AN49150 Schematic and Layout Review Checklist for HX2LP.pdf

AN49150
Schematic and Layout Review Checklist for HX2LP
Author: Vihang Trivedi
Associated Project: No
Associated Part Family: CY7C65620, CY7C65630
Software Version: NA
Related Application Notes: AN5044
AN49150 discusses the schematic and layout review checklist for the EZ-USB HX2LP™ family.
Introduction
Figure 1. Eye Diagram Result 1
The Cypress CY7C65620 and CY7C65630 USB 2.0 hubs
are high performance, low cost solutions for USB. The
CY7C656xx USB 2.0 hubs integrate 1.5 kΩ upstream
pull-up resistors for host notification. All downstream
15 kΩ pull-down resistors and series termination resistors
are also integrated by the hubs on all upstream and
downstream D+ and D- pins. This results in system cost
optimization by providing built in support for the USB 2.0
specification.
Hardware Considerations
The hardware considerations for using HX2LP in your
design are as follows.
Trace Length and Width

It is important to check the data lines (D+ and D-) and
verify how the traces are laid out and to which part
they are nearer. Make sure they stay within the
impedance matching.

The trace length and width, and their impedance
matching play a significant role in signal quality.
Figure 1 shows the eye diagram results of the high
speed signal quality test of a two layer board with thin
and short traces; no impedance matching is
performed. The traces’ lengths are less than an inch.
It passes the test, although it shows overshoot and
undershoot in the eye diagram that causes excessive
voltages. The eye diagram also shows phase
differences between the two traces resulting in jitter.
www.cypress.com

Figure 2 shows the eye diagram results of the high
speed signal quality test of a four layer board with
long traces; impedance matching is performed. The
traces’ lengths went up to about 9 inches. It passes
the signal quality test and the eye diagram did not
show any overshoot or undershoot (Figure 2).
Document No. 001-49150 Rev. *B
1
Schematic and Layout Review Checklist for HX2LP
The 500 µW value is the power that is driven into the
crystal. This rating is typically overlooked, with designers
using 100 µW or 10 µW, which is not allowed.
Figure 2. Eye Diagram Result 2
Power Requirements
Downstream Pow er Switching
The downstream port power of the hub is allowed to have
different configurations according to the USB specification.
One basic requirement is that all ports must be protected.
Most vendors use a power switch to do that. They have a
current limit of 500 mA. In the case of low cost hubs,
designers use a fuse to meet this requirement. Other
options are ganged power and individually controlled
power.
Crystal Requirements
The crystal is one component that must be checked
thoroughly. Some requirements such as crystal drive
strength are often overlooked. This indicates how much
power the crystal can tolerate. A crystal with a low drive
level may result in accelerated aging or may burn out and
not function.
Also, load capacitance is not in the normal parallel design.
In a parallel design with equal load caps, load rating minus
the stray capacitance is taken. The result is doubled to
arrive at the load capacitance. This means that for a 12 pF
load rating with 3 pF stray capacitance, there are 18 pF
capacitors. However, this part uses a crystal with a load
capacitance rating of 12 pF and still requires both these
capacitors to be 12 pF. In a normal parallel design, you
end up driving more than 500 µW into the crystal.
Depending on the crystal, it may age early or burn out.
This is not a typical crystal design, but it works perfectly
for the chip. It is slightly off the center frequency but
maintains a lower drive power into the crystal and still
stays within the bit rate specification from the USB-IF.
Following are the parameters to remember when selecting
the crystal:





12 pF rating on the crystal
24 MHz ± 0.05%
Parallel resonant, fundamental mode
12 pF capacitors for load caps
500 µW drive
The over current inputs in its default configuration need a
pull-up resistor and most switches do not have an internal
pull-up resistor. A pull-up resistor is required only if you
are doing an active low on the signal. If the over current is
high, then a pull-down is required. Each port must have
some capacitance, according to the USB 2.0 specification,
(minimum of 120 µF). Otherwise, enough current is not
supplied at in-rush time to the device. Each capacitor must
have enough out-rush current to be able to supply enough
in-rush current to the device, while maintaining voltage
levels to the device.
3.3 V Regulator
It is important that the power from a 3.3 V regulator is
noise free. If the circuit does not have sufficient bypass
capacitors on the chip, then the noise generated by the
switching frequency of the regulator shows up on the eye
diagram as jitter. The specific recommendations for
ceramic capacitors nearest to the regulator are mentioned
in Table 1 of AN5044.
V B U S P ow er M o n i t o r i n g
The hub chip has a VBUS pin called VBUSPower. This pin
must be connected to VBUS through a resistor bridge to
ground. The resistor to ground allows the power to bleed
when you unplug the hub; otherwise, it still considers the
hub as connected. The hub may be powered by the host
supply (that is, designed with the host and only D+ and Drouted). In this case the hub is powered when the host is
on and switched off when the host is off. In this case, it is
best to tie the VBUSPOWER pin to the reset pin and have
it high only after reset.
Reset Design
One use of the VBUS pin is to inform the hub chip when
the cable is connected to the upstream port. If the cable is
hardwired and no longer removable and the power is
directly derived from the upstream power source, it is
better to tie the VBUS power pin to the reset pin. This
helps to avoid issues with the delays on the regulators.
SPI Interface
SPI_SD line must be pulled low using a 15 kΩ pull-down
resistor.
www.cypress.com
Document No. 001-49150 Rev. *B
2
Schematic and Layout Review Checklist for HX2LP
Debugging HX2LP Chips
Summary
To bring up the HX2LP chips, the board must have the
load capacitors for the crystal, the crystal itself, and pull-up
resistors for the over-current attached to the HX2LP. This
must be the minimum amount of hardware provided on the
board to enumerate as a hub. It brings up the chip in its
default hub configuration because the board does not
have an EEPROM assembled on it.
After the communication between the host and hub is
established, the next step is to attach an EEPROM with its
external resistors so that you can test the programming of
the EEPROM. If a blank EEPROM is connected initially,
the hub chip enumerates as a vendor specific device and
not a hub. After that you can program the EEPROM using
the Blaster program from the CY4605/4606 kits and
enumerate it as a hub defined by the EEPROM
configuration by cycling the power to the hub. When this is
done, start adding the downstream components to the hub
to finish the rest of the circuit design.
www.cypress.com
This application note helps you to produce a compliant
high performance USB 2.0 hub design. It also provides an
overview of probable design issues and remedies.
About the Author
Name:
Vihang Trivedi.
Title:
Applications Engineer Sr
Background:
Vihang Trivedi supports USB products
at Cypress Semiconductor in San
Diego, CA. He holds a Masters degree
in
Electrical
Engineering
from
California State University, Chico.
Document No. 001-49150 Rev. *B
3
Schematic and Layout Review Checklist for HX2LP
Document History
Document Title: Schematic and Layout Review Checklist for HX2LP - AN49150
Document Number: 001-49150
Revision
ECN
Orig. of
Change
Submission
Date
Description of Change
**
2572965
VGT
09/30/2008
New application note.
*A
3468951
DBIR
12/19/2011
Updated Hardware Considerations:
Updated Power Requirements:
Added SPI Interface.
Updated to new template.
*B
4590229
DBIR
12/09/2014
Updated to new template.
Completing Sunset Review.
www.cypress.com
Document No. 001-49150 Rev. *B
4
Schematic and Layout Review Checklist for HX2LP
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Document No. 001-49150 Rev. *B
5
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