Taskit ARM9 CPU-module with Linux Stamp9261-series Stamp9261 (64F/64R) 542310 Data Sheet
Product codes
542310
4.24. USB Device Port (UDP)
a) External Parts
A few external parts are required for the proper operation of the UDP:
A few external parts are required for the proper operation of the UDP:
-
Pull-down resistors on each line of approximately 330 kΩ. These should be installed even if
the UDP is not to be used at all in order to keep the signals from floating.
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).
-
A voltage divider on the 5V USB supply voltage V
BUS
converting this voltage to 3.3V, e.g.
27 kΩ / 47 kΩ, for the V
BUS
monitoring input (USB_CNX).
-
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.
contact. The restrictions with regard to capacitive loading have to be applied when selecting a
protection device.
The USB specification (revision 2.0 of April 27, 2000, chapter 7.1.1.1) demands a cable with a
characteristic impedance of 90 Ω ± 15% . Matching this impedance would require a driver output
impedance of 45 Ω on both the negative and the positive driver. Nevertheless, the SAM9261 manual
demands external resistors of 27 Ω. These would correspond to intrinsic resistances of the drivers of
18 Ω.
characteristic impedance of 90 Ω ± 15% . Matching this impedance would require a driver output
impedance of 45 Ω on both the negative and the positive driver. Nevertheless, the SAM9261 manual
demands external resistors of 27 Ω. These would correspond to intrinsic resistances of the drivers of
18 Ω.
The USB specification demands a switchable pull-up resistor of 1.5 kΩ on USB-Device+ which
identifies the UDP as a full speed device to the attached host controller. On the AT91SAM9261
processor, this resistor is integrated on the chip. It can be switched on or off using the "USB Pad Pull-
up Control Register", which is part of the "Bus Matrix User Interface" (not the "USB Device Port User
Interface", as one might expect). This pull-up resistor is required to be switchable in order not to
source current to an attached but powered down host. This would otherwise constitute an irregular
condition on the host. The software has to take care of this fact.
identifies the UDP as a full speed device to the attached host controller. On the AT91SAM9261
processor, this resistor is integrated on the chip. It can be switched on or off using the "USB Pad Pull-
up Control Register", which is part of the "Bus Matrix User Interface" (not the "USB Device Port User
Interface", as one might expect). This pull-up resistor is required to be switchable in order not to
source current to an attached but powered down host. This would otherwise constitute an irregular
condition on the host. The software has to take care of this fact.
The capacitors are intended to improve the signal quality (edge rate control) in dependency of the
specific design. They are not mandatory. The total capacitance to ground of each USB pin, the PCB
trace to the series resistor, and the capacitor must not exceed 75pF.
specific design. They are not mandatory. The total capacitance to ground of each USB pin, the PCB
trace to the series resistor, and the capacitor must not exceed 75pF.
b) Operation with V
BUS
as a Supply
Special care has to be taken if the Stamp9261 is powered by the V
BUS
supply. Please refer to the
appropriate rules in the USB specification with regard to inrush current limiting and power switching.
As the Stamp9261 draws more than 1W in normal mode, it is a "high-power" device according to the
specification (<100mA = "low-power", 100..500mA = "high-power"). It therefore requires staged
switching which means that at power-up it should draw not more than 100mA on V
As the Stamp9261 draws more than 1W in normal mode, it is a "high-power" device according to the
specification (<100mA = "low-power", 100..500mA = "high-power"). It therefore requires staged
switching which means that at power-up it should draw not more than 100mA on V
BUS
. The capacitive
load of a USB device on V
BUS
should be not higher than 10µF.
c) Layout considerations
The external resistors should be placed in the vicinity of the Stamp9261 connector. The traces of the
differential pair (USB-Device+ and USB-Device- ) 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.
The external resistors should be placed in the vicinity of the Stamp9261 connector. The traces of the
differential pair (USB-Device+ and USB-Device- ) 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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