USB Data Line Pullup

I'm discussing the data line pullup on a USB dev with someone and I'm 
not clear on one thing... or maybe two.  The pullup is added to one or 
the other of the two differential data lines to indicate the speed of 
the interface being either low or full.  This pullup is supposed to be 
1.5 kohms to 3.3 volts.  Once the pullup is detected is there any need 
to remove it?  It does create a slight imbalance although that would be 
very small.  I know some USB chips control this through a pin, but all 
that I have seen are either high speed or OTG controllers.

How exactly is the presence of the pullup detected?  I can see telling 
if one or the other is present by the polarity of the bias on the D 
lines.  But if no pullup is present the host has to know that.  I 
suppose the differential pair is also read separately as a pair of 
single ended inputs to detect the three combinations?

-- 

Rick C

On Tue, 25 Apr 2017 13:05:00 -0400, rickman wrote:

> I'm discussing the data line pullup on a USB dev with someone and I'm
> not clear on one thing... or maybe two.  The pullup is added to one or
> the other of the two differential data lines to indicate the speed of
> the interface being either low or full.  This pullup is supposed to be
> 1.5 kohms to 3.3 volts.  Once the pullup is detected is there any need
> to remove it?  It does create a slight imbalance although that would be
> very small.  I know some USB chips control this through a pin, but all
> that I have seen are either high speed or OTG controllers.
> 
> How exactly is the presence of the pullup detected?  I can see telling
> if one or the other is present by the polarity of the bias on the D
> lines.  But if no pullup is present the host has to know that.  I
> suppose the differential pair is also read separately as a pair of
> single ended inputs to detect the three combinations?


A lot (all?) of the 1.5Mb/s (low speed) and 12Mb/s (full speed) USB 
transceivers ICs have three receiver outputs: RCV, Vp and Vm, for that 
very purpose.

See for example:
https://www.fairchildsemi.com/datasheets/US/USB1T11A.pdf


Regards,
Allan