emulatingrs-232overusb_121004

Microchip Technology Inc.WebSeminar

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© 2004 Microchip Technology Incorporated. All Rights Reserved. Emulating RS-232 Over USB with PIC18F4550 1

Emulating RS-232 over USB with PIC18F4550

December 10th, 2004

Welcome to Microchip Webseminar

My name is Rawin Rojvanit and I am an applications engineer here at Microchip Technology.

With the introduction of the PIC18F4550 family, which has a full-speed USB peripheral, many more USB applications have become possible.

One such application is the emulation of RS-232 over USB.


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© 2004 Microchip Technology Incorporated. All Rights Reserved. Emulating RS-232 Over USB 2

Agenda

l Brief overview of RS-232 emulation over USB

l PICDEM™ FS USB Demo Board

l Where to find more information

This presentation is only about 20 minutes long and not every thing can be explained here. The goal of this presentation is to describe what the RS-232 Emulation over USB is, what it is useful for, what Microchip is providing, and where to find more information.


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USB Terminology

l USB – Universal Serial Bus

For those that are new to USB, I would like to introduce a few terminologies.

USB stands for Universal Serial Bus. USB has literally become the standard peripheral interface for every personal computer.


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USB Terminology

l USB – Universal Serial Busl CDC – Communication Device Class

CDC stands for Communication Device Class. This is a device level protocol specification defined by the USB association body. It defines the rules of how a USB host and a USB peripheral should communicate as a communication device. Specifically, the CDC specification defines a wrapper protocol layer around other communication protocols allowing them to be transported over the USB interface. A standardized specification also allows a USB host and a USB peripheral to be developed independently.

As I will explained later on in this presentation, no work or modification is required on the PC side to implement the RS-232 emulation over USB. This is so because Windows 2000 and XP already come with a driver which provides the RS-232 emulation capability as defined in the Communication Device Class Specification.

Therefore the only things left to do is for the embedded device to have the hardware and firmware supports that conform to the CDC specification.


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USB Terminology


l VID – Vendor ID

VID stands for Vendor ID.

Vendor ID is a 16-bit number assigned by the USB organization body. This number has to be purchased by each manufacturer in order tomarket and sell a product.


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USB Terminology


l VID – Vendor IDl PID – Product ID

PID stands for Product ID and it is also a 16-bit number.

Therefore each manufacturer has 65,536 Product IDs to use.


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USB Terminology


l VID – Vendor IDl PID – Product IDl Every product is required to have a unique

combination of VID and PID


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USB Terminology


l VID – Vendor IDl PID – Product IDl Every product is required to have a unique

combination of VID and PIDl www.usb.org

More information can be found on www.usb.org


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RS-232 Emulation over USB

HyperTerminal

UART Driver

UART Hardware

UART Hardware

UART APIs

Firmware Application

RS-232

PIC18F4550

The emulation of RS-232 over USB provides an easy migration path for an embedded application. The serial communication has traditionally been the most common method of transferring data between a PC and an embedded device.

This slide shows that traditional method of transferring data over the RS-232 interface. The top half represents a personal computer and the bottom half represents an embedded application. The HyperTermina l is a Windows program that is used here as an example of an end application.

Many computers no longer have a serial port. This fact causes a headache for many embedded system designers and a new solution is needed.

An answer is to use USB.


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RS-232 Emulation over USB

HyperTerminal

UART Driver

UART Hardware

UART Hardware

UART APIs


RS-232

HyperTerminal

UART Driver

USB Hardware

USB Hardware

USB UART APIs


USB

CDC Driver

PIC18F4550

This slide shows the migration path from RS-232 to USB.

The UART physical layer is replaced with the USB hardware layer.

Since CDC is a standard USB class, Microsoft has implemented a driver which supports the RS-232 interface emulation. On the PC side, the CDC driver provides the linking layer between the USB hardware and the UART driver. This allows the end application to remain unchanged because from its perspective, it still sees the RS-232 interface.

As for the embedded application side, the PIC18F4550 has a full-speed USB peripheral. In addition, Microchip also provides the CDC RS-232 Emulation Reference Project which provides necessary applicationprogramming interfaces.


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PICDEM FS USB Demo Board

Potentiometer

Switch 2 & 3

Serial Port

18F4550

POT

S2 S3

RST

Power Connector

USB Port

ICD2Connector

RS232

Reset Switch

PICDEMTM FS USB Demo Board

l Assume that the CDC RS-232 Emulation Reference Project is programmed on the microcontroller

I would like to introduce you to the PICDEM FS USB Demo Board. This board can be purchased from buy.microchip.com or any Microchip’s distributors. It provides a hardware platform for demonstrating the USB capability of the PIC18F4550 family of devices.

Assuming that the board has the CDC RS-232 Emulation Reference Project loaded, I will next show you how the demo board would berecognized by Windows Operating System.


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Device Manager

l Two physical RS-232 portsl PICDEM FS USB Board is not connected


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Installing CDC driver in Windows® XP

l Connect the demo board to a PCl Windows should detect a new USB device


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Specifying INF file location

l C:\MCHPFSUSB\fw\CDC\inf\win2k_winxp

A standard INF file for the Communication Device Driver is not provided by Microsoft.

One is provided by microchip.

Example INF file is part of the CDC RS-232 Emulation Reference Project. Once installed, it can be found in directory listed on this slide.

USB Product ID and Vendor ID are also contained in the INF file.

This means when you use the reference project, you should change the IDs in both the firmware and INF files.


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Installation Complete

l Two physical RS-232 portsl PICDEM FS USB Board is connected


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Using HyperTerminal

l Launch HyperTerminall Type in a name for a new connection, and

hit “OK”


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HyperTerminal: Connect To

l Select the new COM port that was added during USB CDC installation and hit “OK”


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HyperTerminal: COM Properties

l Hit “OK”l Terminal is now ready


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Default Firmware Demonstration

l Now, hit switch S2 on the demo board, you should see the message below…


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HyperTerminal: Important Note

l Before physically disconnecting a USB device, you must hang up the connection first, otherwise, the program must be closed and opened again the next time a USB device is connected.

l Let’s take a look at the APIs

Let’s take a look at the APIs provided by the CDC reference project.


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CDC APIs

void putrsUSBUSART(const rom char *data);

void putsUSBUSART(char *data);

void mUSBUSARTTxRom(rom byte *pData, byte len);

void mUSBUSARTTxRam(byte *pData, byte len);

byte getsUSBUSART(char *buffer, byte len);

byte mCDCGetRxLength(void);

Detailed information of each function can be found in Application Note AN956


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Reference Project is a Tutorial

Reset main InitializeSystem

MainLoop

user.cExercise_01();Exercise_02();

…

USBTasks()

ProcessIO()

main.cUSBDriverService();

CDCTxService();…


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Performance Analysis

l Speedl 640 Kbits/s = 80 Kbytes/sl Faster than RS-232 (UART)l This solution is not possible with a low-

speed USB device because it does not have a bulk endpoint.

l Program Memory Usage: ~ 3 KB


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Information on the Web


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AN956 - Migrating Applications to USB from RS-232

l Communication Device Class (CDC) firmware


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CDC_RS232_Emulation.EXE

Workspace file: MCHPUSB.mcw

Output file: MCHPUSB.hex

INF file for Windows Installation

l C:\MCHPFSUSB\


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RS-232 to USB Migration

l Emulates RS-232 over USB = Easiest Transition

l No change for the PC applicationl Continues using COM1, COM2, etc.

l CDC RS-232 Emulation Reference Project is freely downloadable from www.microchip.com

emulatingrs-232overusb_121004

Documents

communication

speed usb

inf file

embedded device

reference

embedded application

cdc rs

physical rs