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8051 Microcontroller: Serial Communication

Virendra Singh

Associate Professor Computer Architecture and Dependable Systems Lab

Department of Electrical Engineering Indian Institute of Technology Bombay

http://www.ee.iitb.ac.in/~viren/ E-mail: viren@ee.iitb.ac.in

EE-309: Microprocessors Lecture  13  (18  Aug  2014)  

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8051 Pin

Diagram

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21

P1.0 P1.1 P1.2 P1.3 P1.4 P1.5 P1.6 P1.7 RST

(RXD)P3.0 (TXD)P3.1

(T0)P3.4 (T1)P3.5

XTAL2 XTAL1

GND

(INT0)P3.2 (INT1)P3.3

(RD)P3.7 (WR)P3.6

Vcc P0.0(AD0) P0.1(AD1) P0.2(AD2) P0.3(AD3) P0.4(AD4) P0.5(AD5) P0.6(AD6) P0.7(AD7) EA/VPP ALE/PROG PSEN P2.7(A15) P2.6(A14) P2.5(A13) P2.4(A12) P2.3(A11) P2.2(A10) P2.1(A9) P2.0(A8)

8051 (8031) (8751) (8951)

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Data Communication

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8051 and PC •  The  8051  module  connects  to  PC  by  using  RS232.  •  RS232  is  a  protocol  which  supports  half-­‐duplex,  synchronous/asynchronous,  serial  communicaLon.  

•  We  discuss  these  terms  in  following  secLons.  

PC 8051

COM 1 port

RS232

MAX232 UART

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Parallel vs. Serial Data Transfer •  Computers  transfer  data  in  two  ways:  

–  Parallel  • data  is  sent  a  byte  or  more  a  Lme  (fast)  • Only  short  distance  between  two  systems  • The  8-­‐bit  data  path  is  expensive  • Example:  printer,  hard  disks  

–  Serial  • The  data  is  sent  one  bit  at  a  Lme  (slow)  • Long  distance  (rarely  distorLon)  • cheap  •  The  data  can  be  transferred  on  the  telephone  line  (by  using  modem.)  

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Serial vs Parallel Data Transfer

Sender Receiver Sender Receiver

Serial Transfer Parallel Transfer

D0

D7

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Serial vs. Parallel Data Transfer

Sender Receiver Sender Receiver

Serial Transfer Parallel Transfer D0-D7 D0

Other control lines

Other control lines

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Serial Communication

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Serial Communication •  How  to  transfer  data?  

–  Sender:  • The  byte  of  data  must  be  converted  to  serial  bits  using  a  parallel-­‐in-­‐serial-­‐out  shiV  register.  

• The  bit  is  transmiWed  over  a  single  data  line.  

–  Receiver  • The  receiver  must  be  a  serial-­‐in-­‐parallel-­‐out  shiV  register  to  receive  the  serial  data  and  pack  them  into  a  byte.  

11101000001011

‘A’

register 8-bit character

register 8 1

parallel-in serial-out

serial-in parallel-out

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Asynchronous Serial Communication •  With  asynchronous  communicaLon,  the  transmiWer  and  receiver  do  not  share  a  common  clock  

Transmitter Receiver +

1 byte-wide Data

Data –

1 byte-wide Data

The Receiver ¨  Extracts the data using its own

clock

¨  Converts the serial data back to the parallel form after stripping off the start, stop and parity bits

The Transmitter ¨  Shifts the parallel data onto

the serial line using its own clock

¨  Also adds the start, stop and parity check bits

Add: Start, Stop, Parity Bits Remove: Start, Stop, Parity Bits

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Synchronous Serial Communication •  In  the  synchronous  mode,  the  transmiWer  and  receiver  share  a  common  clock  •  The  transmiWer  typically  provides  the  clock  as  a  separate  signal  in  addiLon  to  

the  serial  data  

Transmitter Receiver

Data

Clock

The Receiver ¨  Extracts the data using the

clock provided by the transmitter

¨  Converts the serial data back to the parallel form

The Transmitter ¨  Shifts the data onto the serial

line using its own clock

¨  Provides the clock as a separate signal

¨  No start, stop, or parity bits added to data

1 byte-wide Data 1 byte-wide Data

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Asynchronous vs. Synchronous •  Serial  communicaLon  uses  two  methods:  

–  In  synchronous  communicaLon,  data  is  sent  in  blocks  of    bytes.  

–  In  asynchronous  communicaLon,  data  is  sent  in  bytes.  

byte byte byte byte 01011111

preamble 01010101

sender receiver

byte

sender receiver start bit stop bit

byte byte

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UART and USART •  It  is  possible  to  write  soVware  to  use  both  methods,  but  the  programs  can  be  tedious  and  long.  

•  Special  IC  chips  are  made  for  serial  communicaLon:  – USART  (universal  synchronous-­‐asynchronous  receiver-­‐transmiWer)  

– UART  (universal  asynchronous  receiver-­‐transmiWer)  

•  The  8051  chip  has  a  built-­‐in  UART.  

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UART Block Diagram

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8051 Serial Communication •  The  8051  has  serial  communicaLon  capability  built  into  it.    

•  How  to  detect  that  a  character  is  sent  via  the  line  in  the  asynchronous  mode?  – Answer:  Data  framing!  

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Framing •  Framing:  Each  character  is  placed    in  between  start  and  stop  bits  –  Framing  ASCII  “A”  (41H)  

stop bit

start bit mark 0 0 0 0 0 0 1 1

D7 D0 goes out last goes out first

Time (D0 first)

mark

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Framing • We  have  a  total  of  10  bits  for  each  character:  

–  8-­‐bits  for  the  ASCII  code  –  2-­‐bits  for  the  start  and  stop  bits  –  25%  overhead  

•  In  some  systems  in  order  to  maintain  data  integrity,  the  parity  bit  is  included  in  the  data  frame.  –  In  an  odd-­‐parity  bit  system  the  total  number  of  bits,  including  the  parity  bit,  is  odd.  

– UART  chips  allow  programming  of  the  parity  bit  for  odd-­‐,  even-­‐,  and  no-­‐parity  opLons.  

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Data Transfer Rate •  How  fast  is  the  data  transferred?  •  Three  methods  to  describe  the  speed:  

–  Baud  rate  is  defined  as  the  number  of  signal  changes  per  sec.  • The  rate  of  data  transfer  is  stated  in  Hz  (used  in  modem).  

–  Date  rate  is  defined  as  the  number  of  bits  transferred  per  sec.  • Each  signal  has  several  voltage  levels.  • The  rate  of  data  transfer  is  stated  in  bps  (bits  per  second).  

–  EffecLve  data  rate  is  defined  as  the  number  of  actual  data  bits  transferred  per  second.  • Redundant  bits  must  be  removed  

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Data Transfer Rate •  The  data  transfer  rate  depends  on  communicaLon  ports  incorporated  into  that  system.  –  Ex:  100-­‐9600  bps  in  the  early  IBM  PC/XT    –  Ex:  56  kbps  in  PenLum-­‐based  PC    –  The  baud  rate  is  generally  limited  to  100kHz.  

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Example of Data Transfer Rate •  Data  is  sent  in  the  following  asynchronous  mode:  

–  2400  baud  rate  –  each  signal  has  4  voltage  levels  (-­‐5V,  -­‐3V,  3V,  5V)    –  one  start  bit,  8-­‐bit  data,  2  stop  bits  

mark stop bit

start bit mark 00 10 01 10 00 11 11 11

Time (D0 first)

8-bit character

stop bit

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Example of Data Transfer Rate •  2400  baud  =  2400  signals  per  second  =2400  Hz  •  4  voltage  level:  Log24=2  

–  2  bits  is  sent  in  every  signal  change  •  Data  rate  =  2  *  2400  Hz  =  4800  bps  •  EffecLve  raLo  =  8  /  (1+8+2)  =8/11  •  EffecLve  data  rate  =  data  rate  *  effecLve  raLo  =  4800  *  8  /11=3490.9  

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RS232 Standard •  RS232  is  an  interfacing  standard  which  is  set  by  the  Electronics  Industries  AssociaLon  (EIA)  in  1960.  –  RS232  is  the  most  widely  used  serial  I/O  interfacing  standard.  

–  RS232A  (1963),  RS232B  (1965)  and  RS232C  (1969),  now  is  RS232E  

•  Define  the  voltage  level,  pin  funcLonality,  baud  rate,  signal  meaning,  communicaLon  distance.  

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RS232 Voltage Level •  The  input  and  output  voltage  of  RS232  is  not  of  the  TTL  compaLble.  –  RS232  is  older  than  TTL.  

• We  must  use  voltage  converter  (also  referred  to  as  line  driver)  such  as  MAX232  to  convert  the  TTL  logic  levels  to  the  RS232  voltage  level,  and  vice  versa.  – MAX232,  TSC232,  ICL232  

logic 0

-3V

-25V

3V

25V

logic 1

undefined

RS 232 Voltage

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MAX232 • MAX232  IC  chips  are  commonly  referred  to  as  line  drivers.  

PC 8051

COM 1 port

RS232

MAX232 UART

RS232 voltage level

TTL voltage level

TTL voltage level

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RS232 Pins for DTE Pin   Descrip0on  1   ProtecLve  ground  2   TransmiWed  data  (TxD)  3   Received  data  (RxD)  4   Request  to  send  (RTS)  5   Clear  to  send  (CTS)  6   Data  set  ready  (DSR)  7   Signal  ground  (GND)  8   Data  carrier  detect  (DCD)  9/10   Reserved  for  data  tesLng  11   Unassigned  12   Secondary  data  carrier  detect  13   Secondary  clear  to  send  

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14

1

25

13

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RS232 Pins for DTE Pin   Descrip0on  14   Secondary  transmiWed  data  15   Transmit  signal  element  Lming  16   Secondary  received  data  17   Receive  signal  element  Lming  18   Unassigned  19   Secondary  request  to  sent  20   Data  terminal  ready  (DTR)  21   Signal  quality  detector  22   Ring  indicator  23   Data  signal  rate  select  24   Transmit  signal  element  Lming  25   Unassigned  

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14

1

25

13

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RS232 Handshaking Signals • Many  of  the  pins  of  the  RS232  connector  are  used  for  handshaking  signals.  – DTR  (data  terminal  ready)  – DSR  (data  set  ready)  –  RTS  (request  to  send)  –  CTS  (clear  to  send)  –  RTS  and  CTS  are  hardware  control  flow  signals.  – DCD  (carrier  detect,  or  data  carrier  detect)  –  RI  (ring  indicator)  

•  They  are  not  supported  by  the  8051  UART  chips.  19 Aug 2014 EE-309@IITB

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Thank You

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