I'm just starting out in the world of FPGAs. I've picked up a Mojo running a Spartan-6 from Xilinx and I'm attempting to create an arbitrary clock for use with a SENT peripheral. At this point I haven't made it beyond attempting to generate a tick clock (eventually 1.2us) from the base clock (50MHz). I'm using the divider below to take the 50MHz clock as an input and currently divide by 500 which should provide a 100kHz output that I'm feeding directly to a pin connected to my logic analyzer.

File: divider.v

module divider (
    input clk,  // clock
    input rst,  // reset
    output reg out

    parameter DIVIDER = 500;

    reg [15:0] count = 0;

    always @(posedge clk) begin

        if (rst == 1) begin
            out <= 0;
            count <= 0;
        else begin
            if (count == (DIVIDER - 1)) begin
                out <= 1;
                count <= 0;
            end else begin
        out <= 0;
                count <= count + 1'b1;


File: mojo_top.luc

module mojo_top (
    input clk,              // 50MHz clock
    input rst_n,            // reset button (active low)
    output led [8],         // 8 user controllable LEDs
    input cclk,             // configuration clock, AVR ready when high
    output spi_miso,        // AVR SPI MISO
    input spi_ss,           // AVR SPI Slave Select
    input spi_mosi,         // AVR SPI MOSI
    input spi_sck,          // AVR SPI Clock
    output spi_channel [4], // AVR general purpose pins (used by default to select ADC channel)
    input avr_tx,           // AVR TX (FPGA RX)
    output avr_rx,          // AVR RX (FPGA TX)
    input avr_rx_busy,      // AVR RX buffer full
    inout sent0
  ) {

  sig rst;                  // reset signal
  sig data[12];

  .clk(clk) {
    // The reset conditioner is used to synchronize the reset signal to the FPGA
    // clock. This ensures the entire FPGA comes out of reset at the same time.
    reset_conditioner reset_cond;

  divider tick_clock(.clk(clk), .rst(rst));

  always {
    sent0.enable = 1;
    sent0.write = tick_clock.out;

    reset_cond.in = ~rst_n; // input raw inverted reset signal
    rst = reset_cond.out;   // conditioned reset

    led = 8h00;             // turn LEDs off
    spi_miso = bz;          // not using SPI
    spi_channel = bzzzz;    // not using flags
    avr_rx = bz;            // not using serial port

What I'm seeing though is that while the clock signal outputs a 100khz waveform as expected it drops out periodically. I haven't been able to draw any conclusions as to why this is happening.

Logic capture showing breaks

I am expecting a semi-clean 100khz waveform with the supplied code.

I've been banging my head hard against this for a while but I can only assume that it's my lack of Verilog experience that's getting me and not anything to do with the hardware that I am attempting to use. This seems to be similar to existing examples for UART and I2C master modules. But there must be a small detail that is missing.

Can anyone offer any advice regarding producing a clean clock signal to drive an output pin?


1 Answer 1


Your output only goes high for 1 cycle out of every 500.

Your graphical display probably doesn't have the resolution to show this correctly. If you zoom in on one of the regions that seems to have no transitions, you will probably be able to see the missing high pulses.

If you want to create an output clock with 50% duty cycle, then you should be setting out <= ~out when the terminal count is reached, and not changing out otherwise. You would need to halve the terminal count value to have the same period.

  • \$\begingroup\$ Oh, duh... That makes perfect sense! I'm presume that the sample points for my logic analyzer occasionally synchronize with the tiny high pulses to produce what looks like periodic functionality. In other words, my logic analyzer is lying to me =). For some reason I was thinking that I wanted the short high pulse but I'll adjust for a 50% duty cycle and test again. \$\endgroup\$
    – julienj
    Oct 5, 2017 at 15:43

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