Drive strength settings

[Altera_Forum]

Hello:  

 Just starting with FPGA design   When the FPGA (cyclone II) drives pins into a device with a specific loading value (i.e 10pf) , is there a calculation or rule thumb to estimate what the drive strength should be set in Quartus for those pins? I know Quartus defaults to maximum and sometimes not all signals are available to connect a scope. Other than simulation, is there way to estimate this?  thank you

[Altera_Forum]

No one has any hint for this??  

thanks in advance

[Altera_Forum]

There can't be a rule of thumb because the best setting also depends on signal speed and required rise time respectively acceptable signal delays. I usually choose minimum current for general purpose I/O, higher values possibly for clock lines or fast memory interfaces.  

   --- Quote Start ---  sometimes not all signals are available to connect a scope  --- Quote End ---   I think, it's sufficient to connect one signal to an oscilloscope to see the basic effects of current strength settings.

[Altera_Forum]

--- Quote Start ---  There can't be a rule of thumb because the best setting also depends on signal speed and required rise time respectively acceptable signal delays. I usually choose minimum current for general purpose I/O, higher values possibly for clock lines or fast memory interfaces.    --- Quote End ---     I thought based on the parameters that you indicated (rise time , load capacitance, etc) that an estimated calculation can be made as a start up value. There are few cases as well where there is no chance to get a test point to put a scope on a particular signal. It will be nice in this case to use a theorical estimation instead of using the default setting even if the system is running ok.   thanks

[Altera_Forum]

You can extract the exact output characteristic for each current strength setting from the IBIS files and calculate the respective dynamical response or use a tool, that does the calculation.

[Altera_Forum]

--- Quote Start ---  You can extract the exact output characteristic for each current strength setting from the IBIS files and calculate the respective dynamical response or use a tool, that does the calculation.  --- Quote End ---     Would you mind elaborating in what tools and a little more details on how you do this exactly?   thanks again

[Altera_Forum]

A simple option is to calculate a (dynamic) output impedance from dV/dI gradients in Ibis files. Ther are also signal integrity tools that import Ibis data, but I don't use them.