Serial port

1
Serial port

• Communicating
• via the serial port

2
Serial ports

• 1 bit wide
• Simple, but slow

3
Serial port
TxD
RxD
?P
Peripheral
C/GND
4
Signal transmission
01110010
signal
MSB
LSB
1
0
t (s)
1 byte (data)
5
Transmission modes

• Synchronoussynchronized by bytes or clock
  signal line
• Asynchronousevery byte transmitted is
  synchronized by start and stop bits

6
Signal transmission
Bit transfer time
Signal (level)
Start bit
Parity bit
1
Stop bit
0
t (s)
8 data bits
7
Start bit

• Used to synchronize receiver
• Level 0 since idle is level 1

8
Stop bit(s)

• 1 (, 1.5) or 2 bits
• Level 1 to ensure return to idle (level 1)

9
Parity bit

• 1 bit to ensure reliable transfer of data
• Even paritydata bits and parity bit should
  contain even number of 1s
• Odd paritydata bits and parity bit should
  contain odd number of 1s

10
Transfer rates

• Bit rate number of bits transferred per second
  including start, parity and stop bits. Unit
  bits/s
• Baud rate number of possible level transitions
  per second. Unit baud
• Data bit rate number of data bits transferred
  per second.

11
Transfer rates
Signal (level)
T 1/baud
T s/bit
Start bit
Parity bit
1
Stop bit
0
8 data bits, 8 T s/data bit
t (s)
11 bits, 11 T s/bit
12
Serial port
TxD
8
Data bus
?P
RxD
Peripheral
UART
Address and control bus
C/GND
13
UARTUniversal Asynchronous Receiver Transmitter

• Transmit register
• Receive register
• Buffers
• Control register
• Status register

14
Transmit register

• Contains byte to send
• Byte is shifted out with least significant bit
  first

15
Receive register

• Contains last received byte
• Least significant byte is shifted in first
• Considered empty until last (most significant)
  bit is received

16
Buffers

• Internal in UART
• Receive buffer contains received bytes
• Transmit buffer contains bytes to transmit

17
Control register

• Number of databits (5, 6, 7 or 8),
• Parity (none, even, odd),
• Number of stop bits (1, 1.5, 2),
• Baudrate (multiple of 150 up to 230400, e.g.
  9600)
• Receive enable

18
Control register

• Interrupt (received byte, transmit buffer empty)
• Interrupt level register (at which level will
  interrupt occur)

19
Status register

• Receive register full (byte received)
• Transmit register empty
• IRQ (Interrupt Request)
• Error bits

20
Status register error bits

• Parity error the parity bit was not according to
  specification, i.e. a bit was not correctly
  transmitted.
• Overrun error the processor did not read
  received byte until after the next byte arrived
  (byte overwritten by next byte)
• Framing error mismatch between start bit and
  stop bit(s)

21
RS-232-C

• A standard for asynchronous serial communication
• Standardized by EIA (Electronics Industry
  Association)

22
RS-232-C

• 1 is transmitted as 12 volt (lt -3 volt)
• 0 is transmitted as 12 volt (gt 3 volt)
• A voltage level converter is needed between
  TTL-levels (0, 5 volt) and RS-232-C-levels (12,
  -12 volt)

23
RS-232-C serial port
TxD
8
TxD
Data bus
?P
RxD
Voltage level converter
RxD
Peripheral
UART
Address and control bus
C/GND
C/GND