USB System Hardware Design
龚昊然
The UTMI (Universal Transceiver Macro Interface) of the USB controller
is unique. It is sometimes called the PHY section of the USB controller.
Its features allow direct connection to a USB OTG cable connector. System
hardware requirements are discussed below.
clock is needed. The default value would be a 24 MHz clock to the USB_XI
pin or a 24 MHz crystal circuit used with USB_XI and USB_XO. If using a
crystal, use the same circuit as shown in the datasheet for CLKIN and XTAL.
Other frequencies can be used to generate the internal clock of exactly
960 MHz. Use the formula:
960 MHz = frequency of USB_XI × 2 × m where m is an integer.
for integrated circuits. External protection diodes should be added near
the connector for DP, DM, ID and VBUS. There are several sources of ESD
protection designed specifically for USB2.
5 volt supply at 8 ma or more. The 5 volts can be provided with a
"charge pump" or a normal voltage regulator depending on the available
input voltages available for the application. In either case the 5 volt supply
must be enabled and disabled in software using a GPIO with a resistor to
set the initial value to disable the external 5 volt source.
DP and DM are intended for direct connection to the D+ and D- of a USB
cable connector. They do not require any pullup or pulldown resistors as
these are applied internally by the UTMI in accordance with the programmed
application mode. Note also that like any USB design, DP and DM
Processor Hardware Reference 19-15 should be routed as a differential pair
with 90 to 100 ohms mutual impedance.
If using the USB in device mode only, you may put a pullup resistor on
USB_ID pin or leave the pin disconnected. Either pullup or disconnected
will work. The USB_RSET pin can be left open.
is unique. It is sometimes called the PHY section of the USB controller.
Its features allow direct connection to a USB OTG cable connector. System
hardware requirements are discussed below.
clock is needed. The default value would be a 24 MHz clock to the USB_XI
pin or a 24 MHz crystal circuit used with USB_XI and USB_XO. If using a
crystal, use the same circuit as shown in the datasheet for CLKIN and XTAL.
Other frequencies can be used to generate the internal clock of exactly
960 MHz. Use the formula:
960 MHz = frequency of USB_XI × 2 × m where m is an integer.
for integrated circuits. External protection diodes should be added near
the connector for DP, DM, ID and VBUS. There are several sources of ESD
protection designed specifically for USB2.
5 volt supply at 8 ma or more. The 5 volts can be provided with a
"charge pump" or a normal voltage regulator depending on the available
input voltages available for the application. In either case the 5 volt supply
must be enabled and disabled in software using a GPIO with a resistor to
set the initial value to disable the external 5 volt source.
DP and DM are intended for direct connection to the D+ and D- of a USB
cable connector. They do not require any pullup or pulldown resistors as
these are applied internally by the UTMI in accordance with the programmed
application mode. Note also that like any USB design, DP and DM
Processor Hardware Reference 19-15 should be routed as a differential pair
with 90 to 100 ohms mutual impedance.
If using the USB in device mode only, you may put a pullup resistor on
USB_ID pin or leave the pin disconnected. Either pullup or disconnected
will work. The USB_RSET pin can be left open.
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