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Adventurer
Adventurer
8,581 Views
Registered: ‎03-01-2010

Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Is it possible to have a 74LVC4245A, logic level interface, built into "the fabric" of CPLD's and FPGA's? As a hobbyist my "retro" designs are still using 5V signals. Presently using an XC9572. Soon I plan on experimenting with a Spartan 3 to replace, at first, then improve upon an old 8 bit CPU design. If this idea were possible, I'd like to know... Maybe a buffer with a certain logic level constraint?

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Instructor
Instructor
10,904 Views
Registered: ‎08-14-2007

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Jim,

 

That's a good reference for Spartan 3 / 3E, but it does not apply to the Spartan 3A.

Spartan 3A does not have the protection diodes to Vcco, so you need to be

careful not to drive its inputs above the absolute maximum voltage from the

datasheet.  Removing the diodes to Vcco makes it easier to mix input standards

below 3.3V, but as you can see complicates the connection for 5V sources.

 

By the way Xilinx still makes and supports earlier logic families such as

Spartan 2 (not 2e) with 5V tolerant inputs.  You can find webpack software for those

parts in the ISE "classics" download pages.  Whether you can use them

will depend to some extent on the complexity of design.  They have less

fabric logic as well as fewer and smaller block RAM's than Spartan 3, but still a lot more

capability than the 9500XL series CPLD's.

 

Regards,

Gabor

-- Gabor
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8 Replies
Instructor
Instructor
8,561 Views
Registered: ‎08-14-2007

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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The short answer is no.  Basically newer devices have an absolute maximum input

voltage tolerance somewhere around 3.6V (or less).  Depending on the series of

FPGA or CPLD, you may be able to get away with only resistors external to the

part.  There are many app notes on 5V signal interface, and the best method to use

depends on your device.  Outputs from the FPGA can directly drive TTL logic, since

the high threshold for TTL is only 2V, however if you need to drive 5V CMOS, you'll

need external level shifters.

 

For hobbyists I would recommend looking at the products on this site:

 

http://www.enterpoint.co.uk/

 

Regards,

Gabor

-- Gabor
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Contributor
Contributor
8,544 Views
Registered: ‎06-12-2009

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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I came across a Xilinx answer that may be helpful ...

 

   http://www.xilinx.com/support/answers/19146.htm

 

This mentions the Spartan 3 and 3E, but I suspect it may apply to the newer 3A as well - check the datasheet.  So for FPGA inputs you just need a series resistor.  A 1K would adequately limit the current and not slow down rise/fall times all that much.

 

So as mentioned above, your main problem is with the FPGA driving a legacy part with a high input threshold, and for this you could use a translator like 74HCT series.

 

Jim

 

 

 

 

 

 

 

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Instructor
Instructor
10,905 Views
Registered: ‎08-14-2007

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Jim,

 

That's a good reference for Spartan 3 / 3E, but it does not apply to the Spartan 3A.

Spartan 3A does not have the protection diodes to Vcco, so you need to be

careful not to drive its inputs above the absolute maximum voltage from the

datasheet.  Removing the diodes to Vcco makes it easier to mix input standards

below 3.3V, but as you can see complicates the connection for 5V sources.

 

By the way Xilinx still makes and supports earlier logic families such as

Spartan 2 (not 2e) with 5V tolerant inputs.  You can find webpack software for those

parts in the ISE "classics" download pages.  Whether you can use them

will depend to some extent on the complexity of design.  They have less

fabric logic as well as fewer and smaller block RAM's than Spartan 3, but still a lot more

capability than the 9500XL series CPLD's.

 

Regards,

Gabor

-- Gabor
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Adventurer
Adventurer
8,505 Views
Registered: ‎03-01-2010

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Gabor, That is exactly the info I was looking for!

 

I posed the question as a possibility for the fine Xilinx engineers to put this ability to accept these higher volt signals into the more advanced families. I was not aware they were available in the Spartan 2 family. That is excellent info. I was looking into coolrunner series since recently because I've been running out of macrocells on the XC9572, especially with a 16 bit MUX. Now I'll be looking into Spartan 2 series. Still Spartan 3 for CPU... Thanks again!

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Adventurer
Adventurer
8,488 Views
Registered: ‎03-01-2010

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Elsewhere on the forum they were talking about the Coolrunner XPLA3 series, which has 5V I/O tolerant pins. Can't design with Spartan 2...

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Instructor
Instructor
8,481 Views
Registered: ‎08-14-2007

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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I'm not sure what you mean by "Can't design with Spartan 2...".

 

Spartan 2 is still supported in ISE 10.1.03 and it certainly has 5V tolerant I/O's.

-- Gabor
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Adventurer
Adventurer
8,470 Views
Registered: ‎03-01-2010

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Is the Spartan II still being produced?... I've found versions being sold at low QTY, but  I just don't want to base a new design, even though retro 5V, on a family of IC's no longer being produced... Unless of course there are other properties on the IC missing from the newer families. I'll look over the spartan2_scm.pdf tomorrow...

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Instructor
Instructor
8,464 Views
Registered: ‎08-14-2007

Re: Built-in Logic Level Shifters for Xilinx CPLD's and FPGA's

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Older parts than this are still in production.  Here's a link to Digikey where a

quick search turned up some XC2S30 parts in stock:

 

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=122-1318-ND

 

Just be aware that the Spartan 2e series is not 5V tolerant, only Spartan 2 is 5V tolerant.

 

I have used the XC2S30 in some products where I placed a PicoBlaze processor

in the part to do some of the more complex logic.  Even the smallest XS2S15

can fit a PicoBlaze.

 

On the other hand for some one-up projects where you might need to use

more recent parts you should probably consider the legacy products from

Enterpoint, some of which fit in a DIP socket and provide not only the

FPGA, but all level shifters, a clock and memory.  Definitely good enough

for more ambitious projects like legacy CPU replacements.

 

Regards,

Gabor

-- Gabor
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