Tuesday, 27 October 2015

USB Softcore for FPGA : Extra resistor on FPGA transmitter pin


Have anyone used this USB softcore before ?


Why do we need extra resistance on the tx pin ONLY ?



Whichever pins you transmit on need to have resistors after them. The exact values will depend on the internal resistance of the pins; usually something around 27 ohms will be ok.



According to section 7.1.1 (Tx) and 7.1.2 (Rx) of the USB 2.0 specification , it seems like we need some resistance for impedance matching for BOTH Tx and Rx.



enter image description here


1) is it correct that I only need TWO 27 ohm resistors in series (to compensate the low output impedance, say 73 ohm of FPGA pins) with the FPGA usbcorev softcore D+ and D- pins ?


2) And another 1.5 kilo-ohm pull-up resistor for D+ line ?


3) Since D+ and D- are bi-directional, do we need 15 kilo-ohm pull-down resistors for both D+ and D- line at FPGA pins ?


4) From on-chip termination section within intel cyclone IV IO inteface spec (have anyone used on chip termination ?) , I am not sure if I need to use external series termination resistors for both D+ and D- line. When I loop deeper into cyclone IV IO spec, it only supports 25 ohm or 50 ohm on-chip termination. This means this on-chip termination is not suffice for 90 ohm transmission line in USB spec . Could anyone advise ?




No comments:

Post a Comment

arduino - Can I use TI's cc2541 BLE as micro controller to perform operations/ processing instead of ATmega328P AU to save cost?

I am using arduino pro mini (which contains Atmega328p AU ) along with cc2541(HM-10) to process and transfer data over BLE to smartphone. I...