Spartan3 IOB without supply

[Kolja Sulimma]

Austin,

you are probably the right person to answer these questions about
Spartan-3:

1.
If I use no IOBs of a given Bank at all, can I leave VCCIO of that
bank unconnected?

2.
Can I still use VCCAUX based IO standards like LVDS?

3.
Can I use open collector like outputs without VCCIO connected?
(Eg conditionally pull an output to ground)

This request might sound strange, but we have a board where VCCIO of
two banks is supplied by another board. No a new application was
brought up were the second board is not needed except for the
generation of VCCIO. SO I would like to get rid of it.

Thank you in advance,

Kolja Sulimma

 

[Austin Lesea]

Kolja,

Not recommended. If nothing is used, the current consumed is ~ 2mA per
bank, so why do you want to leave them disconnected? You do not have to
provide bypass caps if they are not used, so there is little penalty.
Conencting it up provides the return paths for the ESD protection, etc.

Per question 2: I do not understand. Vccaux is 2.5V, so only 2.5V LVDS
IOB primitive is supported. Since LVDS is a standard, 2.5V or 3.3V
makes no difference at all: a 2.5V powered LVDS buffer receives or
drives a 3.3V LVDS buffer.

And finally, no, you must connect Vcco is you are to use the IOB at all,
for any reason.

Austin

Kolja Sulimma wrote:

Austin,

you are probably the right person to answer these questions about
Spartan-3:

1.
If I use no IOBs of a given Bank at all, can I leave VCCIO of that
bank unconnected?

2.
Can I still use VCCAUX based IO standards like LVDS?

3.
Can I use open collector like outputs without VCCIO connected?
(Eg conditionally pull an output to ground)

This request might sound strange, but we have a board where VCCIO of
two banks is supplied by another board. No a new application was
brought up were the second board is not needed except for the
generation of VCCIO. SO I would like to get rid of it.

Thank you in advance,

Kolja Sulimma

 

[Kolja Sulimma]

In this particular board VCCIO is connected to 2.5V or 3.3V depending
on what mezzanine board is connected.
Now I want to use the existing board without a mezzanine board.
Of course I can solder shorts on the connector. But thats ugly.
Decoupling caps are there, so at least a high frequency return path is
available.

For a later revision of the board:
2mA could be provided by a 1k resistor to 3.3V. And I still could
connect 2.5V (at a standby power penalty).
Or would the impedance of the return path be to large in that case?

Question 2:
I thought I read somewhere that the for LVDS standard the IO drivers
are powered by VCCAUX. But looking at the datasheet again this does
not seem to be the case.

Thank you for your help,

Kolja Sulimma

Austin Lesea <austin@xilinx.com> wrote in message news:<btmis4$1vo1@cliff.xsj.xilinx.com>...

Kolja,

Not recommended. If nothing is used, the current consumed is ~ 2mA per
bank, so why do you want to leave them disconnected? You do not have to
provide bypass caps if they are not used, so there is little penalty.
Conencting it up provides the return paths for the ESD protection, etc.

Per question 2: I do not understand. Vccaux is 2.5V, so only 2.5V LVDS
IOB primitive is supported. Since LVDS is a standard, 2.5V or 3.3V
makes no difference at all: a 2.5V powered LVDS buffer receives or
drives a 3.3V LVDS buffer.

And finally, no, you must connect Vcco is you are to use the IOB at all,
for any reason.

Austin

Kolja Sulimma wrote:

Austin,

you are probably the right person to answer these questions about
Spartan-3:

1.
If I use no IOBs of a given Bank at all, can I leave VCCIO of that
bank unconnected?

2.
Can I still use VCCAUX based IO standards like LVDS?

3.
Can I use open collector like outputs without VCCIO connected?
(Eg conditionally pull an output to ground)

This request might sound strange, but we have a board where VCCIO of
two banks is supplied by another board. No a new application was
brought up were the second board is not needed except for the
generation of VCCIO. SO I would like to get rid of it.

Thank you in advance,

Kolja Sulimma