I am implementing a seconds counter on the Altera DE-1 Educational Board powered by the old Cyclone 2 FPGA. My plan is to make a 'down-clocker' that takes the on-board 50 MHz clock and produces a 1 Hz clock signal (referred to as the 'pulse'), which will then be used to drive the normal counter. The down clocker module is as follows:
module downClockerTest(pulse, clk, reset); output reg pulse; reg [25:0] count; input clk, reset; always @(posedge clk or negedge reset) begin if(~reset) begin count <= 26'h0; pulse <= 1'd0; end else if(count == 26'd49999999) count <= 26'd0; else begin count <= count + 26'h1; pulse <= (count > 26'd24999999); end end endmodule
Notice that the output 'pulse' is registered. This design works fine; the counter counts as it should. However, if I try and remove the register at end by attempting to drive the 'pulse' output by a single assign statement, the seconds-counter seems to begin jumping by 1 and then 4 in 1 second, indicating 5 pos-edges from the down-clocker, where there should only be one.
The slightly modded down-clocker is as follows:
module downClockerTest(pulse, clk, reset); output pulse; reg [25:0] count; input clk, reset; assign pulse = (count > 26'd24999999); // The counter seems to increment by 1 and then 4 in quick succession (1 second). always @(posedge clk or negedge reset) begin if(~reset) begin count <= 26'h0; end else if(count == 26'd49999999) count <= 26'd0; else begin count <= count + 26'h1; end end endmodule
Why is it that the down-clocker only works right when there is a register at the end? Is the register performing some king of 'debouncing'? Does 'bouncing' occur in non-mechanical switching circuits as well? What could be the possible reason from an electronics point of view?
It might be prudent to add that the TimeQuest Timing analysis fails in both cases with critical warning: 'Timing Requirements are not met'. But still, one works where other does not.
I am guessing this bizarre behavior has to do with a race condition caused when a sequence of bits drastically changes its 1's and 0's after an increment. For instance, 110111 becoming 111000 after increment-by-1. Since there is no telling which of the flip-flops - storing the individual bit positions in the sequence - will update their values first, the number (read: bit sequence) might fluctuate for an instant before attaining a stable value. There might exist numbers (bit-sequences) that, while tending towards their stable values, fluctuate through the value being compared (24999999, in our case) and cause the comparator to output Logic-HI. A register at the end would certainly solve this issue. However, all this is guess work, with bases in little experimentation. Any learned opinion would be welcome.