User manual
4.23. USB Host Ports (UHP)
Two independent USB 2.0 Full Speed host ports are integrated in the SAM9261 processor. According
to the USB specification, the term "Full Speed" denotes a transmission rate of 12 MBit/s. A "High
Speed" mode (480 MBit/s) is not available with the SAM9261.
a) External Parts
A few external parts are required for the proper operation of the UHP:
- Pull-down resistors on each line of approximately 15 kΩ. These should be installed even if the
UDP is not to be used at all in order to keep the signals from floating.
- Series resistors of 27 Ω (5%) on each line.
- 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 V
BUS
supply voltage.
b) 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.
c) Layout considerations
The external resistors should be placed in the vicinity of the Stamp9261 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 large areas on the base board, in order to reduce signal distortion and noise.
The are preferably routed closely in parallel from the Stamp9261 to the USB connector.
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