At this point in my digital design adventure, I wanted to get my feet wet learning how to debug on live hardware. My goal was simple, I want to be able to interactively write to and read from a single register running on an implemented design. This post documents the process that can be followed to replicate my end results with Vivado and an Alpha-Data KU3 card.
I like to start learning new things with a fairly bare-bones approach. So my first step here is to create a new project in Vivado. I named my project ku3-peeking, chose
RTL Project for my project type and picked the FPGA part that’s on the KU3,
Adding some source
Next I’ll add a couple SystemVerilog files. One for my top level source and one for the module that I’ll be peeking and poking at through the debugger.
My test module is pretty simple, just a positive edge triggered register.
`timescale 1ns / 1ps module testmodule ( input clk, input data_in, output logic data_out ); always_ff @ (posedge clk) begin data_out <= data_in; end endmodule
My top module initially will just instantiate my
testmodule and a few wires to eventually hook everything up.
`timescale 1ns / 1ps module top (); wire clk; wire data_in; wire data_out; testmodule my_test ( .clk(clk), .data_in(data_in), .data_out(data_out) ); endmodule
By default Vivado will look for the ‘top’ module when building the project, so this is good for now.
Adding a debug core
With the basic design in place, you can pull up the IP Catalog and select from a few different debug cores. For my goal, the VIO core is nice as it’ll let me read and drive signals.
Double clicking on the VIO core will bring up a prompt of options that can be set for the debug core.
All of these defaults are fine for my case of a 1 bit register, but you can see you can easily add additional input and output probes. After hitting OK here you’ll get a prompt for starting an out-of-context synthesis run, which will synthesize the VIO core while you continue to work in Vivado.
Within the IP Sources tab, you can drill down into the hierarchy and find templates to instantiate this new core in your design files.
So I’ll modify my top file, to add a new block that wires the VIO core to the clock and my
vio_0 my_test_vio ( .clk(clk), .probe_in0(data_out), .probe_out0(data_in) );
For me, pulling in the clock for a legit hardware design was the most difficult part to figure out. I spelunked documentation and did much Googling, but eventually I reached out to some more experienced folks that helped lead me in the right direction.
The first step in this process is to check the users manual for the device at hand. In this case I’m looking to use the Fabric Clocks described in section 3.2.2 of Alpha Data’s ADM-PCIE-KU3 User Manual. The relevant bits here say there are 2 available fabric clocks, a 200MHz and a 250MHz clock. It lets me know that these pins use the LVDS I/O standard, what pins are available for each clock, and it also notes that I must set a constraint to set the
TERM_100 as there is a requirement that these clocks are terminated within the FPGA. I don’t fully comprehend what that means quite yet but the doc tells me to do it so I oblige.
To use the LVDS structures built into the FPGA, I need to instantiate a module that can take the LVDS input signals and provide a simple clock output. The module for this is called a Differential Input Buffer; it can be found in Vivado’s
Language Templates window.