Spartan-3E Slave Serial Configuration

Sep 14, 2007 5 Replies
A

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



AVR 3.3V FPGA VCC0 3.3V FPGA VCCAUX 2.5V



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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


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S

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.

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A

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 Tel: +44(0)1904 432828 Mailto: ajg112@ohm.york.ac.uk Fax: +44(0)1904 432335 Web: http://www.bioinspired.com/users/ajg112
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B

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.

Brian

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J

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

John

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A

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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