AN15454
Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
Author: Prajith Cherakkoda
Associated Project: No
Associated Part Family: CY7C65620/30, CY7C65632/42
Software Version: N/A
Related Application Notes: None
If you have a question, or need help with this application note, contact the author at
prji@cypress.com.
With the arrival of the HX2LP/HX2VL families of hubs, you can design a hub for bus-powered operations. HX2LP
consumes less than two unit loads for the core of the hub, which leaves three unit loads for downstream ports. This
means that the hub can be configured to have two or three downstream ports and supply 100 mA per port. HX2VL
consumes less than one unit loads for the core of the hub, which leaves four unit loads for downstream ports. This
means that the hub can be configured to have two or three or four downstream ports and supply 100 mA per port.
AN15454 guides the designer in using these families of hubs to create a bus-powered hub.
Introduction
Bus-Power Versus Self-Powered
When designing a bus-powered hub, you should consider
size, cost, and port configurations. These three items
interact such that one cannot be considered independently
of the others. The starting point should be the number of
ports you want in the configuration. The HX2LP family in a
bus-powered configuration can support at most three
downstream ports and HX2VL family can support at most
four downstream ports; some applications may choose to
implement only two or three ports. These considerations
affect the chip that should be used within the families.
For a bus-powered hub the limiting factor is the amount of
power available from the upstream port. In general, the
upstream port has a 5-unit load. After considering the
number of ports, you must determine the power source for
the hub. A hub design can be made to be self-powered.
For this, you must always have a power outlet. Because
USB supplies bus power up to 500 mA (a 5-unit load), it is
possible to run a hub from just this power source. The
limitation of this is the downstream ports are only allowed
to have 100 mA each. If the device you are trying to run
requires more, it will not function under a bus-powered
configuration. Because of this, it may be desirable to
configure the hub to run in either bus or self-powered
configuration.
Note: In this application note, HX2LP will be used to refer
both HX2LP and HX2VL except where noted.
Power
Bus-powered hubs draw all of their power for any internal
functions and downstream-facing ports from the bus
power on the hub's upstream port. Hubs that can limit their
power consumptions to less than three unit loads are able
to supply a single-unit load to two downstream ports.
The HX2LP family of chips has several configuration
options that help in controlling the power consumption of
the hub design. If the power consumed by the chip is less,
the design for the bus-powered application is better.
These options are:


The HX2LP family of hubs has description entries within
an initialization EEPROM (of CY7C65620/30) and pin
inputs to define the ability of the hub. The SELFPWR pin
defines the mode in which the hub is functioning. If this pin
is high, the hub defines in its descriptors a “self-powered”
configuration. If this pin is low, the hub will define in its
descriptors a “bus-powered” hub.
The designer can either wire this input for a given
configuration or attach a switch/logic control to allow the
user to change the configuration of the hub. Figure 1
shows one such configuration.
Downstream power configuration
Indicators modulation (in CY7C65620/30)
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Document No. 001-15454 Rev. *D
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Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
Figure 1. Sample Configuration
In CY7C65620/30, along with this SELFPWR pin, there is
a requirement of setting up the EEPROM load parameters
such that they support the power configuration of the hub.
In a 0xD4 load configuration, there are entries for the
power consumption of the design (refer to the EZ-USB
HX2LP™ Low Power USB 2.0 Hub Controller Family
datasheet). These entries are: Maximum Power (FullSpeed) and Maximum Power (High-Speed). The values
must be set to the worst-case values of a bus power
configuration. The self-power hub draws its power not
from the bus, but from the AC adapter attached to the hub.
Therefore, the worst-case condition should be the buspowered mode.
If the hub is to be capable of dynamically switching
between bus-powered and self-powered (such as with the
above circuit), an additional bit called Self Powerable in
byte 18 should also be set.
Downstream Power Configuration
When configured as a bus-powered hub the downstream
power will be limited to 100 mA; therefore, some devices
(those consuming more than 1 unit load) do not function
when the hub is configured for bus-powered configuration.
When configured as self-powered, these ports receive a
full 5-unit load (500 mA) of power.
When the SELFPWR pin is changed from either self or
bus-powered mode, the hub disables downstream devices
and reports a configuration change to the host. The host
then gets the new configuration from the hub and
enumerates each device on the hub. This switching can
be accomplished by either a switch that configures
bus/self power, or logic circuits that detect the self power
input and automatically set the SELFPWR pin to the
proper condition (see Figure 1).
www.cypress.com
Indicator Modulation
The CY7C65620/30 also has a mode to modulate the
indicators with a square wave (50 percent duty cycle)
signal. The result of doing this is to reduce the power
consumption of the hub. The maximum amount of current
required does not change in this configuration. What
changes is the amount of average power required. This is
useful when the hub is operating in a bus-powered
configuration while on a battery-powered computer. This
reduction in average power extends the operating time of
the computer.
This option is set through the 0xD4 EEPROM load option.
It is found in byte 18 and is called Modulate Indicators.
Setting this bit modulates the indicator’s control signal.
Descriptors and EEPROM Settings
The descriptors reported to the host computer can be
altered through the use of an EEPROM or through the use
of fuse ROMs. The hub allows the designer to configure
necessary items such as the power consumption of the
hub along with configuring the downstream ports. These
options need to be set for each hub design to obtain
compliance. The number of ports configured affects the
power consumption of the hub, along with configuration
LED modulation (in CY7C65620/30).
Document No. 001-15454 Rev. *D
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Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
For lower volume designs, the cost-effective method of
setting these values is through the configuration
EEPROM. The bus used to communicate with the
EEPROM is a SPI bus (CY7C65632/42 supports I2C
EEPROM as well). If the design is a higher volume design,
there are fuse-ROMs, which can be programmed by
Cypress manufacturing to configure the hub with the
designer's required configuration parameters. For these,
contact
your
Cypress
FAE
through
at
http://www.cypress.com.
About the Author
Name:
Prajith Cherakkoda
Title:
Applications Engineer
Contact:
prji@cypress.com
Summary
Currently, Cypress offers a family of high-speed hubs that
can be configured in either single-transaction translator
(TT) mode or the multiple-TT mode. These hubs support
both bus and self-powered configurations. The designer
needs to first derive the requirements for the environment.
A hub with a single full-speed or low-speed port need not
use a multi-TT configuration while a hub with multiple fullspeed devices transferring larger quantities of data has
throughput benefits from using the multi-TT configuration.
After the requirements are defined, you can easily
configure these hubs for bus-powered use, and minimize
the power consumption to support a lower power
configuration. Contact Cypress technical support
(www.cypress.com/support) for more information on this
product.
www.cypress.com
Document No. 001-15454 Rev. *D
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Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
Document History
Document Title: AN15454 – Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
Document Number: 001-15454
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
3071630
AASI
10/26/2010
New application note.
*A
3174854
AASI
02/16/2011
Updated sample configuration image to show a simpler implementation. Updated
template.
*B
3210564
LRDK
03/30/2011
Fixed typographical error. No content changes.
*C
3228918
AASI
04/15/2011
Added information on HX2VL
*D
4153589
PRJI
10/10/2013
Updated in new template.
Completing Sunset Review.
www.cypress.com
Document No. 001-15454 Rev. *D
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Bus-Powered USB Hub Design Using EZ-USB HX2LP™/HX2VL
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Document No. 001-15454 Rev. *D
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