Removing the need to reset the device before using it - simulation

I'm having trouble implementing a controller block for an 8-bit multiplier. It works normally, but only if I turn the reset wire on, then off, such as in the following stimulus (which works fine):
`timescale 1ns / 100ps
module Controller_tb(
);
reg reset;
reg START;
reg clk;
reg LSB;
wire STOP;
wire ADD_cmd;
wire SHIFT_cmd;
wire LOAD_cmd;
Controller dut (.reset(reset),
.START(START),
.clk(clk),
.LSB(LSB),
.STOP(STOP),
.ADD_cmd(ADD_cmd),
.SHIFT_cmd(SHIFT_cmd),
.LOAD_cmd(LOAD_cmd)
);
always
begin
clk <= 0;
#25;
clk <= 1;
#25;
end
initial
begin
LSB <= 0;
START <= 0;
reset <= 1;
#55;
reset <= 0;
#10;
START <= 1;
#100;
START <= 0;
LSB <= 1;
#200;
#20;
#100;
end
initial
$monitor ("stop,shift_cmd,load_cmd, add_cmd: " , STOP,SHIFT_cmd,LOAD_cmd,ADD_cmd);
endmodule
Here's the simulation result for the working stimulus:
Now, when I set the reset to zero, without ever bringing it high, here's what happens:
Clearly, I'm using the reset wire to bring my Controller to the IDLE state. Here's the code for the controller block:
`timescale 1ns / 1ps
module Controller(
input reset,
input START,
output STOP,
input clk,
input LSB,
output ADD_cmd,
output SHIFT_cmd,
output LOAD_cmd
);
//Five states:
//IDLE : 000 , INIT: 001, TEST: 011, ADD: 010, SHIFT: 110
localparam [2:0] S_IDLE = 0;
localparam [2:0] S_INIT = 1;
localparam [2:0] S_TEST = 2;
localparam [2:0] S_ADD = 3;
localparam [2:0] S_SHIFT = 4;
reg [2:0] state,next_state;
reg [3:0] count;
// didn't assign the outputs to wire.. if not work, check this.
assign ADD_cmd = (state == S_ADD);
assign SHIFT_cmd = (state == S_SHIFT);
assign LOAD_cmd = (state == S_INIT);
assign STOP = (state == S_IDLE);
always #(*) begin
case(state)
S_INIT: begin
count = 3'b000;
end
S_SHIFT: begin
count = count + 1;
end
endcase
end
always #(*)
begin
next_state = state;
case (state)
S_IDLE: next_state = START ? S_INIT : S_IDLE;
S_INIT: next_state = S_TEST;
S_TEST: next_state = LSB ? S_ADD : S_SHIFT;
S_ADD: next_state = S_SHIFT;
S_SHIFT: next_state = (count == 8) ? S_IDLE : S_TEST;
endcase
end
always #(posedge clk)
begin
//state <= S_IDLE;
if(reset) state <= S_IDLE;
else state <= next_state;
end
reg [8*6-1:0] statename;
always #* begin
case( state )
S_IDLE: statename <= "IDLE";
S_INIT: statename <= "INIT";
S_TEST: statename <= "TEST";
S_ADD: statename <= "ADD";
S_SHIFT: statename <= "SHIFT";
default: statename <= "???";
endcase
end
endmodule
I don't know how to fix this. As you can see from the code above, there is a commented portion which is basically always initializing the state to IDLE. But even that doesn't work. Here's the simulation for the code above removing the comment from '//state <= S_IDLE;':
It's going into a different state than any listed above, and I have no idea why.
So I'd like to know:
Why is it going into an unknown state? Why doesn't my uncommented code work?
What can I change for it to work as I intend?

Your problem is that without a reset or initial value, state and next_state will be X. Your case statement assigning to statename will take the default branch and decode to ???. Since your process that assigns next_state does not handle cases where state is X it will get stuck in this state forever.
Your attempt to fix this will not work:
state <= S_IDLE;
if(reset) state <= S_IDLE;
else state <= next_state;
When reset is low you are making two assignments to state, the first as S_IDLE and the second as next_state. This is not a race condition. The Verilog standard states that:
Nonblocking assignments shall be performed in the order the statements were executed.
Since no re-ordering of the event queue occurs for sequential statements within a process this translates to last assignment wins. Therefore your state <= S_IDLE; is effectively optimised away since regardless of the value of reset the assignment will be overridden.
There are two ways you could fix this so that you don't need a reset:
1. Use the default clause to make your state machine safe
always #(*)
begin
next_state = state;
case (state)
S_IDLE: next_state = START ? S_INIT : S_IDLE;
S_INIT: next_state = S_TEST;
S_TEST: next_state = LSB ? S_ADD : S_SHIFT;
S_ADD: next_state = S_SHIFT;
S_SHIFT: next_state = (count == 8) ? S_IDLE : S_TEST;
default: next_state = S_IDLE;
endcase
end
This will ensure that your state-machine is 'safe' and drops into S_IDLE if state is a non-encoded value (including X).
2. Initialise the variable
reg [2:0] state = S_IDLE;
For some synthesis targets (e.g. FPGAs) this will initialise the register to a specific value and can be used alongside or instead of a reset (see Altera Documentation on power-up values).
A couple of general points:
Depending on your synthesis tool it may be better to use an enumeration rather than explicitly defining values for your states. This allows the tool to optimise based on the overall design or use a global configuration for encodings (for example safe, one-hot).
Using a reset registers holding state is standard practice so you should carefully consider whether you really want to avoid using a reset.

The uncommented code is an example of poor coding practice because you are making 2 nonblocking assignments to state in the same timestep. Synthesis linting tools are likely to warn you of this situation.
Since using a reset is a common, good practice, I don't think you need to fix anything.

Related

NBA assignment of $urandom

Can $urandom be NBA assigned in a for loop to an unpacked array of variables?
module tb();
logic clk [2];
initial clk[0] = 0;
always clk[0] = #1ns !clk[0];
for (genvar i = 1; i < 2; i++)
assign #(1ns/2) clk[i] = clk[i-1];
int tmp [2] [8];
always # (posedge clk[0]) begin
foreach (tmp[0][i]) begin
/*int m;
m = $urandom(); // SECTION 1 - using this code works (commenting out SECTION 2)
tmp[0][i] <= m;*/
tmp[0][i] <= $urandom(); // SECTION 2
end
#1ns;
foreach (tmp[0][i]) begin
$display("%1d", tmp[0][i]);
end
$finish();
end
for (genvar i = 1; i < 2; i++) begin
always_ff # (posedge clk[i]) begin
tmp[i] <= tmp[i-1]; // SECTION 3 (just removing this works too)
end
end
endmodule
Using Cadence tools (xrun 17.09-v002), I get all 8 of tmp[0] ints assigned the same value.
-2147414528
-2147414528
-2147414528
-2147414528
-2147414528
-2147414528
-2147414528
-2147414528
Can someone confirm whether this code is legal?
I have spoken to Cadence and been told this:
R&D’s response.
This use model of having $urandom call inside a non-blocking assignment is wrong.
The scheduling semantics of System Verilog dictates that the RHS is calculated and sampled once in the "inactive region" and then in the "NBA region" it's assigned the ALL of the elements of the foreach at the same time!
There is no difference in calling $urandom in a procedural loop versus serially calling $urandom multiple times. Your code gives the desired results in several tools, including Cadence's on EDAPlayground.com. Perhaps you are not showing is part of your problem. It always helps to show an MCVE, like
module top;
int tmp [2] [8];
bit clk;
initial begin
#1 clk=1;
#1 $display("%p",
tmp[0]);
end
always # (posedge clk) begin
foreach (tmp[,i]) begin
tmp[0][i] <= $urandom();
end
end
endmodule

how to delay the beginning of a clock?

Using system verilog and new to it and to verilog,
I want to delay the start of clock by 46.666ns in a testbench.
for this I declared another signal, toggled it to 1 after 46.666 and gated my clock with it. however it is not working, and I don't understand why. any help would be very appreciated.
the code I am using:
// generate CLKXI and inject to vt
logic clk = 1;
logic clkstart = 0;
initial begin
#46.666ns clkstart = 1;
end
always
begin
if (clkstart && ~clk) #21.25ns clk = ~clk;
else if (clkstart && clk) #20.416ns clk = ~clk;
end
assign test_wrapper.dut_top.CLKXI = clk;
The problem is when clkstart is 0, your always block gets into a zero-delay infinite loop, and time cannot advance. I think what you want is
initial begin
#46.666ns
forever begin
#21.25ns clk = 0;
#20.416ns clk = 1;
end

Good Counter Design or Possible Metastability Issues?

Quick summary of my goal:
Design a counter triggered by a variable length auto-reload timer.
A little more verbose: There will be a register with a value that changes (predictably changes, and is latched before the EN signal for the AutoReloadTimer module) that sets the rate at which the counter increments.
Here's the auto-reload timer:
module AutoReloadTimer( clk, rst, EN, D, done );
input clk;
input rst;
input EN;
input [WIDTH-1:0] D;
output done;
parameter WIDTH = 8;
// OneShot EN -> load
wire load;
OneShotD OneShot_D(
.clk( clk ),
.rst( rst ),
.in( EN ),
.RE( load )
);
reg [WIDTH-1:0] counter, load_value;
always #( posedge clk ) begin
if ( rst ) begin
counter <= {WIDTH{1'b1}};
load_value <= {WIDTH{1'b1}};
end else if ( load ) begin
counter <= D;
load_value <= D;
end else if (counter == 0 ) begin
counter <= load_value;
load_value <= load_value;
end else begin
counter <= counter - 1'b1;
load_value <= load_value;
end
end
assign done = ( counter == 0 );
endmodule
And here is the counter triggered by the done signal from the AutoReloadTimer module:
module Counter( clk, rst, EN, CLR, Q );
input clk;
input rst;
input EN;
input CLR;
output [WIDTH-1:0] Q;
parameter WIDTH = 8;
reg [WIDTH-1:0] ctr;
always #( posedge clk ) begin
if ( rst ) begin
ctr <= {WIDTH{1'b0}};
end else if ( CLR ) begin
ctr <= {WIDTH{1'b0}};
end else if ( EN ) begin
ctr <= ctr + 1'b1;
end else begin
ctr <= ctr;
end
end
assign Q = ctr;
endmodule
And here is a portion of the waveform from a testbench:
What I'm curious about here is my counter's stability - is it an issue that the done signal goes low at the rising edge of the clock? I'm still fairly new to Verilog and digital design. I'm familiar with the term and somewhat the idea of metastability but I'm not fully comfortable with my understanding of it.
Looking for input, criticism, etc.
Edit
I forgot to include what configuration I had the modules in to produce that diagram:
wire ART_done;
AutoReloadTimer ART0 (
.clk( clk ),
.rst( rst ),
.EN( EN ),
.D( 4 ),
.done( ART_done )
);
Counter uut (
.clk(clk),
.rst(rst),
.EN(ART_done),
.CLR(CLR),
.Q(Q)
);
As long as your AutoReloadTimer and Counter modules, as well as any logic that uses the done signal are on the same clock, you won't have any metastability issues. What you would have is a fully synchronous implementation. Naturally, you must also meet the timing requirements of the device your using
The done signal will actually change some small combinatorial path delay after the rising clock edge that causes the counter to hit 0. Any logic that uses the done signal has the rest of the clock period before the next rising edge, to do what it needs to do (more combinatorial logic) and still meet the setup time of any register input that is conditioned by the done signal.
The metastability issues will only arise if the input to any registers are transitioning right as the clock is transitioning. This can happen if the data that's being registered is coming from a register that uses an asynchronous clock, or if the register's setup or hold timing is violated.

Verilog testbench design for my MSB downsampling module

A couple of days ago I asked about a module (here) I wanted to implement which takes the MSB of input samples, accumulates them (by shifting) and combines them into the output sample when the 32 output bit is "filled".
Thanks to the help there, I got this implementation, which doesn't produce any compilation errors and seemed fine with Xilinx 12.1:
module my_rx_dsp0
#(
//frontend bus width
parameter WIDTH = 24
)
(
//control signals
input clock, //dsp clock
input reset, //active high synchronous reset
input clear, //active high on packet control init
input enable, //active high when streaming enabled
//user settings bus, controlled through user setting regs API
input set_stb, input [7:0] set_addr, input [31:0] set_data,
//full rate inputs directly from the RX frontend
input [WIDTH-1:0] frontend_i,
input [WIDTH-1:0] frontend_q,
//full rate outputs directly to the DDC chain
output [WIDTH-1:0] ddc_in_i,
output [WIDTH-1:0] ddc_in_q,
//strobed samples {I16,Q16} from the RX DDC chain
input [31:0] ddc_out_sample,
input ddc_out_strobe, //high on valid sample
output ddc_out_enable, //enables DDC module
//strobbed baseband samples {I16,Q16} from this module
output reg [31:0] bb_sample,
output reg bb_strobe //high on valid sample
);
reg [3:0] i_msb;
reg [3:0] q_msb;
reg [31:0] temp_buff = 0;
reg [1:0] count = 0;
always #(posedge clock) begin
if(ddc_out_strobe) begin
// bit shifter for MSB
temp_buff <= {i_msb,q_msb,temp_buff[31:8]};
// to avoid if-else condition
count <= (count==2'd3) ? 2'd0 : (count+1);
end
end
always #(*) begin
i_msb = ddc_out_sample[31:28];
q_msb = ddc_out_sample[15:12];
// to avoid if-else condition
bb_strobe = (count==2'd3);
bb_sample = bb_strobe ? temp_buff : 32'd0;
end
assign ddc_in_i = frontend_i;
assign ddc_in_q = frontend_q;
assign ddc_out_enable = enable;
endmodule //my_rx_dsp0_custom
Now I wanted to implement a testbench that tests my_rx_dsp0.v with some examples.
I implemented a my_rx_dsp0_tb_2.v, which reads 32 bit samples from a file named my_input.dat to feed to the module as inputs ddc_out_sample.
They are then compared to the correct values stored at my_output.dat.
Note: I did not write this testbench myself, I adapted it from another testbench from an open-source project.
Here is the implementation:
module my_rx_dsp0_tb ( );
reg clk;
reg reset;
reg enable;
reg ddc_out_strobe; //high on valid sample
reg [31:0] ddc_out_sample;
wire [31:0] bb_sample = 32'd0;
wire bb_strobe;
wire ddc_out_enable = 1'b1; //enables DDC module
parameter WIDTH = 24;
parameter clocks = 2; // number of clocks per input
reg endofsim = 0;
integer number_of_errors;
initial number_of_errors = 0;
wire set_stb = 1;
wire [7:0] set_addr;
wire [31:0] set_data;
wire [WIDTH-1:0] frontend_i;
wire [WIDTH-1:0] frontend_q;
wire [WIDTH-1:0] ddc_in_i;
wire [WIDTH-1:0] ddc_in_q;
reg signed [31:0] compare_out;
// Setup the clock
initial clk = 1'b0;
always #5 clk <= ~clk ;
// Come out of reset after a while
initial reset = 1'b1 ;
initial #1000 reset = 1'b0 ;
// Enable the entire system
initial enable = 1'b1 ;
// Instantiate UUT
my_rx_dsp0 #(.WIDTH(WIDTH)) UUT_rx_dsp0
( .clock(clk), .reset(reset), .clear(clear), .enable(enable),
.set_stb(set_stb), .set_addr(set_addr), .set_data(set_data),
.frontend_i(frontend_i), .frontend_q(frontend_q),
.ddc_in_i(ddc_in_i), .ddc_in_q(ddc_in_q),
.ddc_out_sample(ddc_out_sample), .ddc_out_strobe(ddc_out_strobe), .ddc_out_enable(ddc_out_enable),
.bb_sample(bb_sample), .bb_strobe(bb_strobe) );
//-------Setup file IO-------//
//
integer i, r_in, r_out, infile, outfile;
initial begin
infile = $fopen("my_input.dat","r");
outfile = $fopen("my_output.dat","r");
$timeformat(-9, 2, " ns", 10) ;
// for n=9,p=2 digits after decimal pointer
//min_field_width=10 number of character positions for %t
end
//-------Get sim values and display errors-------//
//
initial begin
// Initialize inputs
ddc_out_strobe <= 1'd0;
ddc_out_sample <= 32'd0;
// Wait for reset to go away
#(negedge reset) #0;
while(!endofsim) begin
// Write the input from the file or 0 if EndOfFile(EOF)
#(posedge clk) begin
#1
ddc_out_strobe <= 1'b1;
if(!$feof(infile))
r_in = $fscanf(infile,"%b\n",ddc_out_sample);
else
ddc_out_sample <= 32'd0;
end
//
// Clocked in; set the strobe to 0 if the # of clocks/sample
// is greater than 1
if( clocks > 1 ) begin
#(posedge clk) begin
ddc_out_strobe <= 1'b0 ;
end
// Wait for the specified # of cycles
for( i = 0 ; i < (clocks-2) ; i = i + 1 ) begin
#(posedge clk) #1 ;
end
end
//
//
// Print out the number of errors that occured
if(number_of_errors) begin
$display("FAILED: %d errors during simulation",number_of_errors) ;
end else begin
$display("PASSED: Simulation successful") ;
end
//
end
end
//-------Comparison btwn simulated values vs known good values-------//
//
always #(posedge clk) begin
if(reset)
endofsim <= 1'b0 ;
else begin
if(!$feof(outfile)) begin
if(bb_strobe) begin
r_out = $fscanf(outfile,"%b\n",compare_out);
if(compare_out != bb_sample) begin
$display("%t: %b != %b",$realtime,bb_sample,compare_out);
number_of_errors = number_of_errors + 1;
end else begin
$display("%t: %b = %b",$realtime,bb_sample,compare_out);
end
end
end else begin
// Signal end of simulation when no more outputs
endofsim <= 1'b1 ;
end
end
end
endmodule // my_rx_dsp0_tb
When simulating with ISim from Xilinx ISE Suite Edition 12.1 I do not get the desired functionality from the module. I am afraid the output contains several x states (unknown states), instead of 1s or 0s as expected.
Question Is this due to:
1) The way the files are being read with $fscanf?
2) Did I wrong by initializing reg [31:0] temp_buff = 0?
3) Or does someone have an idea on what went wrong?
The error prompts from the testbench are (as an example):
xx000x00xxx00x0xx000x0x000000000 != 10000110111001011100010001101100
The X is from having multiple conflicting drivers on bb_sample and ddc_out_enable. The wire type merges the drivers, conflicting bit values of the same strength resolve as X.
UUT_rx_dsp0 is the intended diver. However you added and additional drivers from the way you declared your wires.
...
wire [31:0] bb_sample = 32'd0; // "= 32'd0" is a continuous driver
wire bb_strobe;
wire ddc_out_enable = 1'b1; // "= 1'd1" is a continuous driver
...
What you want is:
...
wire [31:0] bb_sample;
wire bb_strobe;
wire ddc_out_enable;
...
Correcting the above will resolve the X issue. Based on the example error it looks like are data miss matches. With the provided information, it is hard to tell it if it a test-bench or design issue. Could be just clock or propagation skew.

ONE clock period pulse based on trigger signal

i am making a midi interface. UART works fine, it sends the 8 bit message along with a flag to a control unit. When the flag goes high, the unit will store the message in a register and make a clr_flag high in order to set the flag of UART low again. The problem is that i can not make this clr_flag one period long. I need it to be ONE period long, because this signal also controls a state machine that indicates what kind of message is being stored (note_on -> key_note -> velocity, for example).
My question here is, how can a signal (flag in this case) trigger a pulse just for one clk period? what i have now makes almost a pulse during a clock period, but i does it twice, because the flag has not become 0 yet. ive tried many ways and now i have this:
get_data:process(clk, flag)
begin
if reset = '1' then
midi <= (others => '0');
clr_flag <= '0';
control_flag <= '0';
elsif ((clk'event and clk='1') and flag = '1') then
midi <= data_in;
clr_flag <= '1';
control_flag <= '1';
elsif((clk'event and clk='0') and control_flag = '1') then
control_flag <= '0';
elsif((clk'event and clk='1') and control_flag = '0') then
clr_flag <= '0';
end if;
end process;
the problem with this double pulse or longer than one period pulse(before this, i had something that made clr_flag a two period clk pulse), is that the system will go though two states instead of one per flag.
so in short: when one signal goes high (independent of when it goes low), a pulse during one clock period should be generated.
thanks for your help.
The trick to making a single cycle pulse is realising that having made the pulse, you have to wait as long as the trigger input is high before getting back to the start. Essentially you are building a very simple state machine, but with only 2 states you can use a simple boolean to tell them apart.
Morten is correct about the need to adopt one of the standard patterns for a clocked process; I have chosen a different one that works equally well.
get_data:process(clk, reset)
variable idle : boolean;
begin
if reset = '1' then
idle := true;
elsif rising_edge(clk) then
clr_flag <= '0'; -- default action
if idle then
if flag = '1' then
clr_flag <= '1'; -- overrides default FOR THIS CYCLE ONLY
idle <= false;
end if;
else
if flag = '0' then
idle := true;
end if;
end if;
end if;
end process;
There are several issues to address in order to make the design for a one cycle
pulse using flip flops (registers).
First, the use of flip flops in hardware through VHDL constructions typically
follows a structure like:
process (clk, reset) is
begin
-- Clock
if rising_edge(clk) then
-- ... Flip flops to update at rising edge
end if;
-- Reset
if reset = '1' then
-- Flip flops to update at reset, which need not be all
end if;
end process;
So the get_data process should be updated accordingly, thus:
Sensitivity list should contain only clock (clk) and reset
The nested structure with if on event should be as above
Only rising edge of clk should be used, thus no check on clk = '0'
Making a one cycle pulse on clr_flag when flag goes high can be made with a
synchronous '0' to '1' detector on flag, using a version of flag that is
delayed a single cycle, called flag_ff below, and then checking for (flag =
''1) and (flag_ff = '0').
The resulting code may then look like:
get_data : process (clk, reset) is
begin
-- Clock
if rising_edge(clk) then
flag_ff <= flag; -- One cycle delayed version
clr_flag <= '0'; -- Default value with no clear
if (flag = '1') and (flag_ff = '0') then -- Detected flag going from '0' to '1'
midi <= data_in;
clr_flag <= '1'; -- Override default value making clr_flag asserted signle cycle
end if;
end if;
-- Reset
if reset = '1' then
midi <= (others => '0');
clr_flag <= '0';
-- No need to reset flag_ff, since that is updated during reset anyway
end if;
end process;
Synchronisation and Edge Detection for FSM
The Rise, Edge and Fall outputs will strobe for one cycle when those events are detected. Inputs and outputs are synchronised for use with a Finite State Machine.
entity SynchroniserBit is
generic
(
REG_SIZE: natural := 3 -- Default number of bits in sync register.
);
port
(
clock: in std_logic;
reset: in std_logic;
async_in: in std_logic := '0';
sync_out: out std_logic := '0';
rise_out: out std_logic := '0';
fall_out: out std_logic := '0';
edge_out: out std_logic := '0'
);
end;
architecture V1 of SynchroniserBit is
constant MSB: natural := REG_SIZE - 1;
signal sync_reg: std_logic_vector(MSB downto 0) := (others => '0');
alias sync_in: std_logic is sync_reg(MSB);
signal rise, fall, edge, previous_sync_in: std_logic := '0';
begin
assert(REG_SIZE >= 2) report "REG_SIZE should be >= 2." severity error;
process (clock, reset)
begin
if reset then
sync_reg <= (others => '0');
previous_sync_in <= '0';
rise_out <= '0';
fall_out <= '0';
edge_out <= '0';
sync_out <= '0';
elsif rising_edge(clock) then
sync_reg <= sync_reg(MSB - 1 downto 0) & async_in;
previous_sync_in <= sync_in;
rise_out <= rise;
fall_out <= fall;
edge_out <= edge;
sync_out <= sync_in;
end if;
end process;
rise <= not previous_sync_in and sync_in;
fall <= previous_sync_in and not sync_in;
edge <= previous_sync_in xor sync_in;
end;
Below is a way of creating a signal (flag2) that lasts exactly one clock period from a signal (flag1) that lasts at least one clock period.
I don't program in VHDL~ here is what I usually do for the same propose in Verilog:
always #(posedge clk or negedge rst) begin
if(~rst) flgD <= 1'b0;
else flgD <= flg;
end
assign trg = (flg^flgD)&flgD;
I am new to verilog and this is the sample code, which I used for triggering. Hope this serves your purpose. You can try same logic in VHDL.
module main(clk,busy,rd);
input clk,busy; // busy input condition
output rd; // trigger signal
reg rd,en;
always #(posedge clk)
begin
if(busy == 1)
begin
rd <= 0;
en <= 0;
end
else
begin
if (en == 0 )
begin
rd <= 1;
en <= 1;
end
else
rd <= 0;
end
end
endmodule
The below verilog code shall hold the value for the signals for one clock cycle exactly.
module PulseGen #(
parameter integer BUS_WIDTH = 32
)
(
input [BUS_WIDTH-1:0] i,
input clk,
output [BUS_WIDTH-1:0] o
);
reg [BUS_WIDTH-1:0] id_1 = 0 ;
reg [BUS_WIDTH-1:0] id_2 = 0 ;
always #(posedge clk)begin
id_1 <= i;
id_2 <= id_1;
end
assign o = (id_1 & ~id_2);
The way to achieve this is to create a debounce circuit. If you need a D flip-flop to change from 0 to 1, only for the first clock, just add an AND gate before its input like the image below:
So here you can see a D flip-flop and its debounce circuit.
P.S. Circuit created using this.
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--input of minimum 1 clock pulse will give output of wanted length.
--load number 5 to PL input and you will get a 5 clock pulse no matter how long input is.
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library ieee ;
use ieee.std_logic_1164.all ;
use ieee.std_logic_unsigned.all ;
entity fifth is
port (clk , resetN : in std_logic;
pdata : in integer range 0 to 5; --parallel data in. to choose how many clock the out pulse would be.
din : in std_logic;
dout : out std_logic
) ;
end fifth ;
architecture arc_fifth of fifth is
signal count : integer range 0 to 5;
signal pl : std_logic; --trigger detect output.
signal sample1 : std_logic;
signal sample2 : std_logic;
--trigger sync proccess.
begin
process(clk , resetN)
begin
if resetN = '0' then
sample1<='0';
sample2<='0';
elsif rising_edge(clk) then
sample1<=din;
sample2<=sample1;
end if;
end process;
pl <= sample1 and (not sample2); --trigger detect output. activate the counter.
--counter proccess.
process ( clk , resetN )
begin
if resetN = '0' then
count <= 0 ;
elsif rising_edge(clk) then
if pl='1' then
count<=pdata;
else
if count=0 then
count<=count;
else
count<=count-1;
end if;
end if;
end if ;
end process ;
dout<='1' when count>0 else '0';--output - get the wanted lenght pulse no matter how long is input
end arc_fifth ;