e009052.pdf

BASI CS

The Universal Serial

Part 1: develop your own applications

By B. Kainka

The USB Interface described elsewhere in this issue demonstrates that the

USB may be used for DIY projects related to measurement and control. But

how does one develop USB compatible peripherals, which conditions should

be satisﬁed, and where can the necessary information be found?

The USB is a serial bus system designed for

connection to and addressing of several

peripheral devices. USB is considerably more

complex than RS232, but also much faster at

up to 1.5 Mbit/s (for low-speed devices) or

12 Mbit/s (for full-speed devices). Further-

more, the designers of the USB have paid a lot

of attention to simplicity and ease of use. It is

fair to say that the concept of Plug and Play,

for the ﬁrst time, actually applies to the USB,

which allows devices to be connected and

automatically initialised with the PC

switched on or off. With USB, you don’t have to

grapple with interrupt conflicts, wrong

addresses and missing drivers.

Making it ‘simpler’ for the user means a

more complex job for the designer. If you

want to design your own USB peripheral

device, you should be ready to delve into the

USB standard, program your own microcon-

troller and write your own driver. The inset

box provides some suggested starting points.

USB cables

and power supply

Two different connector types are around: type

A and type B. the system is designed such

that these can not be mixed up. As opposed to

RS232, the so-called ‘crossed’ cable does not

exist with USB, all cables have simple 1-to-1

Figure 1. The plugs used.

Bus (USB)

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The two datalines D+ and D– may only be

employed in combination with special USB

components, i.e., special microcontrollers,

which, as a matter of course, are powered via

the USB connector. After a special authorisa-

tion request, a peripheral is allowed to con-

sume up to 500 mA.

The supply voltage on the USB is allowed

to rise to 5.25 V and drop to 4.2 V. A good

voltage regulator will have no problems step-

ping these levels down to 3.3 V. The entire

system — cable and devices — is dimen-

sioned such that the minimum supply voltage

can not drop below 4.2 V. Devices drawing

more than 100 mA have to report their

requirement with the system and are only

allowed access to the bus when sufficient

current is available.

A clear distinction is made between Bus

Powered and Self Powered USB peripherals.

In many cases, either of the two modes may

be selected. For example, the device may

have a supply input socket for connection to

an external mains adapter.

According to the USB standard, the cur-

rent consumption from the bus is automati-

cally limited. Therefore, if more current is

drawn than allowed, the supply has to be

switched off.

234

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Figure 2. Connections of plugs type A and B.

USB classes

Figure 3. USB socket type B.

Until recently, you would receive a free driver

disk with a new mouse. Not so if you buy an

USB savvy mouse these days. Having con-

nected the mouse despite doubts and worries

about the missing disk, you will be pleasantly

surprised to see that the Windows 98 operat-

ing system will automatically ﬁnd and install

the driver. In fact, Windows 98 will load the

HID (human interface device) driver, which

has been lurking in your PC until a USB-com-

patible device of the ‘defined’ class is con-

nected up. This class is supported by ready-

made drivers.

HIDs include mice, keyboards, pointer

correspondence between the pins at

either side, and the pin functions are

always the same:

speed is an important factor. The

system adopted for ready-made

cables can be relied on to prevent a

low-speed cable being used where a

full-speed cable is called for. All

cables are full-speed cables unless

permanently attached to the USB

device (like the previously men-

tioned mouse). Where a cable is not

long enough, a special type A-A

extension cable may be used.

While USB plugs are not sold in

the shops, the mating socket is avail-

able. Though a bit thin on the

ground, these (PCB mount) sockets

allow you to experiment with the

USB. The bus will present a +5 V

supply voltage which may be loaded

at up to about 100 mA. Many digital

circuits and microcontrollers operate

from a 5-V supply voltage. By con-

necting such a circuit to the USB you

get the required supply voltage

thrown in as a bonus. One ‘must’

however, is that an efficient short-

circuit protection be provided, like a

regular fuse or a Polyswitch fuse.

1 +5 V

2 Data –

3 Data +

4 Ground

At the rear of a modern PC you will

typically find two sockets of the A

type. These may be used for direct

connection to two devices. Relatively

small low-speed peripherals such as

a mouse generally employ a thin,

ﬁxed cable with a type-A plug. In all

other cases, the peripheral will have

its own USB socket of the B type.

The connection to the PC is then

made by means of a type-A cable.

Such cables are only supplied

ready-made and completely moulded

— spare USB plugs are not available.

The length, cable diameter and

screening of USB cables are subject

to tight speciﬁcations. Here, the dif-

ference between full-speed and low-

100mA

D –

D +

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Figure 4. The USB as a stabilised power supply.

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devices and joysticks. In addition to HIDs

there are also USB classes for soundcards,

printers and many other peripheral devices.

The upshot is that all ‘regular’ devices are

divided into classes, for which suitable dri-

vers are available. This class allocation auto-

matically creates a certain level of standardi-

sation, after all, the device manufacturers

have to make sure their products comply with

the relevant class speciﬁcations.

Standard USB devices are no longer

expensive. Not surprisingly, you may wonder

if it is possible to ‘misuse’ a particular USB

peripheral.

For example, a USB compatible soundcard

will install itself in such a way that can be

used as a regular, internal soundcard. After

all, that is the purpose of the whole exercise:

all available programs should be able to

employ the USB device. The same goes for

programs you make yourself. A quick test

using PORT.DLL from the ‘Compact’ software

supplied by Elektor for the extremely popular

Universal Interface for Windows (December

1999) was a direct success. So, your own pro-

grams written in Delhi should be able to

make use of any USB peripheral connected to

a PC. However, our experience in this area is

still too limited to be able to make the over-

assertive statement that it ‘always works’.

An experiment with a USB game controller

produced the following results: a USB joystick

port may be installed, among others, using

DOS emulation. This allows direct access to

the virtual port addresses of a game card. In

this way, you have added a simple USB inter-

face with four analogue and four digital

inputs to your system.

USB

USB Hub

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Figure 5. Daisy chain structure of the USB with one hub.

USB data packets

1 ms

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Figure 6. Data packets is 1-ms frames.

Bus topology

Upstream and four Downstream

ports. The PC, however, will already

contain a hub to implement two USB

ports. This so-called Root Hub is

located on the PC motherboard.

A further hub may be connected

to the downstream port of a hub. A

total of seven hubs may be cascaded

in this way, resulting in a maximum

of 127 devices. That, however, is a

theoretical number because the

available bandwidth has to be dis-

tributed across all devices connected

to the bus.

The USB is a star-shaped bus with a single

master. To be able to connect several USB

devices you will require a Hub, which is sim-

ply a bus-distributing device with several

ports. This device is literally at the hub of the

bus system. Usually, a USB hub has one

D +

3V3

D -

667 ns

83.3 ns

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Figure 7. Low-speed and high-speed signals.

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3V3

have to synchronise to the datastream.

Because a separate clock signal is not dis-

tributed, the clock rate has to be recovered

from the datastream. The NRZI principle (non

return to zero) is used for that purpose. In this

system, only logic zeroes cause the voltage

level to change — for logic ones, the voltage

remains unchanged.

In general, a USB device has several FIFOs

to receive data in. To the device address is

added an endpoint address which indicates

where the data should arrive or where it orig-

inates from. A USB mouse, for instance,

always has an Endpoint 1 and an Endpoint 0.

The latter is used during the initialisation.

The internal microcontroller writes the actual

data into the Endpoint-1 FIFO at regular

intervals, from where they are transmitted to

the PC.

The USB software forms so-called Pipes to

individual Endpoints. A Pipe may be likened

to a data channel consisting of a single wire.

In actual fact, however, the data intended for

a certain Pipe are transmitted in the form of

data packets squeezed in a millisecond

frame. Next, the hardware employs the End-

point address to actually distribute the data

to real memory devices. To achieve a higher

data rate, a device may occupy several Pipes

at a time.

USB Hub

Fullspeed

USB Device

D +

USB

Transceiver D –

D –

Transceiver

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Figure 8. Recognition of a full-speed device.

3V3

USB Hub

Lowspeed

USB Device

D +

USB

Transceiver D –

D –

Transceiver

000094 - 17

Enumeration

Figure 9. Recognition of a low-speed device

A non-used USB connection is inactive, and

the hub will not send data frames to it. The

two datalines are then Low and represent an

internal resistance of about 15 kΩ. Each USB

peripheral device has an internal resistor of

1.5 kΩ connecting one of the datalines with

+3.3 V. In a full-speed device, the D+ line is

pulled up, in a low-speed device, the D– line.

This information is used by the hub to first

determine the type of peripheral it is talking

to, and then set up the required data rate.

With the enumeration of a device, the suit-

able drivers are automatically loaded. Con-

versely, the system will also detect a device

being disconnected from the bus. In that

case, the driver is removed from memory. In

this way, it is a simple matter to employ a sin-

gle USB device with several different com-

puters. All you have to do is disconnect the

USB signals

The signals on the D+ and D– lines

are difference signals with levels of

0 V and +3.3 V. The microcontroller

in the USB peripheral will typically

operate at a supply voltage of 3.3 V.

The USB is a single-master bus — all

activities originate from the PC. Data

is conveyed in packets of eight to

256 bytes. The PC is authorised to

request data from a peripheral. Con-

versely, a peripheral may transmit

data to the PC.

All data traffic takes place in

frames of about 1 millisecond. Within

a frame, several data packets may be

processed for several devices. Low-

speed and full-speed packets may

occur within the same frame. When

several devices are addressed, a bus

distributing device like the previ-

ously mentioned hub arranges the

data distribution.

Low-speed devices operate at a

data rate of 1.5 Mbit/s, so the length

of a bit is 666.7 ns. On full-speed USB

connections, data travels at a rate of

12 Mbit/s, which means that a bit

last 83.33 ns. The speed is dictated

by the master only. All USB slaves

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Some useful addresses

http://www.usb.org : here, the makers of USB reside, that is, the

representatives of several large companies that teamed up to

deﬁne the USB standard. Their major contribution is the USB

Speciﬁcation, which is available as a downloadable pdf document.

Essentially, this document contains everything. However, it is

debatable if users should stick to just these sources for the infor-

mation they require.

Craig Peacock ( http://www.beyondlogic.com ) wrote his own dri-

ver for the USB thermometer. Craig supplies extremely useful

technical information and documentation on building your own

USB driver. His website on the USB is well worth a visit.

The starter kit supplied by Anchor-Chips

( http://www.anchorchips.com ) (now with Cypress,

http://www.cypress.com ) for the EZ-USB (AN2131) is based on

an 8051-compatible processor with internal RAM and an USB

core. Programs written for the 8051 may be loaded into the

processor RAM via the USB interface, and started straight away.

The USB core is so powerful that very few additional instructions

are required to produce useful USB applications.

The Starter Kit from Cypress ( http://www.cypress.com ) for their

USB microcontroller type CY7C63000 comes with an USB ther-

mometer as an application example. Unfortunately, this relatively

cheap kit is no longer available. The Elektor Electronics USB inter-

face published in this issue was developed on the basis of this

thermometer. So, if you missed your chance to obtain a Cypress

USB kit, an excellent alternative is available.

More information and hyperlinks may be found on the author’s

homepage http://www.home.t-online.de/home/B.Kainka

device from the first PC and insert the plug

into the other one. On the second PC, a new

enumeration procedure is started. In this

way, a printer is easily used on two PCs.

PC1

D –

D +

The USB switch

PC1

For the serial (RS232) and parallel (Centron-

ics) port, switches are available that obviate

the need to plug and unplug cables when you

want to use a different peripheral. Because

only very few bus lines are involved, a simi-

lar switch for USB is easy to build. This

switch should, however, mimic a USB cable

being unplugged and plugged in again. Also,

to make sure the peripheral is properly reset,

sufficient time should be allowed to elapse

before it is connected up to the bus. After all,

the internal microcontroller has to be

restarted to enable a new enumeration

process to be initiated. From the above it

should be clear why the USB switch has a

neutral position in which the printer is not

connected to either of the two PCs.

Another small detail should be observed.

If you look closely at a USB plug, you will

notice that pins 1 and 4 for the positive sup-

ply voltage and ground are a little longer than

pins 2 and 3 for the datalines. When the cable

is plugged in, the supply pins are connected

ﬁrst. Consequently, the USB peripheral is reli-

ably powered before the datalines are con-

nected. This system also reduces the risk of

damage caused by static charges building up

on peripheral devices, or equalisation current

that may flow between devices if there is a

problem with the earth connections. In brief,

voltage surges caused by connecting devices

can not cause damage to the electronics con-

nected to the datalines. By the way, a paral-

lel printer cable has an extended metal cover

which touches the computer before the plug

contacts are actually inserted.

D –

D +

Printer

D –

D +

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Figure 10. The USB printer switch.

Equalisation currents between

USN devices could also be prevented

by connecting all ground lines

together in the USB switch. This,

however, causes a large earth loop

via the protective earth lines con-

nected to the PC. Under adverse

conditions, earth loops could lead to

serious interference and reduced

reliability of the entire computer sys-

tem. Arguably, that should be

avoided, hence all four USB lines are

switched together in the switch box.

The special behaviour of the

longer connecting pins may be mim-

icked by a switch with ﬁve contacts.

When switching over to another USB

device, the supply voltage is con-

nected first, then the datalines D–

and D+. The centre position is ‘neu-

tral’. Depending on the USB periph-

eral involved, a delay of one second

may be required.

It should be noted that this is an

experimental proposition which

could not be extensively tested

before publication of this article. In

particular, the exact type of printer

used will determine if fast or slow

switching should be used, or if the

delay with the centre position is at

all required. Fortunately, it should

not be too difficult to ‘automate’ the

switch box using two relays and a

small circuit.

(000094-1)

Continued next month

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