vccaux and vccint

I'm looking at the Spartan-3e datasheet and I'm surprised in the number of 
power pins in the PQ208 package.
There are
* 4 VCCINT pins
* 8 VCCAUX pins
* 12 VCCO pins

The number of VCCO makes sense (3 per banks), VCCINT could be (maybe the 
device consumes little), but why 8 VCCAUX?
VCCAUX is just used by just 6 pins (4 JTAG pins + DONE and PROG_B). 8 power 
pins seems overkill for just powering 6 pins.

Am I missing something? 

Jean Nicolle wrote:
> I'm looking at the Spartan-3e datasheet and I'm surprised in the number of
> power pins in the PQ208 package.
> There are
> * 4 VCCINT pins
> * 8 VCCAUX pins
> * 12 VCCO pins
>
> The number of VCCO makes sense (3 per banks), VCCINT could be (maybe the
> device consumes little), but why 8 VCCAUX?
> VCCAUX is just used by just 6 pins (4 JTAG pins + DONE and PROG_B). 8 power
> pins seems overkill for just powering 6 pins.
>
> Am I missing something?

VVCAUX also powers DCMs and more see the datasheet

-Lasse

Jean Nicolle wrote:
> I'm looking at the Spartan-3e datasheet and I'm surprised in the number of
> power pins in the PQ208 package.
> There are
> * 4 VCCINT pins
> * 8 VCCAUX pins
> * 12 VCCO pins
>
> The number of VCCO makes sense (3 per banks), VCCINT could be (maybe the
> device consumes little), but why 8 VCCAUX?
> VCCAUX is just used by just 6 pins (4 JTAG pins + DONE and PROG_B). 8 power
> pins seems overkill for just powering 6 pins.
>
> Am I missing something?

I know how you feel.  I have often wondered why there are so many power
and ground pins on these devices.  But if you dig into the signal
integrity issues you will find that it is amazing that even with this
many power and ground pins that the parts can work at all!  The
packages are not really up to the task of the really high speed I/Os if
you are using a lot of them.  If you are pushing the quantity and speed
of the I/Os at all, you should never try to use a QFP package because
of the large inductance.

Although the VCCAUX only powers a few I/O pins, it powers a fair amount
of internal logic.  Things like the DCMs are especially sensitive to
power supply noise so I would not begrudge them a single power pin.  I
am a bit suprised that they even share the I/O supply with the internal
logic since the I/O pins can make some hefty current spikes.  It may be
that some of the VCCAUX pins are just for I/Os and some are for the
internal logic.

rickman wrote:

> Jean Nicolle wrote:
> > I'm looking at the Spartan-3e datasheet and I'm surprised in the number of
> > power pins in the PQ208 package.
> > There are
> > * 4 VCCINT pins
> > * 8 VCCAUX pins
> > * 12 VCCO pins
> >
> > The number of VCCO makes sense (3 per banks), VCCINT could be (maybe the
> > device consumes little), but why 8 VCCAUX?
> > VCCAUX is just used by just 6 pins (4 JTAG pins + DONE and PROG_B). 8 power
> > pins seems overkill for just powering 6 pins.
> >
> > Am I missing something?
>
> I know how you feel.  I have often wondered why there are so many power
> and ground pins on these devices.  But if you dig into the signal
> integrity issues you will find that it is amazing that even with this
> many power and ground pins that the parts can work at all!  The
> packages are not really up to the task of the really high speed I/Os if
> you are using a lot of them.  If you are pushing the quantity and speed
> of the I/Os at all, you should never try to use a QFP package because
> of the large inductance.
>
> Although the VCCAUX only powers a few I/O pins, it powers a fair amount
> of internal logic.  Things like the DCMs are especially sensitive to
> power supply noise so I would not begrudge them a single power pin.  I
> am a bit suprised that they even share the I/O supply with the internal
> logic since the I/O pins can make some hefty current spikes.  It may be
> that some of the VCCAUX pins are just for I/Os and some are for the
> internal logic.

On the S3, at least, VccAux powers the JTAG chain, part of the init
setup and the DCMs. I don't believe it powers anything else. One might
surmise there is at least one power pin (VccAux) that comes in at the
bond wire close to each DCM; the DCMs are physically located in the 4
corners and routing power internaly would add noise and other
undesirable effects. That would specify a minimum of 5 VccAux pins (4
DCMs, 1 JTAG in anything larger than the XC3S50 which has two DCMs,
IIRC).

I don't think I've used a non-BGA FPGA in the last 5 years, because of
the lead inductance issues amongst other things (physical space v. pins
is also an issue for me).

Cheers

PeteS

Makes sense. Thanks guys.

"PeteS" <PeterSmith1954@googlemail.com> wrote in message 
news:1164583963.122840.298060@j72g2000cwa.googlegroups.com...
> rickman wrote:
>
>> Jean Nicolle wrote:
>> > I'm looking at the Spartan-3e datasheet and I'm surprised in the number 
>> > of
>> > power pins in the PQ208 package.
>> > There are
>> > * 4 VCCINT pins
>> > * 8 VCCAUX pins
>> > * 12 VCCO pins
>> >
>> > The number of VCCO makes sense (3 per banks), VCCINT could be (maybe 
>> > the
>> > device consumes little), but why 8 VCCAUX?
>> > VCCAUX is just used by just 6 pins (4 JTAG pins + DONE and PROG_B). 8 
>> > power
>> > pins seems overkill for just powering 6 pins.
>> >
>> > Am I missing something?
>>
>> I know how you feel.  I have often wondered why there are so many power
>> and ground pins on these devices.  But if you dig into the signal
>> integrity issues you will find that it is amazing that even with this
>> many power and ground pins that the parts can work at all!  The
>> packages are not really up to the task of the really high speed I/Os if
>> you are using a lot of them.  If you are pushing the quantity and speed
>> of the I/Os at all, you should never try to use a QFP package because
>> of the large inductance.
>>
>> Although the VCCAUX only powers a few I/O pins, it powers a fair amount
>> of internal logic.  Things like the DCMs are especially sensitive to
>> power supply noise so I would not begrudge them a single power pin.  I
>> am a bit suprised that they even share the I/O supply with the internal
>> logic since the I/O pins can make some hefty current spikes.  It may be
>> that some of the VCCAUX pins are just for I/Os and some are for the
>> internal logic.
>
> On the S3, at least, VccAux powers the JTAG chain, part of the init
> setup and the DCMs. I don't believe it powers anything else. One might
> surmise there is at least one power pin (VccAux) that comes in at the
> bond wire close to each DCM; the DCMs are physically located in the 4
> corners and routing power internaly would add noise and other
> undesirable effects. That would specify a minimum of 5 VccAux pins (4
> DCMs, 1 JTAG in anything larger than the XC3S50 which has two DCMs,
> IIRC).
>
> I don't think I've used a non-BGA FPGA in the last 5 years, because of
> the lead inductance issues amongst other things (physical space v. pins
> is also an issue for me).
>
> Cheers
>
> PeteS
> 

PeteS wrote:
> On the S3, at least, VccAux powers the JTAG chain, part of the init
> setup and the DCMs. I don't believe it powers anything else.
It usually supplies input and output buffers for LVDS as well (at least
it does in Virtex2 Pro and Virtex 4), even if the bank those IOs are in
is supplied with i.e. 3.3V.

cu,
Sean

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"Sean Durkin" <news_nov06@durkin.de> wrote in message 
news:456a8e5d$1@news.fhg.de...
> PeteS wrote:
>> On the S3, at least, VccAux powers the JTAG chain, part of the init
>> setup and the DCMs. I don't believe it powers anything else.
> It usually supplies input and output buffers for LVDS as well (at least
> it does in Virtex2 Pro and Virtex 4), even if the bank those IOs are in
> is supplied with i.e. 3.3V.
>
> cu,
> Sean
>
...except for the differential termination bit. That seems to be powered 
from VCCO. Pure genius.
Cheers, Syms. 

Symon wrote:
> "Sean Durkin" <news_nov06@durkin.de> wrote in message
> news:456a8e5d$1@news.fhg.de...
> > PeteS wrote:
> >> On the S3, at least, VccAux powers the JTAG chain, part of the init
> >> setup and the DCMs. I don't believe it powers anything else.
> > It usually supplies input and output buffers for LVDS as well (at least
> > it does in Virtex2 Pro and Virtex 4), even if the bank those IOs are in
> > is supplied with i.e. 3.3V.
> >
> > cu,
> > Sean
> >
> ...except for the differential termination bit. That seems to be powered
> from VCCO. Pure genius.

I can't say I understand this.  Can you explain?

"rickman" <gnuarm@gmail.com> wrote in message 
news:1164628372.027773.61250@h54g2000cwb.googlegroups.com...
> Symon wrote:
>> "Sean Durkin" <news_nov06@durkin.de> wrote in message
>> news:456a8e5d$1@news.fhg.de...
>> > PeteS wrote:
>> >> On the S3, at least, VccAux powers the JTAG chain, part of the init
>> >> setup and the DCMs. I don't believe it powers anything else.
>> > It usually supplies input and output buffers for LVDS as well (at least
>> > it does in Virtex2 Pro and Virtex 4), even if the bank those IOs are in
>> > is supplied with i.e. 3.3V.
>> >
>> > cu,
>> > Sean
>> >
>> ...except for the differential termination bit. That seems to be powered
>> from VCCO. Pure genius.
>
> I can't say I understand this.  Can you explain?
>
Hi Rick,
You can use LVDS receivers in a Vcco = 3.3V bank. As Sean says, it appears 
they're powered from Vccaux. However, you can only use LVDS_DT receivers if 
Vcco = 2.5V. Check out the note on Figure 31, DS083. I've posted here a 
couple of times about this, I don't think I ever found out why it is so.
Cheers, Syms. 

Symon wrote:
> Hi Rick,
> You can use LVDS receivers in a Vcco = 3.3V bank. As Sean says, it appears 
> they're powered from Vccaux. However, you can only use LVDS_DT receivers if 
> Vcco = 2.5V.
Well, you CAN use them (i.e. the tools don't stop with an error or give
you w warning or anything), but the termination value won't be 100 Ohms.
Xilinx doesn't specify what the actual value might be, since they don't
recommend using the terminations that way.
I've used it successfully on 2 boards with VCCO=3.3V. "Successfully"
meaning that it works and the signal at the balls doesn't look that bad.

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