Lpc2000

> > 3) Is there any suggested pcb layout for the USB lines of the 
> > LPC2148 from philips ? Ok, i think i must use the shortest lines 
for 
> > D+ and D-, but is there a need for ground planes, filters,... ? 
Just 
> > for a quick prototype, to get it up and running, is this also 
> > needed ? I think bits are going over D+ and D- with 480 MBit/s, 
even 
> > when only 12 MBit/s are used for LPC2148. Is this correct ?
> 
> Keep D+ and D- together and equal in length. 

Per the USB Spec, these need to be matched differential pairs at 90 
ohms of impedance.  This can be very difficult to achieve on a two 
layer 0.062 PCB board.


> > 4) What is "Soft Connect USB Feature" in detail ?
> 
> That means that you can control USB connection with software. I 
don't
> think this is the case here. External resistor and switch are
> connected (to pull up D+ i.e.) to output pin of LPC214x while in
> ISP1181 you don't need all that (I heard somewhere that ISP1181 is
> inside of LPC214x). 

A Full Speed USB function device anounces it self to the USB 
controller, by pulling D+ hi with about a 1.5K ohm resister.  The 
Softconnet feature allow you to control when this happens...

> 
> > 5) As earlier mentioned in a posting in the mailinglist, is it 
> > correct, that the two 33 Ohm resistors sit in line of D+ and 
Vbus 
> > and not in lines D+ and D- ? What is Vbus in generally used, 
just to 
> > detect the presence of power from host ? So in general every 
> > interrupt line can be used ? Or is there something special, e.g. 
to 
> > detect if voltage is enough ?
> 
> Put two resistors in line of D+ and D-. As I can see in datasheet 
the
> correct value is 18 Ohm. Total driver resistance is between 29 and 
44
> Ohm with note that this includes 18 Ohm external resistors on D+ 
and D-.
> Vbus is used for detection and power supply (read the specification
> for limitations). 
> 

I guess I missed the section in the datasheet that showed 18ohms.  
The included ref circuit shows 33ohms on D+/D-.  On VBus it depends 
if you are using a self powered device vs. a Bus powered device.  On 
self powered, I usually add a 10k, and just use this in my own 
SoftConnect functionality.  I don't bring it to Vcc (5v) as I don't 
what my powersupply, and the pc's duking it out... so to speak.  On 
bus powered devices, this is where you can draw your power.  Again 
by spec, you cannot draw more than 100ma, until you have been given 
permission by the host controller.  The most that you can draw is 
500ma, if the host controller grants this...

If you are planning on doing USB, please get the spec and join 
USB.org.  There are a lot of resources over there.  (Plus you will 
need your own manufacturer's id.)

Tim Larson
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