wb2axip VS vgasim

I've always been partial to my own skidbuffer article and implementation. (You'd expect me to be, they're my own ...) I get your point, though, about some applications needing a registered output. I've come across many, as requirements change from one project to the next. This is why, in my own implementation, I have parameters allowing me to adjust which implementation I'm using. In this case in particular, I have a parameter adjusting whether or not the output is registered. (The outgoing READY signal, though, is always registered--that's the point of the skid buffer in the first place, and what keeps it from being a regular buffer.)

Ref: [1], [2]

Have you considered this article? It goes over the basics of the autocorrelation function, while also illustrating how you can build one with a Wishbone interface. Further, the Verilator logic for this function is kept and maintained on github here. Sure, it uses Wishbone. If you want to use AXI you can either use a bridge, or rework the the interface (it's not that hard ...).

If this is what you want, though, then consider checking out the apply_wstrb function in my wb2axip repository. Perhaps the easyaxil might be the easiest one to start with.

The worst thing that can happen on a bus is a protocol error that causes the device to lock up. Sadly, such errors have been common when using AXI or AXI-Lite--especially since traditional "verification IP" bus models won't catch all circumstances. This is why I make a general (personal) rule that nothing should touch a bus unless it has been formally verified. This check basically guarantees that every bus request gets one (and only one) response. It's not sufficient to verify an IP fully, but it's a good start. You can find my formal verification models for AXI-Lite, Wishbone, and (to some extent) Avalon in this repository. I've also been known to make my full AXI model available to Patreon sponsors of my blog.

As an experiment, I tried a formal first approach using u/ZipCPU's faxil_slave.v.

I then moved on to an internal desk check of the design ... 1. skydivertricky is right: AXI does not allow timeouts. If you implement an AXI timeout, you will risk crashing any system using your design. Your timeout implementation is therefore broken, and will risk crashing any system attempting to interact with your design. This applies to both your read and write paths. 2. Sure enough, your write state machine looks only for the write address strobe during the idle state. If write data shows up early, the write data will be forgotten causing the system to hang. This was one of the bugs caught by SymbiYosys and mentioned above. 3. Your design has a nasty sequencing bug within it: wren is set on the clock _after the transaction takes place. As a result, the s_axi_awaddr and s_axi_wdata values used within your design may contain garbage within them. Indeed, they may be already set for the next transaction, or the master may put some other value in them. As a result, writes to your design may work for some masters, or more likely will not work at all. (They won't work with any of my own open source AXI-lite masters.) 4. I would recommend implementing WSTRB. There are plenty of implementations out there that will use this value, particularly when crossing from one bus width to another, but also when the CPU attempts to write less than a full word. I've been burned more than once by acting on a write when WSTRB was zero. 5. As predicted by SymbiYosys, your read data calculation is also broken. Specifically, it depends upon the value in ARADDR on the cycle following the cycle where ARVALID && ARREADY. This breaks protocol, and is likely to give you some very unexpected results. 6. While not technically broken, I would recommend that your reads ignore the bottom two bits of address. If you do this, the reads will then work if the CPU issues a load-byte or load-halfword instruction. The same applies to the writes--you should be looking at all but the bottom two bits (for a bus with 32-bit data words, like AXI-lite).

Can a FIFO be generated to use the AXI stream protocol? Absolutely! Check out this FIFO: - S_AXIS_VALID can be fed to the write port, i_wr - S_AXIS_READY is the compliment of the FIFO full output, o_full - S_AXIS_DATA can be fed to the incoming data port, i_data - M_AXIS_VALID is the compliment of the FIFO empty output, o_emtpy - M_AXIS_READY can be fed directly to the read port, i_rd - M_AXIS_DATA can be read directly from the outgoing data port, o_data - If you want to support other AXI Stream signals, they can simply be concatenated with the data signals above.

I recently connected my own IP to a Microblaze core via the AXI-lite interface. Vivado has a tool to generate AXI IP, but it's broken so I used this implementation of an AXI-lite slave. It comes with some registers that'll be accessible in the processors address space. Here is a more detailed explanation of the linked implementation.

Here's an example of an FWFT FIFO written in a vendor independent fashion. Yes, it requires a bit of wizardry to get right--primarily in how the read address is calculated, but I've used it for years now and it's held up nicely for me.

I have built a lot of open-source C++ tooling for design verification. You can find a lot of my C++ models posted on my Github. Example C++ models include: UART, SPI/DSPI/QSPI Flash, SD-Card (SPI-based interface), VGA Video, Ethernet MDIO, PS/2 mouse, OLED display, SDRAM and more. (I've even simulated PLLs using C++ models ...) I have also written extensively about doing so at ZipCPU.com.

Most of the graphical images I've seen won't fit in block RAM on on an FPGA. (Think of an 800x600 pixel image, with 8bits per pixel, and it only gets worse from there.) The image needs to be stored elsewhere. That means, you need ports associated with feeding your image to your Sobel processor. This can happen one of two ways. You can either use a external Video frame buffer reader, or you can drive the memory bus yourself. You haven't said what type of memory bus your system has, so let me instead assume the external reader.

Definitely. To see how this might build up, consider this video sprite module. At each stage, counting from the end, the READY backs up.

Ahm ... I've certainly added C components to my test bench to create and simulate graphical interfaces. Here's one for VGA, and another for HDMI. This isn't really a "nobody does this" task. It's much easier to debug a graphical component graphically than it is to debug it with a wave file. Indeed, I owe my success in one particular video decompression example to being able to stop the simulation in real time in order to find and trace a bug.

Yes--I've done that with both VGA and HDMI. You can find the example here if you want to see how I did it.

Check out this page describing these techniques, or even the repository containing my simulator.

Why not try a Verilator based simulation like this one? You'd then be able to see the (broken) design on a window of your simulation host's screen, and capture a VCD file to see what's going on (or not)? You should be able to just place the VGA outputs into the VGASIM class to be able to see the image on your screen. Multiple video modes are supported, so select the one you need. (The demo works with all modes, but the memory mapped frame buffer's image is only built for 1280x1024, and hence the requirement in the demo).

For an example, you might wish to take a look at the histogram design I posted. Other valuable examples might include the slow filter, the slow linear phase filter, the downsampler, or even the FFT Window function. I've also got a sprite video design that I'd like to write about, but haven't had the chance to test yet. All of these designs need to deal with and work around these issues with internal block RAM.

I have a variety of C++ simulation sources that I use which can create either VGA or HDMI signals to input into a test bench--together with a couple example designs that demonstrate these test benches. They work nicely with Verilator.

This demo shows how the AXI stream framed data can be turned to pixels and sent to a screen--should that be your wish.