I am using a FPGA to control a 4-channel ADC (ADC084S101) to sample four different analog voltages. In order to tell the ADC which channel to sample next, there is a control register that can be written with 00, 01, 10, or 11 to sample channels 0, 1, 2, or 3, respectively (See the table below). Control Register Bits

I'm not sure if the approach that I'm taking with this module is correct. What my intention is that the module will take the same CS and SCLK signals that are going to the ADC and also a 2-bit number that corresponds to the next channel that is to be sampled. Then, upon each falling edge of CS, reset a counter to 0. On each rising edge of SCLK, clock out the appropriate bit of the stream using the incremented count as an index, either 0 if it's don't care or the 2 bits that correspond to the next channel if at the appropriate index.

module GenerateNextAddr(
input cs,
input sclk,
input [1:0] addr,
output reg dout

 reg [4:0] count;
 reg [1:0] currentAddr;

 always @(posedge sclk or negedge cs) begin
    if (cs) begin
        count <= 0;
        currentAddr <= addr;
    end else    begin
        count <= count + 1;
        if ((count != 4'b0011) && (count != 4'b0100)) begin //If we're clocking out any of the 'dont care' bits of the ADC
            dout <= 0;
        end else if (count == 4'b0011) begin
            dout <= currentAddr[1];
        end else if (count == 4'b0100) begin
            dout <= currentAddr[0];

I am struggling to get the Verilog implemented for outputting this bitstream (I am a software developer that has only been working with Verilog for a few months). The Verilog module that I have written above is not synthesizable and I'm not sure why (my development board is a Nexys 3 from Digilent). The error message is, "Assignment under multiple single edges is not supported for synthesis."

  • 1
    \$\begingroup\$ Try changing your first if statement to if (~cs) begin .... You're trying to make a low-active asyncronous reset (because of the negedge cs sensitivity) but then you actually do the reset action when cs is high. \$\endgroup\$
    – The Photon
    Mar 23, 2016 at 17:27

1 Answer 1


It looks like you're trying to make logic with an asyncronous reset signal called cs.

But you mismatched the edge sensitivity for the reset signal. You're watching for the falling edge of cs, but the logic calls for resetting when cs is high.

You need to either have sensitivity to posedge cs and reset the registers when cs is high, or have sensitivity to negedge cs (like you have) and reset the registers when cs is low.

For example,

always @(posedge clk or negedge cs) begin
    if(~cs) begin
         count <= 0
    end else begin
  • \$\begingroup\$ Thank you! That was the problem. I thought that the if(cs) meant if the block is currently being driven by the falling edge of cs. \$\endgroup\$
    – zr870
    Mar 23, 2016 at 17:47
  • \$\begingroup\$ Nope, cs is just being used to refer to the logic level of the cs signal. For a syncronous reset, you wouldn't have even mentioned cs in the sensitivity list, just refer to it in the if block the same way you do addr and currentAddr. \$\endgroup\$
    – The Photon
    Mar 23, 2016 at 18:00

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