Digital Serial Input/Output

1
Digital Serial Input/Output

• Two basic approaches
• Synchronous
• shared common clock signal
• all devices synchronised with the shared clock
  signal
• data transfer occurs on clock signal usually a
  clock transition
• Asynchronous
• no shared clock signal
• all devices have there own clock signal
• all devices must use the same clock frequency
• clock frequencies may differ slightly between
  devices
• device clock signals are out of phase with each
  other not synchronised hence the term
  asynchronous
• receiver must synchronise with transmitter in
  order to receive data correctly

2
Serial I/O configurations and standards

• Simple point to point communications e.g.
  RS232, RS423 and RS422.
• Bus based with
• one master and one or more slaves e.g
• RS485
• SPI(Serial Peripheral Interface) bus Serial
  data out, serial data in and serial clock)
• I2C (Inter-IC bus)
• Universal Serial Bus (USB)
• multiple masters and one or more slaves e.g. I2C
  bus
• Peer to peer arrangement e.g. CAN(Control Area
  Network) bus

3
Baud Rate

• Baud rate defines the switching speed of a
  signal (i.e. the Baud rate indicates how often a
  signal changes state).
• Bit rate defines the rate at which information
  flows across a data link. measured in
  bits/second(bps)
• For a binary two-level signal, a data rate of
  one bit per second is equivalent to one Baud

11
10
Bit rate 2 x Baud rate
Data
01
00
Time
00 01 11 10 11 00 01
Note For a binary signal Baud rate bit rate
1/bit time
4
Universal Asynchronous Receiver/Transmitter(UART)

• Most UARTS are full duplex they have separate
  pins and electronic hardware for the transmitter
  and receiver that allows serial output and serial
  input to take place in parallel
• Based around shift registers and a clock signal.
• UART clock determines baud rate
• UART frames the data bits with
• a start bit to provide synchronisation to the
  receiver
• one or more (usually one) stop bits to signal end
  of data
• Most UARTs can also optionally generate parity
  bits on transmission and parity checking on
  reception to provide simple error detection.
• UART often have receive and transmit buffers as
  well as the serial shift registers

5
UART - Transmitter

• Transmitter (Tx) - converts data from parallel to
  serial format
• inserts start and stop bits
• calculates and inserts parity bit if required
• output bit rate is determined by the UART clock

6
Asynchronous serial transmission
7
UART - Receiver

• converts serial format back to parallel data
• Rx synchronises with transmitter using the start
  bit
• samples data line at UART clock rate (normally a
  multiple of baud rate, typically 16)
• reads value in middle of bit period (Infineon 167
  uses a majority decision of the 7th, 8th and 9th
  sample to determine the effective bit value.
• calculates parity and checks against received
  parity bit

8
Asynchronous serial reception
Idle
etc.
Start bit
waiting for start bit
1 First data bit
Start detected
0
9
Asynchronous

• Usually used on simple systems
• Typically point to point
• Various different formats and protocols
• Normally 8bit data format with one start and one
  stop bit
• Standards E.g. RS232
• defines connector type, pin assignments, voltage
  levels, max bit rate, cable length etc.
• Min. 3 pins TxD, RxD, Ground
• Other pins for data flow control.
• Some common baud rates - 300,1200,9600,19200

10
Infineon 167 Asynchronous serial channel 0 (ASC0)
11
ASC0 Control Register S0CON
12
S0CON - contd.
13
S0CON - contd.
14
ASC0 Baud Rate Generation

• The serial channel ASC0 has its own dedicated
  13-bit baud rate generator basically a timer
  with reload capability.
• The baud rate generator is clocked with the CPU
  clock divided by 2 (f CPU /2).
• The timer counts downwards and can be started or
  stopped through the Baud Rate Generator Run Bit
  S0R in register S0CON. Each underflow of the
  timer provides one clock pulse to the serial
  channel. The timer is reloaded with the value
  stored in its 13-bit reload register each time it
  underflows.
• The resulting clock is then divided according to
  the operating mode and controlled by the Baudrate
  Selection Bit S0BRS. If S0BRS 1, the clock
  signal is additionally divided to 2/3rd of its
  frequency (see formulas and table).

15
ASC0 Baud Rate Generation contd.

• So the baud rate of ASC0 is determined by the CPU
  clock, the reload value, the value of S0BRS and
  the operating mode (asynchronous or synchronous).

16
Infineon 167 - ASC0 Initialisation

• // This is zero 0, this is 'Oh' O
• //initialise serial port to 9600 baud
• void serial_init()
• P3 0x0400 // SET PORT 3.10 OUTPUT LATCH
  (TXD)
• DP3 0x0400 // SET PORT 3.10 DIRECTION
  (TXD OUTPUT)
• DP3 0xF7FF // RESET PORT 3.11 DIRECTION
  (RXD INPUT)
• S0TIC 0x80 // SET TRANSMIT INTERRUPT
  FLAG
• S0RIC 0x00 // RESET RECEIVE INTERRUPT
  FLAG
• S0BG 0x40 // SET BAUDRATE TO 9600 BAUD
• S0CON 0x8011 // SET SERIAL MODE
• NOTE The Keil 166 C compiler standard I/O
  functions use the ASC0 serial port i.e
  printf,putchar,scanf etc. will use ASC0. But the
  user program MUST initialise the ASC0 before any
  of the functions can be used.

17
User functions for simple polled serial I/O

• char getbyte (void)
• char c
• while (S0RIR0) // or while(!S0RIR)
• c S0RBUF
• S0RIR 0
• return (c)
• void putbyte (signed char c)
• while (!S0TIR)
• S0TIR 0
• S0TBUF c