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Registered: ‎07-28-2018

Are CLB and SLICE on the same hierarchy?

Hi guys,

I'm working on some CAD research about FPGA, and I'm using RapidWright, designed by Xilinx Research Lab.

Before touching RapidWright, what I have understood about an FPGA architecture is that CLBs are building blocks.

After reading RapidWright-related materials, I become confused now. I'm reading the Vivado property reference (

And in the Page 127, it says "SITEs include SLICE/CLBs".

What's the meaning of "SLICE/CLBs"? I thought CLB and SLICE are two different hierarchies, e.g. in 7-series, each CLB contains 2 SLICEs?

So, how should I understand this sentence correctly?

Thank you.

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Xilinx Employee
Xilinx Employee
Registered: ‎06-13-2018

Hi @zouyu930503 

If you open implemented design in Vivado and check the cell properties, you will get pictorial view about the SITE:


Yes, in 7 series, each CLB element contains 2 slices : SLICEL and SLICEM while in Ultrascale, each CLB contains one slice. CLBs made up of the logic elements themselves and these logic elements are grouped together in a "slice". These slices and their CLBs are arranged in columns throughout the device. The two independent slices of the 7 series CLB are combined into one slice in Ultrascale for greater logic and routing efficiency.

Also, to get better undersatnding of CLB architecture, refer UG474 for 7 series device and UG574 for Ultrascale device.




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