Spartan3 DC datasheet

Jan 25, 2006 1 Replies

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Marco 20 years ago

Hi, on page 25 of DS099-3 Spartan3 DC and Switching Characteristics, there's a a formula for the maximum number of SSOs allowed within an I/O bank. I have a XC3S200FT256 working in LVCMOS33 standard (fast,

2mA), its table 22 value is 3, while its table 23 value is 44. Do I really have a limit of 3x44=132 SSOs for each bank? This value excedes by far the number of pin per bank in this package... It is "reasonable" for output currents of 24mA or 16mA, where it limits the SSOs to 9 and
21 respectively. This post is only to check if what I found is correct or if I'm wrong. Thanks, Marco

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Steve Knapp (Xilinx Spartan-3 Generation FPGAs) 20 years ago

Your calculations are correct. It essentially means that you can safely switch all outputs in a bank using the LVCMOS33, 2 mA, FAST I/O standard. If your performance and DC current requirements are met, it's generally best to use the weakest output driver possible. It generally results in improved signal integrity (more SSOs allowed, drive a PCB trace further without worrying about transmission line effects, etc.).

In cases where you mix I/O standards in the same bank, there are benefits to knowing how much additional "headroom" that you have for a given standard. See XAPP689 for additional details.

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--------------------------------- Steven K. Knapp Applications Manager, Xilinx Inc. General Products Division Spartan-3/-3E FPGAs

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--------------------------------- The Spartan(tm)-3 Generation: The World's Lowest-Cost FPGAs.

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