One of the most annoying things when working on an early design on an FPGA development kit is a lack of run-time register interfaces without a lot of effort.

While looking for an interface that would work on basically any Vivado supported Xilinx FPGA I came across the JTAG to AXI Master core supplied by Xilinx. Unfortunately it has a cumbersome interface that is intended for the user to drive from Vivado’s TCL console which is not always the most convenient. Others have been looking for a C API to interact with the hw_server directly. There seems to be someone that has had put together a C library but I was unable to get the files. I wanted something easier to use anyways so I began to look elsewhere for a solution.

Accessing JTAG2AXI from XSDB

I remembered that XSDK, XSCT and XSDB has the ability to read/write memory on the Xilinx SoCs so I thought to try the mrd and mwr in XSDB.

Running xsdb in a terminal.

$> xsdb

Connecting to the hw_server and JTAG cable.

xsdb% connect
tcfchan#0

Searching for debug target.

xsdb% targets
 1  APU
    2  ARM Cortex-A9 MPCore #0 (Running)
    3  ARM Cortex-A9 MPCore #1 (Running)
 4  xc7z010
    5  Legacy Debug Hub
       6  JTAG2AXI

We see above that the JTAG2AXI core we put in our design which we have already programmed to the board shows up so we select it.

xsdb% target 6 

Trying the mrd command results in a valid read!                                              

xsdb% mrd 0
      0:   0000000A

Accessing JTAG2AXI from Python

While performing memory read and write without the TCL commands in Vivado but from XSDB is great… I wanted a way to interact with the JTAG2AXI bridge from other software. While looking for a solution I found pysct, a Python interface to XSDB and Vivado!

After installing pysct, connecting to XSDB is as easy as starting xsdb in a terminal then creating a server and connecting to it from Python.

$> xsdb

xsdb% xsdbserver start -port 3010

from pysct.core import *

xsct = Xsct('localhost', 3010)

xsct.do("connect")
xsct.do("target 6")

print(xsct.do("mrd -value 0"))

# xsct.do("mrd -value 0 256") performs a read burst of 256 words instead of 1. 

By default the mrd command returns data formatted for human reading with addresses and data in HEX format. This slows stuff down a lot. Using the -value or -bin option is recommended for higher speed.

I noticed some issues in pysct and had to modify the recv() function in the Xsct class to have a much larger buffer size, setting it to 32768 allowed AXI4 bursts of 256 to work.

Performance Testing

With the JTAG cable on the Digilent Zybo board set to 30 MHz I ran some performance tests.

Running some performance tests on an AXI4Lite variant of the core in Python results in about 9 kilobytes/s of read transfers.

If we use the AXI4 variant of the core and use mrd -value 0 256 to perform max length bursts we get about 1.2 megabytes/s of read transfers! Pretty decent!