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Topic: [Resolved] Resistors on USB - necessary? (Read 2378 times) previous topic - next topic

liuzengqiang

Feb 16, 2015, 08:32 am Last Edit: Feb 19, 2015, 06:35 pm by liudr
I followed Arduino Micro's design and have 22 ohm resistors on USB D+ and D- lines and a 0 ohm shunt on VUSB. On my prototype I used 150 ohm without problem. I wonder if these resistor are even necessary since the USB ports should have some isolation built in, correct?
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

ron_sutherland

I think the USB line is a transmission line, so it can have noise signals that keep power by having the voltage and current out of phase (reactive power) and bouncing from end to end, but when the noise hits the resistor it can only pass through with voltage and current in phase and suffers real power loss thus dampening out the noise.
I use an R-Pi Zero on an RPUpi shield to have a tool-chain at the network edge.

liuzengqiang

#2
Feb 16, 2015, 11:39 pm Last Edit: Feb 16, 2015, 11:45 pm by liudr
Thanks. I have since read the spec sheet of the FT232R chip that I'm using for the USB. It says termination resistors are integrated. So I don't need the 22 ohm resistors.

http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

Here is a schematic:

I will read about the ferrite bead and soft start in a bit. It seems the ATMEGA8U2 has no integrated termination resistors. I did a search on its spec sheet. Nothing turned up. The first page also didn't show any integrated termination resistors on the features list.

http://www.atmel.com/images/doc7799.pdf
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

ron_sutherland

#3
Feb 17, 2015, 01:56 am Last Edit: Feb 17, 2015, 02:03 am by ron_sutherland
With the FT232RL, I did not use resistors either, and it is working fine. This is how I used the FTDI chip:


In this case it connects to RS485 drivers that can consume a lot of power, and there was something I remember seeing about USB which required it to use low power until initialized, so the P-ch MOSFET (Q1) is off until the USB is configured as a virtual serial port device. There is also a ferrite bead (L1) to dampen noise, but my understanding is that it does not act like a reactive element I think it dissipates noise as heat (like the transmission line resistor), but has little loss at DC (inductor hiding resistor in my schematic).
I use an R-Pi Zero on an RPUpi shield to have a tool-chain at the network edge.

CrossRoads

I use this FTDI module from mouser.com on my boards as I find the FTDI chip difficult to hand assemble with its tight pin spacing.
http://www.mouser.com/ds/2/272/usb_uart_manual_v100-32094.pdf
It has no D+/D- resistors and include a ferrite to the chip's Vcc. I add a PTC resettable fuse to power the board it connects to.
And resistor/LED to P0/P1 for Rx/Tx monitoring.

At the same time, I've also used plenty of FTDI Basic's, which have no ferrite on board, and have not included one on my boards, and have not seen any issues.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

liuzengqiang

Thanks CR. Here is a link for those interested in looking at the product page:

http://www.mouser.com/ProductDetail/mikroElektronika/MIKROE-483/?qs=sGAEpiMZZMuNcqZxhMNT3znYT0k8AsjH

My board is too crowed to use that module. I have the device inside of an enclosure. The top edge is prime estate for connectors and switches since there is an opening (with a panel you can machine and insert). I've completely run out of space on the top edge: 3 DIN-5, 1 power switch, then USB-mini on top layer and micro-SD card slot on bottom layer.

Yes the pitch is pretty fine, maybe the finest I'm willing to do by hand. I put enough solder to all pins making plenty of bridges and then use solder wick to remove excess solder. I have no problem soldering it once it is secured to a board. It takes maybe 10 minutes to do the job. The key is keeping the chip in place by tacking it. A breakout board from banggood that has long pads helped me. My own board's footprint doesn't have long pin pads like the breakout board does and I had trouble placing the chip symmetrically enough.
Serial LCD keypad panel,phi_prompt user interface library,SDI-12 USB Adapter

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