Spartan-3E Slave Serial Configuration

Hi All,

I'm designing a circuit for configuring 4 Spartan-3E FPGA from an Atmel AVR microcontroller.

I want to configure the FPGA in slave serial mode. Because of the difference in the supply voltages, there are some different logic voltages, some 2.5V, some 3.3V


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Xapp453 states that CCLK needs to be driven with 2.5V logic, however, the Spartan-3E datasheet states (Page 68) that CCLK is powered by VCC0_2 which will be 3.3V.

I'm guessing that the contradiction is due to XAPP453 being for Spartan-3, not Spartan-3E.

I just need to know if CCLK needs the logic level changing as well the PROG_B signal.

Could someone just check the following for me:

PROG_B driven by AVR needs to be 2.5V logic DIN driven by AVR 3.3V logic (I think) CCLK driven by AVR 3.3V logic

INIT_B driven by FPGA 3.3V logic DONE driven by FPGA 2.5V logic DOUT driven by FPGA 3.3V logic

Many thanks Andy

Reply to
Andrew Greensted
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Hi Andy, That's what I did on my last S3E design. I pulled PROG_B low through a schottky diode with a pullup to 2.5V at the FPGA. All that said, I would rely on the datasheet more than usenet! HTH, Syms.

Reply to

Symon wrote: > Hi Andy,

Cheers Symon, My plan was to use one of those tiny-logic single gates, they've got 5V tolerant inputs and can be by 2.5V. That should do the level conversion nicely. Andy

Dr. Andrew Greensted      Department of Electronics
Bio-Inspired Engineering  University of York, YO10 5DD, UK
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Reply to
Andrew Greensted

Have a look at pages 101-121 of:

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This info may have all percolated into the datasheet by now, but that's still a most useful document.


Reply to
Brian Davis

That's good. Use a dual schmitt trigger, and clean up CCLK while you're there. Ringing on CCLK can be a real nuisance.


Reply to
John Larkin

Ahh yes, I th> That's good. Use a dual schmitt trigger, and clean up CCLK while

Good plan.

Thanks both Andy

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