Hi All, I have a schematic which was drawn by someone else (sorry, can't post it for now). In it there is a Micro USB connector with two resistors with the value list as '27' on the D+ & D-. Are these 27 Ohm resistors?
Thanks
Hi All, I have a schematic which was drawn by someone else (sorry, can't post it for now). In it there is a Micro USB connector with two resistors with the value list as '27' on the D+ & D-. Are these 27 Ohm resistors?
Thanks
Sounds plausible, but they should be more like 47 ohm if they are the terminators.
gilcd85:
Hi All, I have a schematic which was drawn by someone else (sorry, can't post it for now). In it there is a Micro USB connector with two resistors with the value list as '27' on the D+ & D-. Are these 27 Ohm resistors?Thanks
I suspect they are not 27 ohm resistors. Are you sure it's even the symbol for resistors and not transient suppressors or some other kind of component. D+ and D- are logic signals and would not be designed to draw the kind of current a 27 ohm resistor would draw. See what you can do about posting the drawing even if you just have to take a picture of it.
I guess it would be useful to know how they are wired, from ground or Vcc to the signal or just in series with the signal pins? If just in series then they could be resistors I guess.
Lefty
There are sometimes low voltage zeners used there, Most low voltage zeners aren't very accurate and tend to be high in zener (avalanche) voltage. The USB signal as I remember is about 800 mV differential signalling so 27 could well mean 2.7V zener diodes too...
It depends on what the schematic is for.
Some circuits use resistors on the data lines to indicate the charging current capability of the device.
Many USB interface designs use 22 ohm resistors between the connector and the USB chip.
FT232R has internal series resistor.
CY7C64225 does not, spec sheet calls for 24 ohm series resistor.
MAX3241E, USB Host, calls for 33 ohm series resistor.
So 27 is not out of the realm of possibility.
Thank you all for the answers!
Yes I should have provided additional information on the schematic..
Both D+ and D- are wired from a Micro USB connector to an FTDI chip (FT230XQ) through a 27 (Ohm?) resistor. They go into USBDM (D+) and USBDP (D-) of the FTDI.
So.. could it be 27 Ohm?
gilcd85:
Thank you all for the answers!
Yes I should have provided additional information on the schematic..
Both D+ and D- are wired from a Micro USB connector to an FTDI chip (FT230XQ) through a 27 (Ohm?) resistor. They go into USBDM (D+) and USBDP (D-) of the FTDI.So.. could it be 27 Ohm?
Yes
Straight and simple answer.. I like it lol
gilcd85:
Thank you all for the answers!
Yes I should have provided additional information on the schematic..
Both D+ and D- are wired from a Micro USB connector to an FTDI chip (FT230XQ) through a 27 (Ohm?) resistor. They go into USBDM (D+) and USBDP (D-) of the FTDI.So.. could it be 27 Ohm?
The spec sheet for the FT230X (http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT230X.pdf) calls for 27 Ohm resistors on D- and D+ (as well as 47 pF caps to ground).
So as retrolefty says...yes....
Good luck!
I came here with a similar question. After looking at the schematic again, I noticed that there were also 47 pF capacitors on each data line, so the result was that the signals each had a low-pass filter with a cutoff frequency around 128 MHz, so those resistors were chosen for noise rejection. (Just FYI to those in the future)
They will also be part of the termination network...