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JColvin

Nexys 4 DDR question

Question

A customer on our website asked the following question:

Hi there,

I noticed on the product page for the 'nexyus 4 ddr' board the spec list:
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Features:
Xilinx Artix-7 FPGA XC7A100T-1CSG324C
15,850 logic slices, each with four 6-input LUTs and 8 flip-flops 
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Just wondering how accurate this is as when I researched the 'Xilinx Artix-7 FPGA XC7A100T' FPGA it claims to have 'logic cells: 101,440'. Could you please clarify this for me.

Regards

Marcus

The response is below.

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Hello,

I guess the shortest answer is that logic slices and logic cells are two different things. 

The longer answer is that a logic cell is an abstraction, kinda like how a dozen means 12. A logic cell isn't actually a "thing" inside the FPGA; it is a way to measure the device capacity where a 4-input LUT and a flip-flop equals a logic cell. However, as an abstraction it does not truly reflect the actual size of the FPGA, it's just a convenient measurement that is somewhat close if you're okay with glossing over some design specific details (that I'm not personally familiar with).

A logic slice on the other hand is more architecture specific and accounts for more details. For Xilinx's 7-series FPGAs (as per DS180) a logic slice is made up of 4 LUTs, 8 flip-flops as well as any associated multiplexers and arithmetic carry logic. Two of these slices make up a configurable logic block (CLB). 

Admittedly, these logic slices and CLBs can also seem somewhat vague, but in the end "are the main logic resources for implementing sequential as well as combinatorial circuits" as per Xilinx's CLB User Guide for the 7 Series FPGAs, so that's why our product page lists the 15,850 slices rather than the logic cells.

Let me know if you have any more questions.

Thanks,
JColvin

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