 2004 Microchip Technology Inc.

DS91054C-page 1

TB054

INTRODUCTION

This Technical Brief demonstrates the translation of a

game port game pad to a USB game pad using the

microcontroller (MCU). The purpose of this Technical

Brief is not only to show the translation of a game pad

to USB, but also to show how to successfully develop

a USB peripheral using the PIC16C765. An under-

standing of USB descriptors is the foundation for

successful USB peripheral development.

USB Basics

All USB communication takes the form of frames sent

over the USB bus. Frames are one-millisecond incre-

ments in which the host schedules what device end-

points it will communicate with. At the scheduled time,

the host and device send one another requests, or

data, in the form of packets, which are limited in length

to eight bytes for low-speed devices. All packets are

sent to and received by the device via endpoints. End-

points are buffers where a device either puts data to

wait for a chance to be sent to the host, or where data

received from the host is stored until the device has a

chance to access the data and utilize it. Low-speed

devices have two endpoints that can be configured by

the device as IN or OUT (with respect to the host).

These are Endpoints 1 and 2. Endpoint 0 is reserved

for control transfers between the host and device. This

is the bidirectional avenue through which the host and

device share administrative data. Enumeration is the

process in which a peripheral describes what type of

device it is to the host, and occurs via Endpoint 0.

USB is a master-slave protocol. In other words, the

host (i.e., PC) directs all communications to and from

the peripherals. Peripherals do not send information to

the host unless the host requests the information. How

does a peripheral tell the host what type of device it is

when it is first connected to the host? The answer lies

in the use of “descriptors,” as described in the following

section.

Descriptors

GENERAL

USB descriptors tend to be the biggest stumbling block

for developers that have been recently introduced to

USB. The purpose of descriptors is to communicate the

identity of a particular peripheral to the host. For

instance, the game pad in this technical brief communi-

cates to the host that it is a two axis, six-button game

pad that sends data to the host via Endpoint 1. A device

does not volunteer this information to the host, rather

the host requests this information when it detects that a

new device has been attached to the USB bus. As

mentioned before, the process in which the host

requests and receives a device's descriptors is called

“enumeration.”

Peripherals have more than one descriptor. Each new

descriptor progressively provides the host more infor-

mation about the peripheral or about other descriptors

to follow. Descriptors can be thought of as a hierarchy.

The first descriptor, the device descriptor, is very gen-

eral and conveys the most basic information about the

device. The next descriptor is more specific, and so on,

until the host finally gains all the information it needs to

communicate effectively with the device.

Game pads fall under the Human Interface Device

(HID) class. Mice, keyboards and LED displays are all

examples of other devices that fall into the HID class.

The HID class is unique in that driver support is sup-

tems. These operating systems will custom build a

driver whenever they detect a new HID peripheral. The

driver is constructed from the data format conveyed in

the report descriptor received from that device. Devel-

oping within the HID class specification is the easiest

way to learn how USB works because the developer

doesn't have to be concerned with writing a driver at the

host end.

Note:

This Technical Brief is the first in a series

of five technical briefs. This series is

meant to familiarize developers with USB.

For the best understanding of USB, read

the briefs in order: TB054, TB055, TB056,

TB057 and TB058.

Author:

Reston Condit

Microchip Technology Inc.

An Introduction to USB Descriptors

with a Game Port to USB Game Pad Translator Example