Sunday, 15 May 2016

USB Design


Some specifications of USB can be complicated as USB has a high data rate.



USB 2.0 low speed (small distances)


Data rate is 1.5 Mbit/s. Doesn't need impedance matching and same trace lengths for small distances. Requires a pull up resistor for USB_DM. Some MCUs have pull up resistors from USB_DM (D-) to VDD (voltage of MCU).

USB 2.0 full speed (small distances)

Data rate is 12 Mbit/s. Doesn't need impedance matching and same trace lengths for small distances. Requires a pull up resistor for USB_DP. Some MCUs have pull up resistors from USB_DP (D+) to VDD (voltage of MCU).


USB 2.0 full speed/low speed (long distances)

Here you need to consider trace length and impedance matching. Design steps:
  1. Add ground plane bellow traces (if ground plane not available add a VDD plane)
  2. Find the minimum lengths of D+ and D- traces to MCU
  3. Take the longest of the lengths and make length of the traces match
  4. Use micro-strip calculator to calculate impedance
  5. Adjust width of trace for the impedance between D+ and D- is 90 ohms
  6. Adjust size of ground plane for the impedance of D+ to ground is 30 ohms and D- to ground is 30 ohms









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