Last time I presented a table of address and command codes, one for each of the XBox DVD Remote's 27 buttons. How did I come up with the table?
First, a recap: back in X. Start Making Sense, I displayed disappointment at Signaltap's partial scriptability, and resigned myself to manually processing the data, in heavy interaction with Signaltap's GUI and Excel. This solution, though workable, didn't sit well with me, and fortunately I found a better method.
The Virtual JTAG Interface To The Rescue!
The sld_virtual_jtag megafunction, aka the Virtual JTAG interface (VJI), provides access to the same on-chip hardware resources that signaltap makes use of, but in a far more user-configurable form. Here's what the Mighty Altera Corporation says:
The megafunction can be used to diagnose, sample, and update the
values of internal parts of your logic. With this megafunction, you
can easily sample and update the values of the internal counters and
state machines in your hardware device.
You can build your own custom software debugging IP using the Tcl
commands listed above to debug your hardware. This IP
communicates with the instances of the sld_virtual_jtag
megafunction inside your design.
I used the VJI, suitably wrapped in glue logic, to gather data under control of a tcl script. I'll give a brief textual description of the method I used; check out the attached design files for more details.
Remember that my goal is to measure the durations of the mark and space values emitted by the remote control. The measurement circuit consists of these functional blocks:
- An edge detector, which provides a 1-cycle pulse on each rising or falling edge of the incoming IR signal
- A 20-bit counter, which increments on each clock pulse, and synchronously resets to 0 on each IR edge
- A 20-bit FIFO, which is written with the counter value on each IR edge
Those simple circuit elements, running at 50MHz, write the sequence of IR duration values into the FIFO. The read side of the FIFO is controlled by the VJI, which exposes two values to the outside world:
- rdempty: is the FIFO empty?
- readdata: data from the FIFO
A tcl script (pseudo code, here - the actual script is get_data.tcl) running on the host gathers the FIFO data:
while (FIFO nonempty) read FIFO print FIFO readdata end while
The script process_data.bat transforms data from all raw data files into a single summary file and one processed data file per raw data file.
So, that's the basic operation of the data-acquisition logic. Next time I'll do some analysis on the gathered data.
Postscript: The testbench for this data acquisition fiesta is "tb_6"; here are the files. This testbench could actually be useful to anyone trying to analyze a serial protocol, so it's worthwhile to give a bit of an overview. The zip file contains these files:
- go.sh: Build bash script. Creates the quartus project, compiles the design and configures the FPGA.
- pgm.sh, make_tb.tcl, compile_tb.tcl: Lower-level build tcl scripts.
- top.v: Design top-level. Synchronizes the incoming IR signal to the system clock, instantiates module "sys".
- sys.v: The real guts of the design. Implements the edge detector, the counter and the FIFO. Instantiates the virtual jtag wrapper module (vj_rw). Drives the test bench code ("06.") onto the seven-segment display.
- fifo.v: A dual-clock FIFO, delivered by the FIFO megawizard.
- vj_rw.v: Assorted logic to provide a clean interface to the virtual-jtag-interface, from the outside world. Instantiates the virtual-jtag-interface itself (vj).
- vj.v: The actual virtual-jtag-interface, as delivered by the virtual jtag interface megawizard.
- get_data.tcl: The data acquisition script, as described above. Reads the FIFO, writes the values to the console, slightly formatted (newlines are inserted for "long" durations, which are supposed to be IR transmission boundaries
- process_data.bat: A perl script which transforms the raw data (as delivered by get_data.tcl) into "cooked" data files, a summary file of IR codes, and an html table for pasting here. This file is very XBox-remote-specific.