2024年4月29日发(作者：)

2011 IEEE Symposium on Computers & Informatics

FPGA Implementation of RS232 to Universal

serial bus converter

, (PhD)

2

Rama

Valupadasu

(Ph

D), ,

3

.o

Chunduri, PhD,

Assoc. Professor. VCEW sor, NIT, Warangal Professor, NIT, Warangal

vsrtej@

agnivesh91@ cbrr@

4. 5.

i Rekha, , vi, ,

sor Assoc. Professor. VCEW

LUC, Mantin, Malaysia vaagvijs_15@

madakranthirekha@

Abstract—

Universal Serial Bus (USB) is a new personal computer

interconnection protocol, developed to make the connection of peripheral

devices to a computer easier and more efficient. It reduces the cost for the

end user, improves communication speed and supports simultaneous

attachment of multiple devices (up to127)RS232, in another hand, was

designed to single device connection, but is one of the most used

communication protocols. An embedded converter from RS232 to USB

is very interesting, since it would allow serial-based devices to

experience USB advantages without major changes. This work describes

the specification and development of such converter and it is also a useful

guide for implementing other USB devices. The main blocks in the

implementation are USB device, UART (RS232 protocol engine) and

interface FIFO logic. The USB device block has to know how to detect

and respond to events at a USB port and it has to provide a way for the

device to store data to be sent and retrieve data that have been received

UART consists of different blocks which handle the serial

communication through RS232 protocol. There are a set of control

registers to control the data transfer. The interface FIFO logic has FIFO

to bridge the data rate differences between USB and RS232 protocols.

1

USB is a new personal computer interconnection standard

developed by industry and telecommunication leaders,

which implements the Plug and Play technology. It allows

multiple devices connection (up to 127) ranges. The use of a the

devices attachment to PCs. USB is a low cost, easing solution

and supports transfer rates up to 12Mbs, comprehending the low-

speed and mid-speed data converter from a serial interface to

USB would free a serial communication port to other

applications, allowing a device that uses a serial interface to

communicate using an USB interface. USB on the other hand is a

bus system which allows more than one peripheral to be

connected to a host computer via one USB port. Hubs can be

used in the USB chain to extend the cable length and allow for

even more devices to connect to the same USB port. The

standard not only describes the physical properties of the

interface, but also the protocols to be used. Because of the

complex USB protocol requirements, communication with USB

ports on a computer is always performed via a device driver. This

way, we are not limited to the availability of a serial port and we

can experience the USB advantages. Using a converter allows us

to have the device unchanged, making the converter responsible

for treating the differences between the protocols. This work was

based on protocol engine which can be managed by exchanging

data with a PC across a serial interface. Most of the times, this

communication is not done constantly, since it is necessary to

have a serial port available just for it. This paper presents the

converter implementation, focusing on the development process,

which comprehends the device itself and the PC-side software

that will communicate with it. This methodology can be extended

to other devices. We first present some important USB standard

concepts. Then, we define the system specification, divided on

host and device requirements. After, we describe the hardware

(UART) features and software design and implementation.

Finally, we discuss about achieved results and future work

II.

Index Terms—

First-In-First-Out, RS-232, Universal

Asynchronous Receive Transmit, Universal Serial Bus.

I. INTRODUCTION

This paper describes the specification and implementation of a

converter from RS232 to USB (Universal Serial Bus). This

converter is responsible for receiving data from a peripheral

device’s serial interface and sending it to a computer’s USB

interface. In the same way, it must be able to send data from the

PC’s USB interface to the

device. The problems faced with

the old standards stimulated the development of a new

communication protocol, which should be easier to use,

faster, and more efficient. RS232 is a definition for serial

communication on a 1:1 base. RS232 defines the interface

layer, but not the application layer. To use RS232 in a

specific situation, application specific software must be

written on devices on both ends of the connecting RS232

cable. RS232 ports can be either accessed directly by an

application, or via a device driver in the operating system.

PROBLEM DESCRIPTION

The USB specification describes bus attributes, protocol

definition, programming interface and other features required to

design and build systems and peripherals compliant with the

USB standard. We briefly explain features used in our project.

978-1-61284-691-0/11/$26.00 ©2011 IEEE237

The USB interface does not give this flexibility. When however

an RS232 port is used via an USB to RS232 converter, this

flexibility should be

present in some way. Therefore to use an

RS232 port via an USB port, a second device driver is

necessary

which emulates a RS232 UART, but communicates via USB.

the firmware must also contain a report descriptor that defines

the format for transmitted and received device data.

A. Requests

USB works as a Master/Slave bus, where the USB Host is the

Master and the devices are the Slaves. The only system resources

required by a USB system are the memory locations used by

USB system software and the memory and/or I/O address space

and IRQ line used by the USB host controller. USB devices can

be functional (displays, mice, etc) or hubs, used to connect other

devices in the bus. They can be implemented as low or high-

speed devices. Low-speed devices are limited to a maximum 1.5

Mb/s rate. Each device has a number of individual registers -

known as Endpoints which are indirectly accessed by the device

drivers for data exchange. Each endpoint supports particular

transfer characteristic has a unique address and direction. A

special case is Endpoint 0, which is used for control operations

and can do bi-directional transfers. It must be present in all

devices. According to the device’s characteristics, other types of

endpoints can be defined. USB Host verifies the attachment and

detachment of new devices, initiating the enumeration process

and managing all the following transactions. It is responsible to

install device driver (based on information provided by device

descriptors), to automatically reconfigure the system (hot

attachment) and to collect statistics and status of each device.

USB on the other hand is a bus system which allows more than

one peripheral to be connected to a host computer via one USB

port. Hubs can be used in the USB chain to extend the cable

length and allow for even more devices to connect to the same

USB port. The standard not only describes the physical

properties of the interface, but also the protocols to be used.

Because of the complex USB protocol requirements,

communication with USB ports on a computer is always

performed via a device driver. Device’s descriptors specify USB

devices attributes and characteristics and describe device

communication requirements (Endpoint Descriptors). The USB

host uses this information to configure the device, to find its

driver, and to access it. Devices with similar functions are

grouped into classes [1, 2] in order to share common features and

even use the same device drivers. Each class can define their own

descriptors (class-specific descriptors), as for example, HID

(Human Interface Device) Class Descriptors and Report

Descriptors. The HID class consists of devices used by people to

control computer systems. It defines a structure that describes a

HID device, with specific communication requirements.

According to the converter characteristics, it can be implemented

as a HID device, using already developed HID drivers. A HID

device’s descriptors must support an Interrupt IN endpoint and

The USB protocol is based on requests sent by the host

and processed by the USB devices. These requests can be

directed to a device or a specific endpoint in it. Standard requests

must be implemented by all devices and are used for configuring

a device and controlling the state of its USB interface, among

other features. Two HID-specific requests must be supported by

the converter: Set Report and Get Report. These requests enable

the device to receive and send generic device information to the

host. Set Report request is the only way the host can send data to

a HID device, once it does not have an Interrupt OUT endpoint

B. Communication Flow

USB is a shared bus and many devices might use it at the same

time. The devices share the bandwidth using a protocol based on

tokens and commanded by the host. USB communication is

based on transferring data at regular intervals called frames. A

frame is composed by one or more transactions that must be

executed in a 1 ms time. USB data transfers are typically

originated by a USB Device Driver when it needs to

communicate with its device. It supplies a memory buffer used to

store the data in transfers to or from the USB device. The USB

Driver provides the interface between USB Device Driver and

USB Host Controller, translating transfer requests into USB

transactions, consistent with the bandwidth requirements and

protocol structure. Some of these transfers consist of a large

block of data, which need to be splitted into several transactions.

The Host Controller generates the transaction based on the

Transfer Descriptor, which describes the frame sharing among

the several devices requests. When a transaction is sent to the

bus, all devices see it. Each transaction begins with a packet that

determines its type and the endpoint address. The USB driver

controls this addressing scheme. Inside the device, the USB

Device Layer comprehends the actual USB communication

mechanism and transfer characteristics. USB Logical Device

implements a collection of endpoints that comprise a given

functional interface, which can be manipulated by its respective

USB client.

C. Transfer Types

The USB specification defines four transfer types: Control,

Interrupt, Isochronous and Bulk. Control transfers send requests

and data relating to the device’s abilities and configuration. They

can also be used to transfer blocks of information for any other

purpose. Control transfers consist of a Setup stage, followed by a

Data stage, which is composed of one or more Data transactions,

and a Status stage. All data transactions in a Data Stage must be

in the same direction (In or out). Interrupt transfers are typically

used for devices that need to transfer data at regular period of

time, and consequently must be polled periodically. The polling

interval is defined in the Endpoint Descriptor. The data payload

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