MCF5485 FREESCALE [Freescale Semiconductor, Inc], MCF5485 Datasheet - Page 8

• Manufacturer Part Number: MCF5485
• Description: Integrated Microprocessor Electrical Characteristics
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

Hardware Design Considerations
4.3
4.3.1
4.3.2
Connecting the USBVBUS pin directly to the 5V VBUS signal from the USB connector can cause long-term reliability
problems in the ESD network of the processor. Therefore, use of an external voltage divider for VBUS is recommended.
Figure 4
should connect. Point B, marked in each figure, is where a 3.3V version of VBUS should connect to the USBVBUS pin on the
device.
4.3.3
It is recommended to connect the shield and the ground pin of the B USB receptacle for upstream ports to the board ground
plane. The ground pin of the A USB receptacles for downstream ports should also be connected to the board ground plane, but
industry practice varies widely on the connection of the shield of the A USB receptacles to other system grounds. Take
precautions for control of ground loops between hosts and self-powered USB devices through the cable shield.
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High speed clock and the USBD+ and USBD- differential pair should be routed first.
Route USBD+ and USBD- signals on the top layer of the board.
The trace width and spacing of the USBD+ and USBD- signals should be such that the differential impedance is 90Ω.
Route traces over continuous planes (power and ground)—they should not pass over any power/ground plane slots or
anti-etch. When placing connectors, make sure the ground plane clear-outs around each pin have ground continuity
between all pins.
Maintain the parallelism (skew matched) between USBD+ and USBD-. These traces should be the same overall length.
Do not route USBD+ and USBD- traces under oscillators or parallel to clock traces and/or data buses. Minimize the
lengths of high speed signals that run parallel to the USBD+ and USBD- pair. Maintain a minimum 50mil spacing to
clock signals.
Keep USBD+ and USBD- traces as short as possible.
Route USBD+, USBD-, and USBVBUS signals with a minimum amount of vias and corners. Use 45° turns.
Stubs should be avoided as much as possible. If they cannot be avoided, stubs should be no greater than 200mils.
and
General USB Layout Guidelines
Figure 5
USB D+ and D- High-Speed Traces
USB VBUS Traces
USB Receptacle Connections
depict possible connections for VBUS. Point A, marked in each figure, is where a 5V version of VBUS
MCF5485 Integrated Microprocessor Electrical Characteristics, Rev. 3
(5V)
(5V)
Figure 4. Preferred VBUS Connections
Figure 5. Alternate VBUS Connections
A
A
8.2k
50k
20k
(3.3V)
(3.3V)
B
B
50k
50k
MCF548x
MCF548x
50k
50k
Freescale Semiconductor