The DMA is one of the most critical elements of any FPGA or high speed computing design. It allows data to be transferred from source to memory, and memory to consumer, in the most efficient manner and with minimal intervention from the processor. It’s no wonder then that a tutorial I wrote three years ago about using the AXI DMA IP, is still relevant and still getting thousands of visits per month. I decided to remake that tutorial, this time as a video and using Vivado 2017.2 (just today they released Vivado 2017.3, doh!). Although I prefer doing written tutorials, I think that video tutorials can be very useful in their own way, and they’re a hell of a lot easier for me to produce. I hope you find this one useful.

Video transcript:

Hi I’m Jeff. In this video I’m show you how to a simple example of using the AXI DMA in Vivado. This is going to be based on a tutorial that I did in 2014. Now I’m going to refer to this diagram a few times. So to tell you a little bit about DMA, DMA is basically an interface between a data producer or a consumer, and a memory controller, so you’d need a DMA if for example you had data coming in from an ADC and you need to store it very quickly into memory. Or in the other case when you have a DAC and you have data in a memory and you need to send that data as quickly as possible through to your DAC. In both of these cases, you could always use the processor to do this job so the DMA is not the only solution but obviously using the processor to transfer data from one place to another is very time consuming for the processor and it’s a bit of a waste of the processor. The processor should be left to do intelligent things. So the DMA is really a hardware solution for transferring data from one place to another and it is the most efficient way of doing so.

So I’ll get into the example now. In Vivado we start by creating project. Now I’m going to base this one on the MicroZed 7010 so I’ll call the project mz_7010_dma_test. Next. Now it’s an RTL project and I won’t specify sources at this time because I don’t have sources for this. Here we select the board, so I’m going to select MicroZed 7010 and all that’s going to do is configure this project for the right part depending on the hardware we’re using. So I click finish.

Now I can click Create Block Design, I’ll leave it with the default name. And the first thing I do is add my Zynq processing system to the design. I’m going to click run block automation. Now what the block automation’s going to do in the beginning is apply the board preset on the processing system. So that’s going to configure the Zynq PS for the hardware we’re using. So depending on the board preset that we chose earlier, which means the DDR and whatever other hardware devices we have connected to the Zynq, well the board preset should configure that. So I click OK.

Now in the block diagram I can see that the DDR interface has been externalized, so I know that my Zynq is configured with the DDR to which it is connected to on the MicroZed. Now I also have FIXED_IO port, that’s for all the other devices that are wired to the Zynq on the MicroZed, so if I want to know what they are I could just click on the Zynq PS and have a look at the block diagram here. I can see that I have a UART connected, because I know the MicroZed has a USB UART on there, GPIO probably has some LEDs, has an SD card, so an SDIO interface for that, USB port and Ethernet, so that’s one of the Gigabit Ethernet MACs that’s enabled there and connected to the Gigabit Ethernet PHY and RJ45 connector that’s on the MicroZed. So the other thing the board preset is going to do for us is configure a clock for us, so we can see here that we have one of the fabric clocks that has been configured to 100MHz, so I’m going to use that clock for all of this design. OK so now what I want to do in this design because I have an AXI DMA, AXI DMA is going to need access to the DDR memory controller and it’s also going to need a configuration interface which is AXI lite, so the processor is going to need to configure the AXI DMA through an AXI lite port and the AXI DMA is going to need access to the DDR. That’s the important thing to know because I need to configure the Zynq PS for those things. So If I go to my Zynq block design, firstly for the DMA configuration, the Zynq is going to need a general purpose AXI master port so that it can configure the DMA, when I say configure the DMA, I mean setup DMA transfers and trigger them. That’s what the AXI slave port is for and that’s what I’m going to enable here, so the easy way to do it is I click on this block here and Vivado takes me through to the right setting that I have to enable, so here I can see general purpose master AXI interface and I click, I tick that to enable one, the GP0 interface. So again, that’s my interface that I’m going to use to configure the DMA from the processor. The other interface that I need is to access the DDR controller, the memory controller, and I can see from this diagram what I need to enable, is one of these high performance AXI slave ports. So that would allow the DMA to read and write from the DDR. So to enable one of these, I have to click on that. Tick one of those, there’s four of them, I only need one. So now I’ve got those two ports. The only other thing that I need to configure here, is the interrupts, so I need to enable fabric interrupts, because I’m going to be receiving interrupts from the DMA IP. So I have to enable this IRQ_F2P which means FPGA to Processor system. I enable that. Click OK. So now my Zynq is properly configured. I can start off by connecting the fabric clock, the 100MHz clock through to these AXI interfaces. My two AXI interfaces, the general purpose master AXI interface, and the high performance slave. So here is my high performance slave and here is my general purpose master. So that’s that, now what I can do is add my DMA. There’s a few DMAs there for different applications, the one that I want though is “AXI Direct Memory Access”. So now what I can see is I’ve got an AXI lite interface, that’s for configuration of the DMA, setting up DMA transfers from the processor, and I’ve got these other interfaces here which … M_AXI or just AXI itself is just an AXI memory mapped interface, whereas AXIS is going to be the AXI streaming interfaces. So these AXI memory mapped interfaces are going to need to go through to the DDR controller, or to the high performance port. Whereas the AXI lite is going to need to go to the general purpose AXI master. Then we have the streaming interfaces and in our case, what we want to do is we want to connect the streaming interfaces through an AXI Data FIFO so we can loop back the data, so that way our application is going to just send data from the memory through the DMA, to the FIFO, that’s going to be looped back to the DMA and be written back into the memory, so the processor can just verify that the transfer was successful by comparing the data that was sent and the data that was received. So that only leaves two ports here, which are the AXI streaming status and AXI streaming control ports. We don’t actually need those, those are used in Ethernet applications, so we’re going to disable them. So that gets rid of them, so now I can start connecting my interfaces, so I’m going to run connection automation. I’ll tick on first the AXI lite interface, slave interface. It’s always good when you’re using connection automation to check what Vivado wants to connect things to, but here I can see it wants to connect the AXI lite interface through to the processing system’s general purpose AXI master port – that’s correct, that’s what we want. Now it can’t really make a mistake here because there is only one master AXI interface configured on the Zynq at the moment, the other is a slave interface, the high performance port is a slave interface, so it’s not going to use that. So that’s right, now I can tick on the high performance slave AXI interface of the PS. Vivado wants to connect it to the scatter gather, AXI master interface of the DMA. Now it could’ve chosen any of the other AXI master interfaces, it chose scatter gather, it doesn’t really matter because it’s just going to create either an AXI interconnect, or an AXI smartconnect for this and then the other two ports will be able to go through that as well. So anyway we start things off like that.

OK now if I just run through quickly what that’s done. We just want to make sure that our general purpose AXI master port, that’s going through now to an AXI interconnect, which is called peripheral, so that’s for all of your peripherals. It goes through here. Out here. And it should go through to the AXI lite interface of the DMA, so thats for configuration of the DMA, and for triggering and setting up DMA transfers. So what about the master interfaces of the AXI DMA. We’ve only connected one of them for now, the scatter gather interface, so that goes through here, through to an AXI smart connect, and then this should go through to our high performance slave interface which is basically access to the DMA .. not to the DMA to the DDR sorry, the memory. That’s what that has done. I’m going to run connection automation again to hook up my last two AXI master interfaces of the DMA, that’s going to connect both of them through to the high performance slave port, so click OK. And here they are. And all of that’s done is opened up two more ports on this AXI smartconnect. OK so now I’ve hooked up those things, that leaves my AXI streaming interfaces to connect. So if I go back to the diagram, I’m talking about these two interfaces, so I need to add my AXI data FIFO and connect up my AXI streaming interfaces. So I go plus, FIFO, I want an AXI4 Stream data FIFO. OK and I want to connect the AXI streaming master interface through to the AXI streaming slave interface of the DMA. And I want to connect the AXI streaming master interface of the DMA through to the AXI streaming slave interface of the FIFO. So that’s going to be my loopback, so the data’s going to come out of here, memory mapped to streaming, it’s going to go through the FIFO and it’s going to come out of the FIFO and back into the DMA, the streaming 2 memory mapped interface and be written again to the DDR memory. So what about these things here the FIFO needs to be clocked, we’re going to use the same 100MHz clock that everything else uses. So I hook that through to there. And for the reset, I want to use the reset that the rest of my design is using which is generated by the automatically generated processor system reset. The source of which is the fabric clock reset here. So that’s what I’m going to use. So now that’s all connected properly the only thing that I have to connect up now are the two interrupts of the DMA. I need to connect them through to here, the IRQ_F2P. The way to do that is to use a Concat. So the output of my concat has to go to there. And then my two interrupts have to go to there. And my interrupts are now connected. So that’s my design and I can save the block design now. I can click validate design, to make sure that I haven’t made any mistakes or forgotten to connect any clocks or resets. OK so this is an intermittent problem that sometimes happens with MicroZed designs, it’s something that started I think a couple of versions ago, but you can safely ignore these messages, they’re basically coming from the board preset and Vivado’s complaining about them now whereas it didn’t complain about them at all in previous versions. Anyway so we’ll just ignore those. And save the design again.

For more info regarding this issue, checkout this forum post:

So now the only thing I have to do is to generate my HDL wrapper. So I click on that and I say let Vivado manage wrapper and auto update. Now the only thing I have to do is generate the bitstream.

OK so my bitstream has been generated. I’m going to tick on view reports because I don’t want to open the implemented design. Now what I have to do is I want to bring this hardware design into the SDK so I can run a software application on it and test out the hardware. So to do that I have to say File-Export-Export Hardware and “include the bitstream”. I’ll export it local to the project. I click OK. So now the hardware’s been exported for SDK, I just have to run SDK, so the easy way to do that is go File-Launch SDK. And I exported it local to the project, my SDK workspace I’ll also leave it local to the project, it doesn’t really change much for me here. So OK.

So the SDK workspace at the moment has nothing in it, it should have nothing in it except my hardware platform specification. Which is here. It’s got the name of the block design. So I’ve got to add my application to the SDK at this point. So the way I do that is I say File-New-Application project. Now I can call this dma_test like that. If I just look into what’s going on here, here you can choose the processor that the application is going to run on. Now because the Zynq on the MicroZed has a dual-core ARM processor so we can choose which one we want to use, I’ll just choose that one. You obviously have to specify the hardware platform, or the hardware platform which is defined here. So there’s only one that I’ve got to choose from so that’s why it’s choosing that. Then I click next. What I’m going to do is I’m going to use an empty application for this, that’s going to be an application with no code, I’ve got to supply the code which I’ll do. So I say finish. So in my dma_test application here I’ve got no sources just the linker script and a readme. So to do this, I’m going to add my software application. What I’m going to use as a software application is an example software application that is provided by Xilinx. It is in this folder here Xilinx SDK version number, data, embeddedsw, Xilinx processor IP lib drivers, AXI DMA, examples (actual folder C:\Xilinx\SDK\2017.2\data\embeddedsw\XilinxProcessorIPLib\drivers\axidma_v9_3\examples). So this is in your Xilinx installation files. So what I want to do is use the example scatter gather poll, to begin with, let’s try that. So if I take this file, maybe I can drag it over here. Copy files. Click OK. So that’s copied that into my software application and now.. project build automatically,so I’m using the build automatically setting, so it should have built that project automattically. So let’s try and run that application.

So first of all, you’ve got to make sure that you’ve set the jumpers correctly for the configuration of the Zynq. So here I’ve got the jumpers set for configuration by JTAG. So here I have my JTAG programmer here. Here’s my JTAG programmer that I’m going to use, I’ll just plug it in. And now I can plug in my USB cable. Plug into here. And when I do that, the MicroZed you’ll notice that the LEDs turned ON, that’s because it’s getting it’s power from the USB port. So now what I can do is go back into the SDK, click Xilinx Tools, Program FPGA. Now that’s loaded the bitstream into the FPGA on the Zynq. Now I have to do is run the application, but before I run the application, I’m going to setup a connection to it. So I’ve already set that up earlier, so here is my COMPORT terminal window, connected through to the MicroZed, so when I run my application, I should see some, I should see some text coming up onto my console window. So to run the test, I’m going to click on dma_test application, click on run configurations, and I want to use the System Debugger for this, so I double click on that. And I can then click Run. And it will run the application on the hardware, I can see here that the application was successful.

Now just one last thing, I’ll create another application. I want to run another application but this time using the application using the example application that has interrupts. I’ll again choose an empty application, and move, and copy the application code into the workspace. OK so now I can see in the application I have no code. So I want to grab this, the scatter gather example with interrupts. I’ll drag it over into my application. OK now it’s going to crash (meant to say: fail to compile!). If you go and look and see why it crashed (meant to say: failed!), you’ll find that there are a couple of defines here that Vivado (the SDK) can’t find. So if I hover of that it says to me that this define is undeclared “first use in this function”. SO this is a define that should be in xparameters.h in the BSP. So I’m going to open up the BSP and see why that define isn’t there, and maybe change the, maybe it’s changed names. So I’ll go into xparameters.h, and see.. let’s search for this name maybe I can find it.. OK so here I can see that the defines that this application is looking for have changed names in this version of Vivado (SDK), so all I want to do is take the new names and modify my code with the new names, so this is the MM2S. I’ll change that. and then get the other one, S2MM. And change that one. Then save the file. It builds automatically and I can see that now my, now the SDK can, knows, the interrupt vector IDs. So the application is built, I just have to run it, so I’m again going to say I’m going to click on the application. Click on Run configurations. Double click on system debugger. And then run. OK and when I run that, go back to my terminal window, to see the output. And I see that it says “successfully ran the AXI DMA scatter gather interrupt example” so, that’s my two examples working. At this point I guess I leave you guys to muck around with the example applications, see what you can learn from the code. So thanks for watching and good luck with your projects.

Jeff is passionate about FPGAs, SoCs and high-performance computing, and has been writing the FPGA Developer blog since 2008. As the owner of Opsero, he leads a small team of FPGA all-stars providing start-ups and tech companies with FPGA design capability that they can call on when needed.

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