1. Serial Communications
Introduction to UART and USART
Intel 8250 and 8251

2. Outline Concept of serial communications Synchronous, Asynchronous
RS-232 standard
Hand shaking
UART and USART chips
8250 and 8251 chips

3. What is Serial Communication

4. Serial Communication Types
In Terms of Data on a Line
Asynchronous
Synchronous
In Terms of Number of Senders
Simplex
Half duplex
Full duplex

5. Transfer Types

6. Asynchronous Data Framing
Sending Character ‘A’ = 0x41 on the Line
Idle (high): Mark
Low: Space
Overhead? (parity, start, stop)

7. Vocabulary DTE DCE Data Transfer Rate data terminal equipment
e.g. computer, terminal
DCE
data communication equipment
connects DTE to communication lines
e.g. modem
Data Transfer Rate
Baud Rate Baud (Symbol representing n bits)
BPS Bit per second

8. RS-232 Standard EIA 232 = ITU-T V.24/V.28
Specifies the interface between DTE and DCE:
V.28 : mechanical and electrical characteristics
V.24 : functional and procedural characteristics
Even used in applications where there is no DCE
e.g. connecting computer to printer, magnetic card reader, robot, … etc.
Introduced in 1962 but is still widely used
Stand for Recommended Standard

9. DTE Connections

10. Mechanical Characteristics
25-pin connector
9-pin connector is more commonly found in IBM-PC but it covers signals for asynchronous serial communication only
Use male connector on DTE and female connector on DCE
Note: all signal names are viewed from DTE

11. 25-Pin RS232 Connector

12. 9-Pin RS232 Connector

13. Electrical Characteristics
Single-ended
one wire per signal, voltage levels are with respect to system common (i.e. signal ground)
Mark: –3V to –15V
represent Logic 1, Idle State (OFF)
Space: +3 to +15V
represent Logic 0, Active State (ON)
Usually swing between –12V to +12V
Recommended maximum cable length is 15m, at 20kbps

14. TTL to RS-232
Line drivers and line receivers

15. RS-232 Frame Format Example A 111101000001111 … Start bit ASCII Parity
Stop bit A
Idle

16. RS232 Logic Waveform

17. Function of Signals TD: transmitted data RD: received data
DSR: data set ready
indicate whether DCE is powered on
DTR: data terminal ready
indicate whether DTR is powered on
turning off DTR causes modem to hang up the line
RI: ring indicator
ON when modem detects phone call

18. Function of Signals DCD: data carrier detect RTS: request to send
ON when two modems have negotiated successfully and the carrier signal is established on the phone line
RTS: request to send
ON when DTE wants to send data
Used to turn on and off modem’s carrier signal in multi-point (i.e. multi-drop) lines
Normally constantly ON in point-to-point lines
CTS: clear to send
ON when DCE is ready to receive data
SG: signal ground

19. Flow Control
Means to ask the transmitter to stop/resume sending in data
Required when:
DTE to DCE speed > DCE to DCE speed
(e.g. terminal speed = 115.2kbps and line speed = 33.6kbps, in order to benefit from modem’s data compression protocol)
without flow control, the buffer within modem will overflow – sooner or later
the receiving end takes time to process the data and thus cannot be always ready to receive

20. Hardware Flow Control RTS/CTS
the transmitting end activates RTS to inform the receiving end that it has data to send
if the receiving end is ready to receive, it activates CTS
normally used between computer and modem
computer is always ready to receive data but modem is not, because terminal speed > link speed

21. Software Flow Control Xon/Xoff
when the buffer within the receiving end is nearly full, Xoff is sent to the transmitting end to ask it to stop
when data have been processed by the receiving end and the buffer has space again, Xon is sent to the transmitting end to notify it to resume
advantage: only three wires are required (TD, RD and GND)
disadvantage: confusion arises when the transmitted data (e.g. a graphics file) contains a byte equal to 13H (Xoff)

22. Are you ready to receive?
RS-232 (con)
Communication between two nodes
Software Handshaking
Hardware Handshaking
data transmission
Are you ready to receive?
Software flow control uses special characters to control the flow of data when a device wishes to halt the flow it sends the XOFF character, when flow can resume it sends the XON character. The XOFF character is called DC3 or Device Code 3. It is decimal 19 or control S. The XON character is called DC1 or Device Code 1. It is decimal 17 or control Q.
XOn / XOff is a software flow control mechanism. It can only be used in character oriented data transmissions (not binary transfer) as it relies on the use of a pre-determined XOn character and a pre-determined XOff character. The basic idea is simple, when the receiver buffer fills to the point it cannot receive anymore data it issues an XOff (Transmit Off) to the transmitter. When the transmitter sees the XOff character is stops transmitting. It will only resume once it sees a corresponding XOn. The disadvantage of such a system is that should an issued character become damaged in transit no action will be taken by the transmitter.
RTS
RTS
CTS
CTS No
x-off
receiver
receiver
transmitter
transmitter
x-on
CTS
CTS
Yes TD RD
data transmission
Send character

23. Null Modem Cables Used to directly connect two DTEs together
Many possibilities – depending on whether and how the two DTEs handshake (i.e. doing flow control)

24. Null Modem Cables Examples

25. Other Standards

26. UART in PC
115200*16=

27. 8250/16450/16550 UART

28. Registers Transmitter holding register Receiver buffer register
Interrupt enable register

29. Registers
Interrupt identification register

30. Line Control

31. Modem Registers
Modem Control Register
Modem Status Register

32. Line Status

33. Divisor Register

34. Example Program the divisor Latch for 300 baud. Assume Xin=1.8432MHz
The Base Address: 0x3F8

35. Example 2
Program the divisor Latch for 2400 baud. Assume Xin=1.8432MHz
The Base Address: 0x3F8

36. Example 3
Program 8250 for 2400 baud, 8 data bit, even parity and 1 stop bit.
Assume Xin=1.8432MHz
The Base Address: 0x3F8
MOV AL,80H ; Accessing DLAB
MOV DX,3FBH ;Line Control Register Address
OUT DX,AL
MOV AX,48 ;baud= :48=2400
MOV DX,3F8H ;Low byte of Divisor
MOV AL,AH
INC DX
MOV AL, ; DLAB,Break,Even,1 stop, 8 data
MOV DX,3FBH ;LCR

37. Synchronous Protocols
BCC block check character
STX start of text
Control field contains info like the sequence of data

38. CRC
In SDLC:
G(X) = x**16 + x**12 + x**5 + 1

39. 8251 Block Diagram

40. 8251 Registers

41. Mode Register

42. Mode Instruction (Asynchronous)

43. Mode Instruction (Synchronous)

44. Command Register

45. Status Register

46. 8251 Timing

47. 8251 USART Interface A7 A6 A5 A4 A3 A2 A1 IO/M D[7:0] RD WR A0 C/D CLK
TxC
RxC
TxD
RxD
8251
RS232

48. Programming 8251 8251 mode register 7 6 5 4 3 2 1 Mode register
Mode register
Number of
Stop bits
00: invalid
01: 1 bit
10: 1.5 bits
11: 2 bits
Parity
0: odd
1: even
Parity enable
0: disable
1: enable
Character length
00: 5 bits
01: 6 bits
10: 7 bits
11: 8 bits
Baud Rate
00: Syn. Mode
01: x1 clock
10: x16 clock
11: x64 clock

49. Programming 8251 8251 command register TxE: transmit enableEH IR
RTS ER
SBRK
RxE
DTR
TxE
command register
TxE: transmit enable
DTR: data terminal ready
RxE: receiver enable
SBPRK: send break character
ER: error reset
RTS: request to send
IR: internal reset
EH: enter hunt mode

50. Programming 8251 8251 status register TxRDY: transmit ready
DSR
SYNDET FE OE PE
TxEMPTY
RxRDY
TxRDY
status register
TxRDY: transmit ready
RxRDY: receiver ready
TxEMPTY: transmitter empty
PE: parity error
OE: overrun error
FE: framing error
SYNDET: sync. character detected
DSR: data set ready

51. Simple Serial I/O Procedures
Read
start
Check RxRDY
Is it logic 1?
Read data register*
end
Yes No
* This clears RxRDY
Write
Check TxRDY
Write data register*
* This clears TxRDY