Communicating with your Cyclone II FPGA over serial port, Part 3: Number Crunching

Altera Cyclone II FPGAIn my first blog post on communicating with the Altera Cyclone II FPGA, I demonstrated how to create a serial echo by simply connecting the Tx wire and the Rx wire together.  Then in my second post I demonstrated how to use third-party libraries and a clock to allow that serial line to be translated into a byte stream and back before being re-transmitted to the host.  In this post I’ll show how to write custom Verilog to process the data that is received.

As a software developer, this is the part of FPGAs that interests me the most.  If you’re reading this post, I’m sure you’re aware of the fact that an FPGA can provide massively parallel computation in a way that a CPU or even a GPU is unable to achieve.  The difficulty is in getting started.  You can’t just download a C library and link to it like you can with CUDA.  You instead have to roll up some means of communication and write code to send the raw and processed data back and forth.  For most high-level software designers like myself this is far from straightforward.  Hence, this series.

So, on to the good stuff.  We left off last time with third-party components for a transmitter and a receiver that were hooked up directly to each other.  For this lesson we’re going to create a new Verilog component that takes the first ten bytes of the serial stream, reverses them, and then sends them back to the PC.  The following assumes that you have gone though the previous posts and have finished the Quartus project of Part 2.

The first step is to create the new Verilog file and add it to the project.  So click File > New, and select Verilog HDL File.

Image 10

Go ahead and save the file as “processor.v”.

Image 11

At this point we start coding.  This blog post isn’t intended to be an introduction to Verilog, so I’m not going to go into great detail over each line of code here.  But, the general idea is, we’ve got a state machine with a READ state, a SOLVING state, and two WRITE states.  The READ state reads data until the 10th byte is received, then switches to SOLVING.  The SOLVING state reverses the array (all in one clock tick!) and switches to WRITE1.  Finally the WRITE states wait for the txBusy signal to go away, transmit the next byte, and repeat.  (There’s probably a way to combine WRITE1 and WRITE2 into one state but this is the first thing I got working).

Here is the full source of processor.v

module processor(clk, rxReady, rxData, txBusy, txStart, txData);
  input clk;
  input[7:0] rxData;
  input rxReady;
  input txBusy;
  output reg txStart;
  output reg[7:0] txData;

  localparam READ=0, SOLVING=1, WRITE1=2, WRITE2=3;
  localparam LEN = 10;
  localparam LENMAX = LEN - 1;

  integer ioCount;
  reg[7:0] data[0:LENMAX];
  integer state;

  initial begin
    txStart = 0;
    state = READ;
  end

  always @(posedge clk) begin
    case (state)
      READ: begin
        if (rxReady) begin
          data[ioCount] = rxData;
          if (ioCount == LENMAX) begin
            ioCount = 0;
            state = SOLVING;
          end else begin
            ioCount = ioCount + 1;
          end
        end
      end

      SOLVING: begin
        integer i;
        for (i = 0; i < LEN/2; i = i + 1) begin
          reg[7:0] swap;
          swap = data[i];
          data[i] = data[LENMAX-i];
          data[LENMAX-i] = swap;
        end
        state = WRITE1;
      end

      WRITE1: begin
        if (!txBusy) begin
          txData = data[ioCount];
          txStart = 1;
          state = WRITE2;
        end
      end

      WRITE2: begin
        txStart = 0;
        if (ioCount != LENMAX) begin
          ioCount = ioCount + 1;
          state = WRITE1;
        end else begin
          ioCount = 0;
          state = READ;
        end
      end
    endcase
  end
endmodule

This is different from traditional code in that all this stuff is happening in parallel.  For example in READ, there’s no loop to read 10 bytes.  Instead, this entire block gets executed each clock tick, so you update your data array, increment your counter, and update your state to SOLVING if it’s read 10 bytes.

The exciting part is that the SOLVING block also happens as one clock tick.  Here we’re doing nothing much, just reversing a small array.  However you can imagine the potential, to do enormous computations here and it would just take a few clock ticks.  For instance I have a sudoku solver that typically requires only 6-7 ticks to solve a full puzzle (perhaps a topic for a future post).

Anyway, now that we’ve got the code we have to insert it into the board design.  So, as in Part 2, the first thing you have to do is create a “symbol file”, so click File > Create/Update > Create Symbol Files for Current File.  Now we can add it to the board design, so double-click on the board, and you’ll see your shiny new “processor” symbol.  Select OK and add it.

Image 12

You’ll now want to hook up the processor to the receiver and transmitter.  The links should be self-describing, but if you need help the final design should look like this:

Image 13

Now we compile and load the code on the FPGA just like last time, and that’s it!  Open your favorite terminal, and take it for a spin!

Image 14

As I mentioned earlier, this FPGA example was pretty trivial, but this series has demonstrated an easy way for a beginner to get up to speed using an FPGA for data processing and communicating with it over the serial interface, which is a big hump for software engineers with little low-level hardware exposure.  From here, it should be straightforward for any good developers to experiment with new algorithms or ideas that they have in mind.

This blog post is Part 3 in a series. To review Part 1, you’ll want to Go Here. To review Part 2, you’ll want to Go Here.

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2 Responses to Communicating with your Cyclone II FPGA over serial port, Part 3: Number Crunching

  1. Pingback: Talking RS-232 with Cyclone II FPGA, Part 1 - Sparx Blog

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