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gwideman

Cmod A7 pio[discontinuous range of index!]: what to do?

Question

On Cmod A7, the xdc file lists pio[1] to pio[48], but there are gaps at 15,16 (which are instead analog input pins) and 24,25 (which don't exist).

So when I create a Verilog top module that uses pio, how to I specify these discontinuous ranges? Conceptually I want:

module top ( input sysclk, output [1:14, 17:23, 26:48] )  but obviously that's not valid syntax.

So how DO you use the full set of pio's as written in the xdc file?

The file in question:

https://raw.githubusercontent.com/Digilent/digilent-xdc/master/Cmod-A7-Master.xdc

Edited by gwideman
Added link to xdc file

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Hi,

I split it into pioA, pioB and pioC groups, as follows:

 

module top(input wire CLK12, 
	   output wire [1:0]  LED, 
	   output wire 	      RGB0_Red, 
	   output wire 	      RGB0_Green, 
	   output wire 	      RGB0_Blue, 
	   input wire [14:1]  pioA, // 14 
	   input wire [23:17] pioB, // 7
	   input wire [48:26] pioC, // 23
	   inout wire [7:0]   ja, // 8
	   input wire [1:0]   BTN, 
	   input wire [1:0]   xa_n, 
	   input wire [1:0]   xa_p);

and the constraints file:

...
set_property PACKAGE_PIN L1 [get_ports {pioA[13]}]
set_property IOSTANDARD LVCMOS33 [get_ports {pioA[13]}]
set_property PULLDOWN true [get_ports {pioA[13]}]
set_property PACKAGE_PIN L2 [get_ports {pioA[14]}]
set_property IOSTANDARD LVCMOS33 [get_ports {pioA[14]}]
set_property PULLDOWN true [get_ports {pioA[14]}]
set_property PACKAGE_PIN M1 [get_ports {pioB[17]}]
set_property IOSTANDARD LVCMOS33 [get_ports {pioB[17]}]
set_property PULLDOWN true [get_ports {pioB[17]}]
set_property PACKAGE_PIN N3 [get_ports {pioB[18]}]
set_property IOSTANDARD LVCMOS33 [get_ports {pioB[18]}]
set_property PULLDOWN true [get_ports {pioB[18]}]
...

Not pretty, but at least I don't need to renumber.

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Hi @xc6lx45,

Yeah, I ended up renaming and renumbering as well. I just figured if the xdc was supplied written as a discontinuous array, there must be some way to use it as such.

Graham

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One thing I realized in the course of mulling this question: Part of my early resistance to splitting pio[...] into pioA, pioB and pioC was that I wanted to implement a bus that would span those ranges, and had a mental block about how to achieve that.

It's not very difficult, but in case someone else has the same hiccup, I'll post an example solution here:

module top(
    input   sysclck,
    output  [1:0]   led,
    input  [14:1]   pioA,
    input  [23:17]  pioB,
    input  [48:26]  pioC
    );
    
    wire [8:1] mybus;    
    
    assign mybus[4:1] = pioA[14:11];
    assign mybus[8:5] = pioB[20:17];
    
    assign led[0] = &mybus;
    assign led[1] = &pioA[10:1]; 
    
endmodule

I should note in passing that the supplied xdc file has another problem.

In the supplied xdc file, pio[1]..pio[9] are written as pio[01]..pio[09], as in:

set_property -dict { PACKAGE_PIN M3    IOSTANDARD LVCMOS33 } [get_ports { pio[01] }]; #IO_L8N_T1_AD14N_35 Sch=pio[01]

The xdc file is a TCL language file, and it may not be evident what the { pio[01] } actually means. It looks like it would be an array pio, with index 01, which one might assume is an integer, and thus equivalent to plain 1.

In fact, the {...} is syntax to enclose a string, which is not to be interpolated (no substitutions made). So the result of { pio[01] } is the string:

" pio[01] "

... including the spaces!  In short, the string " pio[01] " is used verbatim as a string key for some array/hashtable/dictionary. It does not imply that there's an array called "pio", from which this code fetches the item at index 1.

But then how does this match up to a particular port object declared in module top's declaration (where pio[14:1] treats the numbers in the index range as integers).

There is some mechanism in get_ports or somewhere that must match this string index ("key") to top's port declaration. So the {...} string as written in the xdc file has to follow the pattern that the matching algorithm uses, which pio[01] does not, apparently.

Consequently, xdc's pio[01] fails to match top.v's pio[1], etc.  (And Vivado will give some obtuse error message about "Unspecified I/O Standard", and fails to generate bitstream.)

Instead, you have to edit the xdc file for all the cases where pio[xx] has a leading zero, and remove it. So, for example,  { pio[01] } should be edited to { pio[1] }. This does satisfy the matching algorithm.

For further illumination on this syntax, you might like to try the following code:

set result A
append result { pioA[01] }
append result B
puts $result

... at an online TCL interpreter, like: https://www.tutorialspoint.com/execute_tcl_online.php

This prints a result "A pioA[01] B", demonstrating that the action of the {...} syntax used in the xdc file is simply to produce a string, which is used verbatim as an argument to get_ports.

Edited by gwideman
Added TCL sample and link to TCL interpreter. Added more explanation.

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Hi,

so I'm not the only one who's fighting with the template constraints files. Slightly annoying, sends you searching in the wrong direction... Well, it's a problem I have avoided so far, not solved.

At least the error message can be explained: For un"constrained" ports, Vivado picks a random package pin, effectively suggests a floorplan. Strange feature, but it's there. Now it doesn't assign random IO standards, that would make little sense. The initial "DEFAULT" isn't valid. Therefore the error message

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