Help me understand this NCO rate issue

I have an NCO based on the equation "desiredFreq = controlword * Clockfrq / 2 power bitwidth of accumulator"

or "controlword = desired freq * 2 power bitwidth / clockfreq"

However when running it appears to run at 1/4 desired freq. When I multiplied the control word by 4 it appears to run the correct rate. This was based on several examples I found. So what exactly is the correct relationship of the control word, accumulator bit width and clock frequency?

module NCO(reset, sysclk, sampleclk, code_freq_ctl, other outputs);
 input reset;
 input sysclk;
 input sampleclk;
 input code_freq_ctl;
 reg  accum_sum; //30bit
 reg   accum_reg; //29bit
 wire chip_advance;
 always @ (posedge sysclk)      //50MHz main clock
 begin
    accum_reg <= accum_sum;
    code_phase <= accum_reg;
    accum_sum <= accum_reg + code_freq_ctl;
  end
always @ (posedge sampleclk)     //from freq divider 5MHz readable 1 sysclk cycle, timed on 10th sysclk cycle
begin
   //on sampleclk check value of chip_advance, do other functions if ==1
end
 assign chip_advance = accum_sum;
endmodule