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Help on understanding HDL module "always" clock transitions

FredcwGO_BR
Beginner
388 Views
Howdy all,
 
I'm a newbee on FPGA, and needing to use QuartusII 13.0.1 for this is the EPM7128 board that i have.
 
The following  code compiles but simulation does not show what i expected. 
In the simulation for  50uS , feeding 
  • clk - 10ns/50% 
  • baudSelector  - whith  whatever value from 3'b000 - 3'b111

The attached simulation of two cases demonstrates. 

Out signal (which drives de board led) is the one i'm interested on; it should be the same frequency for whatever baudSelector input.
 
I expected that the "if xcount2[] == " (line 19)  guard condition, would only fire
at every time the xcount2 counter goes thru it at that count, but it looks that fires along with the clkSTB. 
 
Can't figure what i'm doing wrong. 
 
Quartus II Web Edition  13.0.1  /  13.1sp1
Debian (bookworm) Linux  64bit
Target devices: EPM7128 / EPM240T100
 

 

module counter(clk, out , count, baudSelector , clkSTB, srst, xcount2 );

  input [2:0] baudSelector;
  input srst;
  input clk;
  output reg out;
  output reg[7:0] count;
  output reg[14:0] xcount2;
  output reg clkSTB;

  initial begin
     count = 0;
	  xcount2 = 0;
	  clkSTB = 0;
	  out = 0;
  end

  always @(posedge clk) begin 
  	if( xcount2[10:0] == 11'd433 ) 
	begin
		count <= count + 1; 
	end
	out <= count[4];
  end	
  
  always @(posedge clk) begin 
  	if( srst ) 
	begin
		{ clkSTB , xcount2 } <= 16'd0;
	end

	//  --  115200 ;; 50000000/115200 -> 434
	//  --  57600 ;; 50000000/57600 -> 868
	//  --  38400 ;; 50000000/38400 -> 1302
	//  --  19200 ;; 50000000/19200 -> 2604
	//  --  INVALIDOS :: default 9600 ;; 50000000/9600 -> 5208
	//
	//  3'b000 : { clkSTB, xcount2 } <= xcount2 + 16'd434 ;
	//  3'b001 : { clkSTB, xcount2 } <= xcount2 + 16'd868 ;
	//  3'b010 : { clkSTB, xcount2 } <= xcount2 + 16'd1302 ;
	//  3'b011 : { clkSTB, xcount2 } <= xcount2 + 16'd2604 ;
	//  default: { clkSTB, xcount2 } <= xcount2 + 16'd5208 ;
	//	
	case(baudSelector)
	  3'b000 : begin
			if( xcount2 >= 15'd434 ) 
			begin
				clkSTB <= 1;
				xcount2 <= 0;
			end
			else
			begin
				clkSTB <= 0;
				xcount2 <= xcount2 + 1;
			end				
	  end	// { clkSTB, xcount2 } <= xcount2 + 16'd434 ;
	  3'b001 :
	 			if( xcount2 >= 15'd868 ) 
			begin
				clkSTB <= 1;
				xcount2 <= 0;
			end
			else
			begin
				clkSTB <= 0;
				xcount2 <= xcount2 + 1;
			end
	  3'b010 : 
			if( xcount2 >= 15'd1302 ) 
			begin
				clkSTB <= 1;
				xcount2 <= 0;
			end
			else
			begin
				clkSTB <= 0;
				xcount2 <= xcount2 + 1;
			end
	  3'b011 : 
  			if( xcount2 >= 15'd2604 ) 
			begin
				clkSTB <= 1;
				xcount2 <= 0;
			end
			else
			begin
				clkSTB <= 0;
				xcount2 <= xcount2 + 1;
			end
	  default: 
  			if( xcount2 >= 15'd5208 ) 
			begin
				clkSTB <= 1;
				xcount2 <= 0;
			end
			else
			begin
				clkSTB <= 0;
				xcount2 <= xcount2 + 1;
			end
	endcase
	
  end
  
endmodule    

 

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1 Solution
FvM
Valued Contributor III
338 Views

Hi,

it's the design concept.

The condition if( xcount2[10:0] == 11'd433 ) can't produce constant frequency, evenly spaced events when applied to the 15 bit baud rate counter.

You can however easily achieve waht you want by slightly rearrangind the design:

1. have a 115k2 tic counter producing you constant frequency tic.
2. divide the 115k2 tic down according to select baud rate, fortunately they involve integer frequency ratios.

Your time constants are 1 clock period off, by the way.

View solution in original post

3 Replies
sstrell
Honored Contributor III
367 Views

What you're looking for isn't super clear, but it sounds like the "out" assignment is supposed to be between begin/end because otherwise, out is just count[4] every clock cycle.  And if you do that, you should have a concluding else clause:

 

 

always @(posedge clk) begin 
  	if( xcount2[10:0] == 11'd433 ) 
	begin
		count <= count + 1; 
                out <= count[4];
	end
        else...
  end	

 

 

TingJiangT_Intel
Employee
352 Views

We recommend you to assign the initial value via 'always' block with reset signal instead of 'initial'. As 'initial' is unsynthesizable.


FvM
Valued Contributor III
339 Views

Hi,

it's the design concept.

The condition if( xcount2[10:0] == 11'd433 ) can't produce constant frequency, evenly spaced events when applied to the 15 bit baud rate counter.

You can however easily achieve waht you want by slightly rearrangind the design:

1. have a 115k2 tic counter producing you constant frequency tic.
2. divide the 115k2 tic down according to select baud rate, fortunately they involve integer frequency ratios.

Your time constants are 1 clock period off, by the way.

Reply