Ok I must be an idiot :P

Hi Chris:

The problem is in the sensitivity list applied. You really want it evaluated on both rising and falling edges, and tell it to hold in the non-case.

However, that being said. it is much better to do a synchronous design as follows:

module updown (

input clk_i,

input reset_i,

input up_i,

input down_i,

output [11:0] counter_o

);

reg [2:0] up_s;

reg [2:0] down_s;

reg [11:0] counter_r;

wire up_rise_edge_p <= up_s[1] & ~up_s[2];

wire down_rise_edge_p <= down_s[1] & ~down_s[2];

always @(posedge clk_i)

begin

if (reset_i)

begin

counter_r <= 12'd250;

up_s <= 3'b111;

down_s <= 3'b111;

end

else

begin

up_s <= {up_s[1:0],up_i};

down_s <= {down_s[1:0],down_i};

if (up_rise_edge_p)

begin

if (down_rise_edge_p)

counter_r <= counter_r;

else

counter_r <= counter_r + 12'd3;

end

else

begin

if (down_rise_edge_p)

counter_r <= counter_r - 12'd3;

else

counter_r <= counter_r;

end

end

end

endmodule

This takes care of async up and down signals, but glitch eaters could also be added if necessary. This also takes care of a non-count case of both up and down being pressed at the same time.

Pete