GPIO output on DE2_115

Question

I have a DE2-115 board and i'm trying to do some basic functions.

I want provide a high/low input using a switch on the board,and read this high/low input as output onto a GPIO pin. i.e. whenever I 'ON' the switch, '1' is supposed to appear at the GPIO pin,and whenever the switch is 'off' the GPIO should provide '0'.

This is the corresponding verilog code:

module dswitch(SW,CLOCK_50,GPIO);

input [17:0]SW;

input CLOCK_50;

output reg[35:0]GPIO;

always @(posedge CLOCK_50)

begin

GPIO[31:1]= 32'hz;

if(SW[0]==1)

GPIO[0]=1'b1;

else

GPIO[0]=1'b0;

end

endmodule

all the pins are assigned correctly using the pin editor.

The code compiles fine.

But,when i try to read the output from the GPIO[0] pin, on the board, onto a scope,there is no change in activity when the switch is pushed on or off. I am unable to figure out what is wrong.

Am I missing something?Is there any thing else that needs to be done or any other connections to be given?

Also,what would be a good ground on the fpga board that could be used to connect the ground pin of the scope to?

Thanks!

altera_forum · Answer

Your code works (unchanged) on my DE2-115. So, I think you need to look at for something else wrong in your project settings.  However, whilst we're here, a few comments on your code. You are loading registers GPIO[31:1] with 'z'. Ask yourself whether a register (a real one, in a device) can be loaded with a 'z'. How Quartus (or other vendor tools) interprets your statement may not be well defined. So, different tools may produce different results - not a good thing. As you will see from the compilation report the number of registers used for your design is 1, even though you've declared and inferred 36 in your code.  Loading registers with 'z' can be a useful tool when simulating - it allows you to follow a particular path through a design - very useful for fault finding. However, a real register, in a device, can only be loaded with a '0' or '1'.  Secondly, a minor point, you've declared GPIO[35:0] in your port list but only coded for GPIO[31:0]. GPIO[35:32] are left wanting...  I suggest you trim (tidy) your code to this: module dswitch (SW,CLOCK_50,GPIO);
input  wire  SW;
input  wire        CLOCK_50;
output reg    GPIO;
always @(posedge CLOCK_50)
begin
if(SW==1)
   GPIO=1'b1;
else 
   GPIO=1'b0;
end
endmodule ...or if you must tri-state the remaining GPIO... module dswitch (SW,CLOCK_50,GPIO);
input  wire  SW;
input  wire        CLOCK_50;
output wire  GPIO;
reg GPIO_reg;
always @(posedge CLOCK_50)
begin
if(SW==1)
   GPIO_reg=1'b1;
else 
   GPIO_reg=1'b0;
end
assign GPIO=GPIO_reg;
assign GPIO=35'hz;
endmodule  Cheers, Alex

altera_forum · Answer

Thanks for your reply Alex! I will clean up the code.

Did you try looking at the GPIO output by reading it onto a scope? The reason I am asking this is,when I connect the GPIO output to an LED onto the board,to check whether the output is really coming across the GPIO line,the LED works correctly according to the on or off of the switch.

But it doesn't show any change in the signal when connected to an oscilloscope. I wanted to know what could be the reason for this. Would be great if you could help me on that!

Thanks,

Ankita

altera_forum · Answer

Hi Ankita,

have you applied voltage settings on your clock_50 signal in assignment editor ? Try giving the voltage if you havent. This will give you the correct pin names and their voltage level -

ftp://ftp.altera.com/up/pub/altera_material/12.1/boards/de2-115/de2_115.qsf

Also, according to what you want to do, why not use assign statement and remove the clock altogether ? unless you have further code and this is just a small part.

altera_forum · Answer

I assume you did the correct pin assignments?

altera_forum · Answer

Hi Ankita,

I checked my GPIO pin with a DVM (rather than a scope). However, I'm confident it worked.

If you can drive an LED directly from the same GPIO pin (that's what you tired - yes?) then you can be confident the logic level is appearing on the GPIO pin. I'm now suspecting your scope setup if that's not showing the level change.

Cheers,

Alex

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GPIO output on DE2_115

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