Help: How to deal with the IOs that are not used?

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honghongrong

Joined: 02 Dec 2004
Posts: 8

04 Dec 2004 4:01 Help: How to deal with the IOs that are not used?

I am using Virtex II Pro to design a system.
But I only use a few IOs and there are still
many other IOs that are not used. How can I deal
with these unused IOs? Shall I connenet them
to the Ground ?

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nicleo

Joined: 06 Sep 2004
Posts: 717
Helped: 60

04 Dec 2004 4:14 Help: How to deal with the IOs that are not used?

Yup, one way is to connect them to ground.

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

04 Dec 2004 4:19 Re: Help: How to deal with the IOs that are not used?

Feel free to leave them not connected, connected to 0V or to +V..really doesn't matter...

honghongrong

Joined: 02 Dec 2004
Posts: 8

04 Dec 2004 12:41 Help: How to deal with the IOs that are not used?

Thanks for your reply

SolarTorch

Joined: 07 Oct 2004
Posts: 66
Helped: 2

04 Dec 2004 12:46 Re: Help: How to deal with the IOs that are not used?

I think, if the all are i/p, directly connect to GND/VDD. But if not sure, add a resistor to pull each i/o down.

btbass

Joined: 20 Jul 2001
Posts: 1187
Helped: 113
Location: Oberon

04 Dec 2004 13:11 Help: How to deal with the IOs that are not used?

All unused inputs should be tied high or low, a resistor is not needed, all unused outputs can be left floating. This is a rule of thumb for all digital circuits.

Ace-X

Joined: 25 Jan 2002
Posts: 590
Helped: 26

04 Dec 2004 19:03 Re: Help: How to deal with the IOs that are not used?

btbass wrote:

All unused inputs should be tied high or low, a resistor is not needed, all unused outputs can be left floating. This is a rule of thumb for all digital circuits.

Not for all - most of modern ICs have internal weak pull-up or pull-down, otherwise it will be pain for PCB designer to place hundreds of resistors for unused pins.

honghongrong,
you DON'T need external pull-downs for usual I/O pins. Check this link: _www.xilinx.com/xlnx/xil_ans_display.jsp?getPagePath=8272
There are some requirements for unused RocketIO pins - check this link: _www.xilinx.com/xlnx/xil_ans_display.jsp?getPagePath=14747 .
Also, if your IC comes in BGA package, it is good idea to place solder pads on PCB for all pins, including unused, otherwise the solder will float around and can make shortcuts.

Ace-X.

echo47

Joined: 07 Apr 2002
Posts: 4206
Helped: 566

05 Dec 2004 1:45 Help: How to deal with the IOs that are not used?

Ace-X, your first link does not mention Virtex II Pro. The V2 data sheet says unused pins have internal pulldown resistors, but the V2P data sheed does not say that. Or it's hidden very well.

If it were my project, I'd leave the pins unconnected on the PC board, and configure them with internal pull-down resistors. They will float briefly during configuration time, but most applications won't care.

Ace-X

Joined: 25 Jan 2002
Posts: 590
Helped: 26

05 Dec 2004 9:40 Re: Help: How to deal with the IOs that are not used?

echo47 wrote:

Ace-X, your first link does not mention Virtex II Pro. The V2 data sheet says unused pins have internal pulldown resistors, but the V2P data sheed does not say that. Or it's hidden very well.

V2Pro is based on V2. Also, you can check its datasheet page 1:

Quote:

Virtex-II Pro devices are built on the Virtex-II FPGA architecture.
Most FPGA features are identical to Virtex-II devices.

The only difference concerning our topic:

Quote:

The open-drain output pin TDO does not have an internal pull-up resistor.

Ace-X.