Design Considerations
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5. Design Considerations
5.1. Ethernet Controller (EMAC)
The emac needs an aditional PHY design. The emac supports both, MII and RMII interface.
Please take care of the specific layout requirements of the Ethernet port when designing
a base board. The two signals of the transmitter pair (ETX+ and ETX-) should be routed
in parallel (constant distance, e.g. 0.5mm) with no vias on their way to the RJ45-jack. The
same is true for the receiver pair (ERX+ and ERX-). No other signals should be crossing
or get next to these lines. If a ground plane is used on the base board, it should be omitted
in the vicinity of the Ethernet signals.
A 1nF / 2kV capacitor should be connected between board ground and chassis ground
(which is usually connected to the shield of the RJ45-jack).
5.2. USB
5.2.1. USB Host Controller (UHP)
External Parts.
A few external parts are required for the proper operation of the UHP:
• No pull-down resistors are needed.
• No series resistors are needed.
• Small capacitors (e.g. 15pF) to ground on each line (optional).
• ESD protection devices are recommended for applications which are subject to external
contact. The restrictions with regard to capacitive loading have to be applied when
selecting a protection device.
• A circuit to generate the 5V VBUS supply voltage.
V
BUS
considerations for USB Host.
A USB host port has to provide a supply voltage
V
BUS
of 5V +- 5% which has to be able to source a maximum of 500mA, or 100mA in case
of battery operation. Please refer to the appropriate rules in the USB specification. A low
ESR capacitor of at least 120µF has to be provided on V
BUS
in order to avoid excessive
voltage drops during current spikes.
V
BUS
has to have an over-current protection. The over-current drawn temporarily on
V
BUS
must not exceed 5A. Polymeric PTCs or solid state switches are recommended
by the specification. Suitable PPTCs are "MultiFuse" (Bourns), "PolyFuse" (Wickmann/
Littelfuse), "PolySwitch" (Raychem/Tyco).
It is required that the over-current condition can be detected by software, so that V
BUS
can be switched off or be reduced in power in such a case.
Layout considerations.
If external resistors are needed, they should be placed in the
vicinity of the module's connector. The two traces of any of the differential pairs (USB-
Host A+ and USB-Host A- , as well as USB-Host B+ and USB-Host B-) should not encircle