Serial Communications Introduction to UART and USART Intel

Serial Communications Introduction to UART and USART Intel 8250 and 8251

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

What is Serial Communication

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

Transfer Types

Asynchronous Data Framing Sending Character ‘A’ = 0 x 41 on the Line Idle (high): Mark Low: Space Overhead? (parity, start, stop)

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

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

DTE Connections

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

25 -Pin RS 232 Connector

9 -Pin RS 232 Connector

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

TTL to RS-232 Line drivers and line receivers

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

RS 232 Logic Waveform

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

Function of Signals • DCD: data carrier detect – 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

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. 2 kbps and line speed = 33. 6 kbps, 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

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

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 13 H (Xoff)

RS-232 (con) • Communication between two nodes Software Handshaking Hardware Handshaking data transmission CTS TD data transmission No Yes Send character RTS CTS RD receiver transmitter x-on receiver transmitter x-off Are you ready to RTS receive?

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)

Null Modem Cables Examples

Other Standards

UART in PC 115200*16=1843200

8250/16450/16550 UART

Registers • Transmitter holding register • Receiver buffer register • Interrupt enable register

Registers • Interrupt identification register

Line Control

Modem Registers Modem Control Register Modem Status Register

Line Status

Divisor Register

Example Program the divisor Latch for 300 baud. Assume Xin=1. 8432 MHz The Base Address: 0 x 3 F 8

Example 2 Program the divisor Latch for 2400 baud. Assume Xin=1. 8432 MHz The Base Address: 0 x 3 F 8

Example 3 Program 8250 for 2400 baud, 8 data bit, even parity and 1 stop bit. Assume Xin=1. 8432 MHz The Base Address: 0 x 3 F 8 MOV MOV OUT MOV INC OUT MOV OUT AL, 80 H DX, 3 FBH DX, AL AX, 48 DX, 3 F 8 H DX, AL AL, AH DX DX, AL AL, 00011011 DX, 3 FBH DX, AL ; Accessing DLAB ; Line Control Register Address ; baud=2400 115200: 48=2400 ; Low byte of Divisor ; DLAB, Break, Even, 1 stop, 8 data ; LCR

Synchronous Protocols

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

8251 Block Diagram

8251 Registers

Mode Register

Mode Instruction (Asynchronous)

Mode Instruction (Synchronous)

Command Register

Status Register

8251 Timing

8251 USART Interface 8251 D[7: 0] RD WR A 0 CLK A 7 A 6 A 5 A 4 A 3 A 2 A 1 IO/M Tx. D RD WR C/D CLK Rx. D Tx. C RS 232

Programming 8251 q 8251 mode register 7 6 5 Number of Stop bits 00: 01: 10: 11: invalid 1 bit 1. 5 bits 2 bits 4 3 Parity enable 0: disable 1: enable Parity 0: odd 1: even 2 1 0 Mode register Baud Rate 00: 01: 10: Character length 11: 00: 5 bits 01: 6 bits 10: 7 bits 11: 8 bits Syn. Mode x 1 clock x 16 clock x 64 clock

Programming 8251 q 8251 command register EH IR RTS ER SBRK Rx. E DTR Tx. E: transmit enable DTR: data terminal ready Rx. E: receiver enable SBPRK: send break character ER: error reset RTS: request to send IR: internal reset EH: enter hunt mode command register

Programming 8251 q 8251 status register DSR SYNDET FE Tx. RDY: Rx. RDY: Tx. EMPTY: PE: OE: FE: SYNDET: DSR: OE PE Tx. EMPTYRx. RDY Tx. RDY transmit ready receiver ready transmitter empty parity error overrun error framing error sync. character detected data set ready status register

Simple Serial I/O Procedures q Read q Write start Check Rx. RDY Check Tx. RDY Is it logic 1? No Is it logic 1? Yes Read data register* end * This clears Rx. RDY No Yes Write data register* end * This clears Tx. RDY