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Honored Contributor I
703 Views

Simple DMA Controller Mystery

I have implemented a qsys design with pcie_hard_ip, Altera's DMA controller and my custom Avalon component as a slave to the DMA controller. For testing, my custom component presents a constant 32-bit value on the readdata lines. My component implements readdatavalid (set to 1) and waitrequest (set to 0) to When I program the DMA controller with a byte count of 4, tell it not to increment the source address, and set the GO bit, I get one 32-bit word transferred to my target (PC Host) and DMA controller status bits DONE and LEN set to 1. As it should be. But, if I set the byte count to > 4, I only get one word transferred, the DMA controller status indicates BUSY and the length register is 4. At this point, I have to reload the sof file and reboot the PC before any DMA controller commands have an effect again. 

Looking at the read line on the o-scope while I run the above, it's high for 4 clocks, in both the count=4 and count > 4 cases. Seems it should be high for just one clock to read a 32-bit word... one byte width enforced somewhere? byteenable bits from master are all high.  

Specs: Stratix IV, Quartus 12.0, DMA bursting enabled, controller-supported data types include 32-bit word. x4 PCIe, 125MHz app clk, reconfig block in place, running on target hardware, not doing simulation. Hardware works with Altera's chaining DMA example. 

To summarize: 

1 - Why can't I transfer more than one 32-bit word? 

2 - Why 4 read cycles for one 32-bit word? 

Hopefully there's enough info here to be of use. Does anybody have any ideas why this isn't working? 

Thanks, 

Tom
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Honored Contributor I
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FIXED (maybe) - I got correct DMA transfers when I selected only the 32-bit transfer support checkbox in the Qsys DMA controller GUI. The docs imply that you can check all the transfer size option boxes and that the size used by your slave (in my case 32 bits) will be the one used. Could be my 'fix' is in reality a work-around for some other lurking oversight...

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