USB1 Series Resistor yes/no?

[thorstenwilmer]

Hi,

in the LPC546xx-540xx_Dev_brd_schematic_Rev-F_180708 Schematic of the Eval board, there is no Series resistor in USB1' DP/DM (See Areas D7, D6, D5). It was added for USB0 (R79 and R78).

The datasheet LPC546XX.pdf Rev 2.6 page 152 does not differentiate between USB0 and USB1.

Table 21 and 49 talk only About USB0, are similar table for USB1 is missing?

Kind Regards

 Thorsten

[Sabina_Bruce]

Hi Thorsten,

The USB1 in the LPC546xx is compliant with the high speed usb specifications and the USB0 is for full speed. 

In the schematic, you can see that the USB0 has two resistors in series. The USB specification requires that the USB device have a 45 Ω single-ended or 90 Ω differential impedance. This impedance includes the impedance of the USB transceiver. The impedance of the USB transceiver is lower than the requirement of the USB specification, so the 33 Ω resistors are required to bring the device impedance within the required specification limits.

On the other hand, a high-speed USB connection is made through a shielded, twisted pair cable with a differential characteristic impedance of 90 Ω. In addition the USB1 is connected to the  internal USB 2.0 PHY, this will  do the impedance matching if necessary, therefore no external resistors are necessary. 

The table for USB1 is not missing, as I mentioned it is compliant with the high speed specifications. You can find more information regarding the electrical specifications for the USB high speed by downloading the USB 2.0 Specification.

Hope this helps and have a great day!

Sabina