spartan 3 designing board


Ive lurked here for a while and have read the many arguments for and against designing a board or purchasing a dev board,

well usually i would agree that a dev board is allot less hassle, however i recently got given a couple of spartan 3's and ive got everything else in my junk box i need to design my own board, so there is no cost issue involved, except my own which i am not concerned about.

As FPGA design is new to me (its something i really want to get into) I was woundering whether you could prehaps clear me up some questions i have?

1) do i need to supply a clock to the spartan 3, or is there an on chip one? I have read the data sheets, prehaps not fully, but i couldnt see any special pins to supply a clock to on there pinout.

2)can i places all the chips for driving the jtag on the board permentantly with the fpga, or is it good practice to keep this off board? if so why?

I think ive got all the important elements sorted, it was just these issues i was interested in,

If anyone else can point me to a website with a basic schematic so that I can verify my design, I would be very grateful :-)



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See below,


Best to have a local clock that can be used for whatever you desire. A string of inverters may be used in a pinch for a low accurcy local clock, but that is hardly useful if you actually need to do something useful. Connect the oscillator to one of the dedicated global clock input pins.

One can supply an input to aqny pin, and route it where it is needed, but the dedicated clock input pins are he best for lowest skew/delay.

One jtag chain of all devices is usual. Simpler, easier, and more convenient than separate chains.

Have you checked out the Signal Integrity Central checklist for pcb design?

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Well huh, no cost issue ... I also have have spare components but the cost of the PCB manufacturing and assembly for just 1 board is pretty high depending on what you want to do (e.g. for a BGA spartan 3, you'll need like 6 layers board and 0402 decoupling on the back )

Use a xtal oscillator chip connected to one of the global clock. Depending on the devices you intend to put on your dev board, you may want to connect the other global clock for theses. Also, if you need de-skew, a DCM feed back pin might be needed.

1 jtag chain, the soft can handle that.

Well, for a FPGA, except for the power supply and configuration, the rest is up to you (depending on the device). And for the power supply of spartan 3, TI has good appnotes and for configuration, just look up the datasheet of the spartan 3, it's basically just pin to pin connection of the configuration PROM.

The other bits are dependent on what devices you want on your devboard. Of you course you can connect all the I/O pins to connectors but be aware that some peripherals are not easy to add on a daughter board ( like RAM)


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Sylvain Munaut

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