Can I connect two FPGA pins together?

[timsanr]

Hi,

can I connect two FPGA pins together? One as input, second as output.

 

[srini.pes]

Re: Connect two FPGA pins

hi.,
u want to connect one pin to input and second one to output...
if so., its quite common...
u r Q not cleared....

 

[timsanr]

Re: Connect two FPGA pins

I want to send signal outside FPGA and via wire put it again into FPGA. So I need something like short circuit (Im not sure if 'short circuit' are proper english words, my dictionary says that).

 

[alexan_e]

Re: Connect two FPGA pins

It is just a connection of two pins, there is no problem unless they both become outputs.

 

[timsanr]

Re: Connect two FPGA pins

Thanks I heard that it is bad to connect two pins but didn't remember when.

 

[alexan_e]

Re: Connect two FPGA pins

Actually even when they are outputs they can be connected together to increase the current they can provide assuming that the are both driven simultaneously from the same signal so they change together

 

[timsanr]

Re: Connect two FPGA pins

It is good to know. It could be useful with high frequency signals. Avoiding time shifts, am I right?

 

[alexan_e]

Re: Connect two FPGA pins

I think I have seen this output connection is some application note but I can't find it

 

[timsanr]

Re: Connect two FPGA pins

Ok, no problem, now not needed, but intresting detail. Thanks for help again

 

[KonstanU]

Re: Connect two FPGA pins

Actually even when they are outputs they can be connected together to increase the current they can provide assuming that the are both driven simultaneously from the same signal so they change together

Please, give link on this information, because I think It is dangerous switching. Even if you apply driven signal simultaneously to both outputs how can you be sure that they will switch simultaneously. At least every output buffer has a bit different capacitance. I mean that there will be time when one output in the high state and another one in the low state.
I am reckon, that in practice that is wrong way. If you need more power/current make boost.

 

[alexan_e]

Re: Connect two FPGA pins

As I said I'm not sure where I have seen it so I can't provide a link nevertheless I'm sure there are restriction to when this can be used.

I dis a search from google and found the following

https://www.latticesemi.com/support/faqs/details.cfm?id=1827
www.siliconbluetech.com/media/downloads/SiliconBlue_AN007.pdf

Alex

 

[KonstanU]

Re: Connect two FPGA pins

Thanks.
So it is possible to use several connected outputs if output is open source or
if it is push-pull output (any another active output) then it is necessary to put additional series resistance to each output.