I have a Xilinx/Digilent Spartan 3E starter board, UG230. I'm designing a PCB for an ADC/DAC board. I'm quite new to FPGAs (with some experience), but I don't want to make any silly mistakes using the Hirose FX2 connector (J3) - see board datasheet pg. 115. I have 2 questions:

1) The board clock is 50 MHz. Can I also output that same 50 MHz onto an IO pin to clock the ADC? I know this will work in theory, I just want to know if the FPGA will be able to do this? I'm used to MCUs where the IO speed can only be a fraction of the master clock.

2) Some of the pins on the Hirose FX2 connector are IO and others are INPUT only. The ADC I have is a 16 bit parallel bus, and I'd like to keep the IO pins for user input/output (display, etc). So the INPUT only pins would be good for the ADC. Are there any tricks to using the input pins (D12, C12, A15, B15, C15)? I would like to clock the ADC at 50 MHz (see Q1).

All advice greatly appreciated. Just don't want to get a board make and have to junk it for a silly mistake because I've miss-understood something trivial.

Cheers folks.


1 Answer 1


1) Yes, absolutely. Most FPGAs can directly route a clock out to a pin. A slightly better idea is to actually use a DDR flip flop at the pin as the relative timing between this and signals registered in IOB flip flops will be the same.

2) Nothing in particular. Just double check that you have them hooked up properly as inputs as the FPGA does not have output drivers on those pins (no, I haven't made that mistake before...)

  • \$\begingroup\$ Perfect answer, @alex.forencich - exactly what I wanted to know. Answer accepted! Cheers! \$\endgroup\$
    – M1GEO
    Commented Mar 13, 2017 at 20:57

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