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Unused clock pins tied inactive?

I'm doing a PCB design for a client which incorporates a Cyclone II.

I have only used two of the 8 dedicated clock pins so had left the rest floating.

During a schematic review one of the client's engineers said that he thought it might be a good idea to tie the unused clock pins to GND.

I have never done this before, I was under the impression that these were weakly held high or low but can't find any reference to this in the documentation.

If these aren't pulled weakly I'd definitely add the extra few components, but don't want to add extra stuff to be placed/routed/built if it's not necessary.

What's the panel's view?

Nial.

Reply to
Nial Stewart
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I'd love to reference the specific App note or data page but this information comes second hand. The same issue came up here a couple days ago where an engineer was told to connect the unused Altera clock inputs to ground. There was a short initial debate among a few of us before another engineer (who had a recent Cyclone-II design) interjected that Altera explicitly calls out that the unused clock pins should be grounded.

Since it's near and dear to the Altera designers (it's been too many years since I've used "Brand A") you might get a quick document reference from your FAE or someone at the Altera Application Hotline.

- John_H

Reply to
John_H

Hi,

You do not need to connect unused general or clock inputs to ground if you configure them as "inputs tri-stated with weak pull-up" or as "outputs driving ground" in Quartus. We do recommend that if your unused inputs are set to "inputs tri-stated" that you connect them to VCC or GND external to the device for better noise immunity.

With some of our FPGAs, there can be signal integrity advantages of physically connecting your unused I/Os to ground (regardless of internal setting), primarily because this provides better ground return current paths in the PCB under the FPGA (assuming you are using through vias). Even if your outputs are set to "outputs driving ground", you won't get much return current through the I/O buffer itself -- but the act of adding the via will reduce the size of inductive loops in the PCB via region, reducing your inductive coupling.

Regards,

Paul Leventis Altera Corp.

Reply to
Paul Leventis

Hi Paul, So, I think you're saying the advantage is in having more ground vias, right? Attaching the I/O to the ground has only a small effect? Cheers, Syms.

Reply to
Symon

Take a look at the .PIN file output and see what it has to say. 'Unused' pins will fall into one of the following categories, do as it suggests.

-- GND+ : Unused input pin. It can also be used to report unused dual-purpose pins.

-- This pin should be connected to GND. It may also be connected to a

-- valid signal on the board (low, high, or toggling) if that signal

-- is required for a different revision of the design.

-- GND* : Unused I/O pin. This pin can either be left unconnected or

-- connected to GND. Connecting this pin to GND will improve the

-- device's immunity to noise.

-- RESERVED : Unused I/O pin, which MUST be left unconnected.

-- RESERVED_INPUT : Pin is tri-stated and should be connected to the board.

-- RESERVED_INPUT_WITH_WEAK_PULLUP : Pin is tri-stated with internal weak pull-up resistor.

-- RESERVED_INPUT_WITH_BUS_HOLD : Pin is tri-stated with bus-hold circuitry.

KJ

Reply to
KJ

Paul,

Looking at the data sheet for CycloneII (I think, it was last week) the dedicated clock inputs didn't look like they had this option.

I've just done a test build setting the unused clock inputs to 'tri-stated with weak pullups' with a CycloneI. In the *.pin file the unused clock was listed as GND+ ....

-- GND+ : Unused input pin. It can also be used to report unused dual-purpose pins. -- This pin should be connected to GND. It may also be connected to a -- valid signal on the board (low, high, or toggling) if that signal -- is required for a different revision of the design.

So it looks like unused clocks _should_ be tied somewhere (through a resistor to keep the assembly test people happy).

Nial

Reply to
Nial Stewart

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