Notiz

FPGA

Based on the FPGA version and the device I am using currently:

• I am using Artix-7 FPGA (xc7a35tcpg236-a(active)) and a 64-bit counter

1. CLB - Configurable Logic Block

   

   A CLB is a physical block inside an FPGA that contains resources for building digital circuits.

   Think of it as a small workshop containing tools for processing and storing binary data.

   • Here, Artix-7 FPGA contains thousands of CLBs.

   • Each CLB contains two slices.

   • Vivado uses resources inside these CLBs to implement the VHDL design.

   For example, when we write a counter in VHDL, Vivado uses CLB resources to implement its counting logic.

2. Slice - A smaller part of CLB: A slice is a subdivision of CLB containing configurable logic and storage resources.

   In our current project for Artix-7 FPGA, one slice contains:

   • 4 LUTS
   • 8 flip-flops
   • 1 CARRY4 carry-chain block
   • Additional multiplexers and connections.

   In Vivado, we can see SLICE_X10Y20, that means a physical slice at column X10 and row Y20.

   A CLB contains two slices, but the two slices do not necessarily have identical capabilities.

3. LUT - Look-Up Table

   A LUT is a small programmable logic circuit that produces an output based on its inputs. Imagine you want to implement an AND gate:

        $[ Y=A\land B ]$  A LUT can store the following truth table:
   
    | A   | B   | Output Y |
    | --- | --- | -------- |
    | 0   | 0   | 0        |
    | 0   | 1   | 0        |
    | 1   | 0   | 0        |
    | 1   | 1   | 1        |
   

   Instead of physically constructing a separate AND gate, the FPGA configures a LUT to produce these outputs.

   Your Artix-7 uses 6-input LUTs, meaning one LUT can implement any Boolean function of up to six inputs with one output.

   In your counter: LUTs help calculate the next counter value, including whether individual bits should change.

4. Flip-Flop - Stores one bit A flip-flop is a small circuit that remembers one bit (0 or 1).

   Unlike a LUT, which calculates an output from current inputs, a flip-flop stores a value over time.

5. Carry Chain - Helps perform arithmetic quickly

   A carry chain is a dedicated hardware path that makes arithmetic operations, such as addition, faster.

The most important distinction to remember:

• LUT = Calculates

• Flip-flop = Remembers

• Carry chain = Speeds up arithmetic

• Slice = Contains these resources

• CLB = Contains slices