Cypress Semiconductor Corporation
• 3901 North First Street • San Jose • CA 95134 • 408-943-2600
April 2, 2002
Choosing the Right Device for Your USB Application
Introduction
This application note guides the USB designer through the
process of determining the most suitable Cypress USB chip
for their product. With the widest portfolio of USB chips,
Cypress Semiconductor has a device for almost every appli-
cation.
USB can be a daunting new technology for designers not fa-
miliar with its details. Most designers will only need to become
familiar in detail with the parts of the USB Specification that
are relevant to their application. To avoid the requirement for
designers to familiarize themselves with the whole USB
Specification simply in order to determine what kind of device
will be required in their application, this application note takes
designers through the process of determining whether they
require a device with Host, Hub or Peripheral functionality. It
also helps designers decide whether their application re-
quires Low, Full or High Speed signaling.
Step #1 - Host, Hub or Peripheral?
Every USB system must have one and only one “host” - that
is, a bus “master.” USB does not support a peer-peer topolo-
gy. A host is said to have a “downstream” port—that is a port
to which peripherals (“slaves”) can be connected either direct-
ly or via one or more hubs. The host initiates all USB traf-
fic—either by transmitting data itself, or requesting data from
a device on the bus. USB host controllers are typically PCI
bus devices, controlled via the PCI bus by a powerful micro-
processor; most PC chipsets now include an integrated USB
host controller. But what happens when the PC is removed
from the system and the application requires a USB host in a
Set-top box or PDA/Cell-phone? Cypress recognizes the par-
adigm shift towards an embedded host market and offers the
SL811HS/T as a solution. To learn more about the
SL811HS/T, turn to the “Cypress USB Embedded Host Se-
lection Guide” section of this application note.
A USB Hub is a “repeating” device that allows multiple down-
stream peripherals to be connected to a single upstream host
or hub port. The USB specification permits up to 5 “tiers” of
hubs between a peripheral and a host. A hub cannot originate
downstream traffic on its downstream ports itself—all down-
stream data traffic is simply retransmission of data the hub
has received on its upstream port. In addition, a hub is re-
quired to behave in a similar manner to a peripheral on its
upstream port, as the hub controller function can itself be re-
garded as a peripheral. Note that it is possible to have a hub
function and an independent peripheral function implemented
in the same device—for example a monitor or keyboard hub.
To select the USB hub device that is right for your application,
please refer to the “Cypress Hub Device Selection Guide”
section of this application note.
A Peripheral has one and only one “upstream” port—that is a
port by which the device is connected to the downstream port
of a host, either directly or via one or more hubs. A peripheral
never transmits data without first receiving a request from the
host to do so. To select the USB peripheral device that is right
for your application, proceed to step #2.
Step #2 - Low-Speed, Full-Speed, or High-
Speed Peripheral?
USB has 3 data rates: “low-speed,” “full-speed,” and “high-
speed.” The signaling rates are 1.5 Mbps, 12 Mbps and
480 Mbps respectively, but these rates do not accurately re-
flect the data throughput available to a single device using
that signaling rate.
For low-speed signaling, the best guaranteed throughput is 8
bytes of payload data every 10 ms, per endpoint—which is
6400 bps. Comparing this to RS-232 with 1 start, 1 stop and
1 parity bits with continuous transmission (no handshaking)
this is equivalent to 8800 baud on a traditional serial link. Low-
Speed devices can have up to 2 data endpoints, so by using
2 endpoints a single USB device may achieve an RS-232
equivalent data throughput of 17600 baud.
If this data throughput is adequate for your application. refer
to the “Cypress Low-Speed USB Peripheral Device Selection
Guide” section of this application note.
For full-speed signaling, the payload data throughput is limit-
ed only by the 12-Mbps signaling rate and the protocol over-
head. The maximum data throughput available shared be-
tween all devices connected to a single USB host is 1.216
MB/s—equivalent to 9.728 Mbps. It is only possible for a sin-
gle device to “reserve” up to 90% of this bandwidth using
“interrupt” or “Isochronous” transfers (please refer to the USB
specification for details). Sustained data throughputs of 8.7
Mbps have been demonstrated, even using Cypress’s lowest
specification AN21xx Full-Speed device. However in general,
in cases where bandwidth requirements approach the full-
speed limit, it is recommended that High-Speed signaling be
selected.
If this data throughput is adequate for your application, refer
to the “Cypress Full-Speed USB Peripheral Device Selection
Guide” section of this application note; otherwise, refer to the
“
Cypress High-Speed Peripheral Device Selection Guide”
section of this application note.
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