• Serial Communications: 2 Flavors: Synchronous –

8/14/2019 Serial Communications: 2 Flavors: Synchronous

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10/19/2005 ECE 340 Fall 2004 Lecture 23 1

Serial I/O

Serial Communications: 2 Flavors:Synchronous Asynchronous

Synchronous Communications

Requires clock signal Data bits may be non-uniformly spaced!

Can operate at faster data rates than

asynchronous serial I/O Clock signal synchronizes data transfers


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Serial I/O

Asynchronous Communications

No clock is required

Additional overhead in message to manage

synchronization

Data bits must be uniformly spaced!

Data rates: 110B 115KB

Common use:

PC serial ports

Point to point communications


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RS232 Standard Reference WEB site: http://www.camiresearch.com/Data_Com_Basi

cs/RS232_standard.html

Voltage levels areNOTTTL!


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Definition: BAUD The inverse of the period of a unit symbol.

Protocol -


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Parity Options: None, Odd, and Even None: Parity bit is omitted. Character

terminated with a stop bit

Even: Logic level of the parity bit is set tomake the total number of logic one states (data

plus parity bit) evenly divisible by 2.

Odd: Logic level of the parity bit is set to make

the total number of logic one states (data plusparity bit)not evenly divisible by 2.


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UART Block Diagram

Baud Rate Generator, Transmitter, and Receiver.

Transmitter transmits using Baud Clock

Receiver Samples using faster clock (8x Baud Clock)


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Reciver

From idle state, waits 4 cc after RXD drops, then samples

every 8 cc

Packet always starts with RXD low (start bit) and ends with

RXD high (stop bit)


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Serial Communications ASCII American Standard Code for

Information Interchange


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Serial Communications

Standard Connectors: 9 pin and 25 pin

(legacy)

DTE Data terminal equipment TheComputer!

DCE Data communications equipment The

modem

Full standard includes clock (for sync),

secondary data channel, and flow control


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Common Configurations

Direct Extension (e.g., DTE -> DCE)

Wire straight across

Loopback

Allows a DTE to talk to itself

Null Modem (e.g., DTE -> DTE)

Connecting two computers via RS232


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Direct Extension (DB9)


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Loopback (DB9)

ConnectsRX to TX


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Null Modem (DTE to DTE)


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Modem to Modem (DCE to DCE)


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Gotcha!!!


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Rabbit - Asynchronous Serial

Communications UI Lab Project board:

Port D DCE

Port C DTE

Rabbit Serial Port Initialization (Port D)#use rs232.lib

#define DINBUFSIZE 64 // Arbitrary

#define DOUTBUFSIZE 64 // Arbitrary

#define _232BAUD 19200

serDopen(_232BAUD);

serDwrFlush();

serDrdFlush();


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CommunicationsserDopen

SYNTAX: int serDopen(long baud);

DESCRIPTION: Opens the D serial port.

This function is non-reentrant.

PARAMETER1: baud : bits per second of data transfer

RETURN VALUE:1: The baud set on the rabbit is the same as the input baud

0: The baud set on the rabbit does not match the input baud

RELATED WORDS: serDgetc, serDpeek, serDputs, serDwrite,

cof_serDgetc, cof_serDgets, cof_serDread, cof_serDputc, cof_serDputs,

cof_serDwrite, serDclose


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CommunicationsserDwrFlush

SYNTAX: void serDwrFlush();

DESCRIPTION: Flushes the serial port D transmit buffer.


PARAMETER1: None

RETURN VALUE: None

serDrdFlush

SYNTAX: void serDrdFlush();

DESCRIPTION: Flushes the serial port D input buffer.


PARAMETER1: None

RETURN VALUE: None


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CommunicationsserDputc

SYNTAX: int serDputc(char c);

DESCRIPTION: Write a character to serial port D write buffer.


PARAMETER1: c: Character to write to serial port D write buffer

RETURN VALUE: 0 if buffer locked or full, 1 if character sent

serDgetc

SYNTAX: int serDgetc();

DESCRIPTION: Get next available character from serial port D read buffer.


PARAMETER1: None

RETURN VALUE: success: the next character in the low byte, 0 in the high byte

failure: the integer -1


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CommunicationsserDpeek

SYNTAX: int serDpeek();

DESCRIPTION: Returns the 1st character of the data in the D input buffer.


PARAMETER1: NoneRETURN VALUE: -1 if the buffer is empty, the 1st character of the data if not

serDputs

SYNTAX: int serDputs(char* s);

DESCRIPTION: Calls serDwrite(s, strlen(s)); does not write null terminator.


PARAMETER1: s: Null terminated character string to write

RETURN VALUE: The number of characters successfully written to serial port D.


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Communications

serDwrite

SYNTAX: int serDwrite(void *data, int length);

DESCRIPTION: Transmits length bytes to serial port D.This function is non-reentrant.

PARAMETER1: data : data structure to write to serial port D

length: number of bytes to write

RETURN VALUE:The number of bytes successfully written to serial port D.


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Formatted Buffer Writes

sprintf

SYNTAX: int sprintf(char *buffer, char *format, ...);

KEYWORDS:

DESCRIPTION: This function takes a "format" string (pointed to by "format"),arguments of the format, and output the formatted string to "buffer"

(pointed to by "buffer"). The user should make sure

(1) there are enough arguments after "format" to fill in the format parameters in the

format string

(2) the types of arguments after "format" match the format fields in "format"

(3) buffer is large enough to hold the longest possible formatted string


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Formatted Buffer Writes

Example: sprintf(buffer,"%s=%x","variable x",256) should put the string "variable

x=100" into buffer.

The macro STDIO_DISABLE_FLOATS can be defined if it is not necessary to

format floating point numbers. If this macro is defined, %e, %f and %g will not be

recognized. This can save thousands of bytes of code space. This function can be

called by processes of different priorities.

RETURN VALUE: The number of characters written


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Example#define CINBUFSIZE 15

#define COUTBUFSIZE 15

#define DINBUFSIZE 15

#define DOUTBUFSIZE 15

// change serial baud rate here#ifndef _232BAUD

#define _232BAUD 115200

#endif

main() {

auto int nIn1, nIn2;

auto char cOut;

long time;

serCopen(_232BAUD); serDopen(_232BAUD);

serCwrFlush(); serCrdFlush(); serDwrFlush(); serDrdFlush();


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for (cOut='a'; cOut

• Serial Communications: 2 Flavors: Synchronous –

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