1. Home
  2. Field-Programmable Gate Arrays
  3. Spartan3 IOB without supply

Spartan3 IOB without supply

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?

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

  2. 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

Reply to
Kolja Sulimma
Loading thread data ...

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.

Aust> Austin,

Reply to
Austin Lesea

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

Reply to
Kolja Sulimma

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.