I'm laying out the USB interface portion of my board right now, and
I've got
a question on the pull-up control. The SAM7 datasheet shows a simple BJT
controlling the pull-up, but the AT91SAM7S-EK board appears to use an
n-channel FET driving a p-channel FET. Why the added complexity? I've used
that type of circuit before, but only for switching a lot of current. This
only needs to handle like 2 ma. Is there any advantage to using this scheme
versus a BJT or logic level FET?
Thanks,
Scott
UDP pull-up control
Started by ●May 9, 2005
Reply by ●May 9, 20052005-05-09
Hi Scott,
> I'm laying out the USB interface portion of my board right now,
and I've got
> a question on the pull-up control. The SAM7 datasheet shows a simple BJT
> controlling the pull-up, but the AT91SAM7S-EK board appears to use an
> n-channel FET driving a p-channel FET. Why the added complexity? I've used
> that type of circuit before, but only for switching a lot of current. This
> only needs to handle like 2 ma. Is there any advantage to using this scheme
> versus a BJT or logic level FET?
> a question on the pull-up control. The SAM7 datasheet shows a simple BJT
> controlling the pull-up, but the AT91SAM7S-EK board appears to use an
> n-channel FET driving a p-channel FET. Why the added complexity? I've used
> that type of circuit before, but only for switching a lot of current. This
> only needs to handle like 2 ma. Is there any advantage to using this scheme
> versus a BJT or logic level FET?
I don't know enough about USB, so leave that for the buffs.
But AFAIK the enumeration process includes an initial limit of current that
the hub is able
to give, and then it tells how much current the USB slave device can
use.
If you don't power anything major through the USB, I don't think
you need major external
current switching at enumeration.
USB buffs, if I'm wrong, please educate us :-)
B rgds
Kris
Reply by ●May 9, 20052005-05-09
I forget what the limits are (the shutdown limits are really a bigger
problem), but this doesn't affect that. At least, not as far as I can see.
The +5V bus voltage goes straight into the regulator, though a diode. The
dual FET circuit only controls insertion of the 1.5k pullup that's used to
signal device connection. See page 19 of the EK manual for the relevant
schematic.
Scott
----- Original Message -----
From: "microbit" <microbit@micr...>
To: <AT91SAM7@AT91...>
Sent: Monday, May 09, 2005 10:50 AM
Subject: Re: [AT91SAM7] UDP pull-up control I don't know enough about USB, so leave that for the buffs.
But AFAIK the enumeration process includes an initial limit of current that
the hub is able
to give, and then it tells how much current the USB slave device can use.
If you don't power anything major through the USB, I don't think you need
major external
current switching at enumeration.
USB buffs, if I'm wrong, please educate us :-)
B rgds
Kris
problem), but this doesn't affect that. At least, not as far as I can see.
The +5V bus voltage goes straight into the regulator, though a diode. The
dual FET circuit only controls insertion of the 1.5k pullup that's used to
signal device connection. See page 19 of the EK manual for the relevant
schematic.
Scott
----- Original Message -----
From: "microbit" <microbit@micr...>
To: <AT91SAM7@AT91...>
Sent: Monday, May 09, 2005 10:50 AM
Subject: Re: [AT91SAM7] UDP pull-up control I don't know enough about USB, so leave that for the buffs.
But AFAIK the enumeration process includes an initial limit of current that
the hub is able
to give, and then it tells how much current the USB slave device can use.
If you don't power anything major through the USB, I don't think you need
major external
current switching at enumeration.
USB buffs, if I'm wrong, please educate us :-)
B rgds
Kris
Reply by ●May 9, 20052005-05-09
--- In AT91SAM7@AT91..., "microbit" <microbit@c...>
wrote:
> Hi Scott, > > I'm laying out the USB interface portion of my board right now,
and I've got
> > a question on the pull-up control. The SAM7 datasheet shows a
simple BJT
> > controlling the pull-up, but the AT91SAM7S-EK board appears to use
an
> > n-channel FET driving a p-channel FET. Why the added complexity?
I've used
> > that type of circuit before, but only for switching a lot of
current. This
> > only needs to handle like 2 ma. Is there any advantage to using
this scheme
> > versus a BJT or logic level FET?
>
> I don't know enough about USB, so leave that for the buffs.
> But AFAIK the enumeration process includes an initial limit of
current that the hub is able
> to give, and then it tells how much current the USB slave device can
use.
> If you don't power anything major through the USB, I don't think you
need major external
> current switching at enumeration.
>
> USB buffs, if I'm wrong, please educate us :-)
The main problem I see with the npn circuit in the datasheetis that it
does not pull up as hard as a pulldown driving pullup circuit. The npn
will only be able to pull up to 3V3-0.6 = 2.7V. The down-up circuit
(which you could do with an npn driving a pnp) will pull to the rail.
This won't consume appreiably more current (perhaps an extra 100uA),
but does drive the pull-up better. This will make the USB more robust
on longer wires and worse noise conditions etc.
> Hi Scott, > > I'm laying out the USB interface portion of my board right now,
and I've got
> > a question on the pull-up control. The SAM7 datasheet shows a
simple BJT
> > controlling the pull-up, but the AT91SAM7S-EK board appears to use
an
> > n-channel FET driving a p-channel FET. Why the added complexity?
I've used
> > that type of circuit before, but only for switching a lot of
current. This
> > only needs to handle like 2 ma. Is there any advantage to using
this scheme
> > versus a BJT or logic level FET?
>
> I don't know enough about USB, so leave that for the buffs.
> But AFAIK the enumeration process includes an initial limit of
current that the hub is able
> to give, and then it tells how much current the USB slave device can
use.
> If you don't power anything major through the USB, I don't think you
need major external
> current switching at enumeration.
>
> USB buffs, if I'm wrong, please educate us :-)
The main problem I see with the npn circuit in the datasheetis that it
does not pull up as hard as a pulldown driving pullup circuit. The npn
will only be able to pull up to 3V3-0.6 = 2.7V. The down-up circuit
(which you could do with an npn driving a pnp) will pull to the rail.
This won't consume appreiably more current (perhaps an extra 100uA),
but does drive the pull-up better. This will make the USB more robust
on longer wires and worse noise conditions etc.
Reply by ●May 9, 20052005-05-09
> The main problem I see with the npn
circuit in the datasheetis that it
> does not pull up as hard as a pulldown driving pullup circuit. The npn
> will only be able to pull up to 3V3-0.6 = 2.7V. The down-up circuit
> (which you could do with an npn driving a pnp) will pull to the rail.
> This won't consume appreiably more current (perhaps an extra 100uA),
> but does drive the pull-up better. This will make the USB more robust
> on longer wires and worse noise conditions etc.
> does not pull up as hard as a pulldown driving pullup circuit. The npn
> will only be able to pull up to 3V3-0.6 = 2.7V. The down-up circuit
> (which you could do with an npn driving a pnp) will pull to the rail.
> This won't consume appreiably more current (perhaps an extra 100uA),
> but does drive the pull-up better. This will make the USB more robust
> on longer wires and worse noise conditions etc.
What about pulling the signal up to VBUS instead of the 3.3v rail. I
assume this will give an even better pull up and make the system more robust or
is there something that I am missing here. Correct me if I am wrong, but
isn't the IO-pins 5v tolerant.
Reply by ●May 10, 20052005-05-10
The SAM7 has 5-volt tolerant I/Os, but it's the host you're
signalling with
the pullup and I don't think that the spec lets you pull the data bus any
higher than 3.3v.
Scott
----- Original Message -----
From: "Lars Andersson" <laran@lara...>
To: <AT91SAM7@AT91...>
Sent: Monday, May 09, 2005 2:13 PM
Subject: RE: [AT91SAM7] Re: UDP pull-up control
What about pulling the signal up to VBUS instead of the 3.3v rail. I
assume this will give an even better pull up and make the system more
robust or is there something that I am missing here. Correct me if I am
wrong, but isn't the IO-pins 5v tolerant.
the pullup and I don't think that the spec lets you pull the data bus any
higher than 3.3v.
Scott
----- Original Message -----
From: "Lars Andersson" <laran@lara...>
To: <AT91SAM7@AT91...>
Sent: Monday, May 09, 2005 2:13 PM
Subject: RE: [AT91SAM7] Re: UDP pull-up control
What about pulling the signal up to VBUS instead of the 3.3v rail. I
assume this will give an even better pull up and make the system more
robust or is there something that I am missing here. Correct me if I am
wrong, but isn't the IO-pins 5v tolerant.