I am using a Xilinx Spartan 3E FPGA kit in my academic project to synthesize a design comprising of a couple of 32 bit internal registers (eg few counters, shifters and some configuration registers) . There is no direct access to the registers using the input/output ports of the design.

My design is in Verilog. Is there any way to probe the values written in the registers through Xilinx ISE tool?

Should I use chipscope for this purpose? For verification, I want to check the internal values in the registers.

I do not have JTAG cable, I am using USB connector to interface the kit with Xilinx ISE.

  • \$\begingroup\$ Does it not support JTAG? \$\endgroup\$ Commented Aug 4, 2017 at 7:45
  • \$\begingroup\$ Yes it supports JTAG but unfortunately I am limited to resources inside the lab and that only has USB connector. \$\endgroup\$ Commented Aug 4, 2017 at 7:49
  • \$\begingroup\$ JTAG is how it's done. \$\endgroup\$ Commented Aug 4, 2017 at 7:50
  • 1
    \$\begingroup\$ Isnt chipscope tool used for the same purpose? \$\endgroup\$ Commented Aug 4, 2017 at 7:50
  • 1
    \$\begingroup\$ The usb cable given in the kit can be used for chipscope debugging. \$\endgroup\$
    – samjay
    Commented Aug 11, 2017 at 6:55

1 Answer 1


One of the most useful ways to gain access to the internal register contents is to design it in!!

First you have to decide the most convenient mechanism with which to access the FPGA. You could use a SPI interface, UART, I2C ... what ever suits connection to your monitoring device.

I one instance I decided to design in monitoring through the use of LEDs connected to the FPGA. I had registers up to 16-bits wide in the FPGA so I connected 16 LEDs up to output pins in the FPGA design. I also equipped the design with a set of four jumpers that were inputs into the FPGA design. In the logical design of the FPGA the LED outputs were connected to an internal MUX that was 16-bits wide. The inputs to the MUX were connected to all of the various registers and counters that I wanted to be able to monitor. The selector controls for the mux channel came from the four jumper pin inputs. This allowed me to monitor up to 16 different internal sources of information (although I only needed to use 11 or 12 for that particular design).

I could easily watch the internal state machines and counters operate by viewing the LEDs. During the debug phase I needed to be able to monitor more closely than what you can view on an LED and so each of the 16 outputs also had a test point attached where I could connect my USB logic analyzer to capture the selected outputs in real time.

In your case most general purpose FPGA evaluation board provide for switches that connect to the FPGA as inputs and a set of LEDs driven by outputs of the FPGA. You can leverage these existing resources to implement exactly what I did (in my case I had made my own board). If your kit support only 8 LEDs and you have larger registers you can design the MUX to show the upper and lower halves of the register via two separate MUX selections from the switches.


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