Mismatch of the LUT sizes between product description and vivado

[raymondlam]

I bought the Arty S7: Spartan-7 FPGA Board for Hobbyists and Makers Arty S7-50T.The product description stated that it has 52,160 Logic Cells. However, from the utilization report in vivado, there is only 32600 LUT available. Could anyone please help me .Thanks.

1. Slice Logic
--------------

+----------------------------+------+-------+-----------+-------+
|          Site Type         | Used | Fixed | Available | Util% |
+----------------------------+------+-------+-----------+-------+
| Slice LUTs*                |  122 |     0 |     32600 |  0.37 |
|   LUT as Logic             |  120 |     0 |     32600 |  0.37 |
|   LUT as Memory            |    2 |     0 |      9600 |  0.02 |
|     LUT as Distributed RAM |    0 |     0 |           |       |
|     LUT as Shift Register  |    2 |     0 |           |       |
| Slice Registers            |  119 |     0 |     65200 |  0.18 |
|   Register as Flip Flop    |  119 |     0 |     65200 |  0.18 |
|   Register as Latch        |    0 |     0 |     65200 |  0.00 |
| F7 Muxes                   |    0 |     0 |     16300 |  0.00 |
| F8 Muxes                   |    0 |     0 |      8150 |  0.00 |
+----------------------------+------+-------+-----------+-------+

[D@n]

@raymondlam,

If you check out Xilinx's 7-series CLB user's guide, you'll read,

Quote

Device capacity is often measured in terms of logic cells, which are the logical equivalent of a classic four-input LUT and flip-flop.

The difference stems from the fact that 7-series devices don't have four input LUTs, they have 6-input LUTs.  This leads to an apparent bit of inflation in the advertising material, until you read the fine print.  However, if you look at the Xilinx 7-series overview datasheet, you'll see that right next to the logic cells number they identify a number of slices found within the design.  In the case of the S7, using a XC7S50 part, there are 8,150 slices within the design.  Each of these "slices" has (roughly) 4 LUTs, and so this matches your design numbers above where you've discovered your design has only 32,600 LUTs available to you.

Dan