed as zeros, 7
  data bits with even parity and 2 stop bits are
  transmitted and received.
• CR5 and CR6 control the transmitter interrupt and
  RTS pin and determine whether or not a break
  character is transmitted. Normally used for
  MODEM.
• CR7, enable IRQ pin. IRQ0 occurs for a received
  byte of data, an overrun error, or positive
  transition on the DCD pin.

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• Status register (SR) is read by selecting the
  device and placing a logic zero on the RS pin.
• This SR indicates error conditions, the
  conditions of some of the MODEM control pins, and
  the general operating condition of the ACIA.
• RDRF indicates that data is available for the
  microprocessor, which must read or removed it
  before the next data/character is received into
  RDR. If not, the OR bit will set to one.

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• RDRF is logic high when full and become empty (0)
  after read operation.
• TDRE, acknowledge to the microprocessor to load
  character/data into TDR when empty (TDRE1). The
  write command cause TDRE0. TDRE automatically
  set to one when the ACIA complete transmit the
  contents of TDR.
• IRQ status bit indicates that the input buffer
  (RDR) is full. CR7 at control register (CR) must
  set to one if we used external interrupt pin
  (IRQ).

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HOW TO ACCES ACIA?

• Before we start writing a program to acces
• the ACIA device, the following procedure
• must be followed.
• Choose the Baud rate clock at baud rate generator
  by selecting the RSA and RSB according to the
  speed what we want to used.
• Initialized the ACIA by putting 03 hex into
  conrol register
• Choose the type of transmission by stored control
  word into control register. The counter divide
  inside control register (CR0 and CR1) must select
  according to the value has been choose at baud
  rate generator (RSA and RSB)

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HOW TO INTERFACE AN ACIA WITH IBM PC?

• The RS-232C interface standard is almost
  universally used for the interconnection of
  terminal equipment such printer, keyboard, MODEM,
  tape recorder, CRT stcs. That receives and
  tranmits serial asynchronous data.
• The standard specifies the pin connections and
  connector to be employed.
• For RS-2332C, there is two types of connector can
  be used
• DB-25 male/female (25pins)
• DB-9 male/female (9 pins)

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