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jhenderson
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Joined: 07 Mar 2006
Posts: 1178
Location: So. Cal. USA
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 Posted: Tue Jul 10, 2007 10:02 am Post subject: Framed acquisition |
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| Quote: | I have been trying to write code for the board and there are a few things that are confusing in the documentation and the sample code. I understand the concept of a framed acquisition. A trigger signal (either from external hardware or software causes the board to collect x number of points. What I don't get is how I tell the board the value of x. Is that the packet size?
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No. The Packet size specifies the size of buffers bus-mastered to/from the Host via the PCI bus. It is relevant to triggering only in the sense that it is often desirable to coordinate the packet size with the frame size, so that when a frame is acquired, it is transferred immediately to the host via bus-mastering. If the packet size is larger than the frame size, then the contents of that frame will sit in the onboard buffer until enough data to satisfy a packet have been collected.
| Quote: | Or the frame count size. I cannot find in the Malibu documentation the method Framed( channel number, frame count) It looks like it is part of the UWB class but I don't see it in the help file.
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The frame count specified by the Framed method controls the number of 256-sample-long frames to be acquired in response to each framed-mode trigger event. The stock logic implementation defines a frame to equal 256 samples from each enabled A/D device.
| Quote: | Assuming that I need to tell the board how many points I need to take upon a trigger is defined by the frame count number what are its restrictions? The code seems to imply that the frame count must be a power of 2 and greater than 1024. Is this true ? I thought I read somewhere (Don't remember where and of course can not find it now) that the frame count must be a multiple of 256. This is not the same as a power of 2 and in my application I need to collect around 6000 points between triggers so I don't want to round up to 8192 if I don't have to.
In the code I see that the call to framed set the size to framecount/256. Is that the multiple of 256 restrictions? Or is it that the frame count is passed as the number of packets because the default packet size is 256 points. Assuming that the packet size is not related to the framed count size what is the best way to set the packet size for maximum speed?
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Due to logic restrictions, the frame count must always be a multiple of 256 samples. Consequently, the example code divides the requested sample count by 256 to generate a frame count, which is passed to the board object as streaming commences.
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Jim |
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jhenderson
Site Admin
Joined: 07 Mar 2006
Posts: 1178
Location: So. Cal. USA
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| Posted: Tue Jul 10, 2007 3:50 pm Post subject: More framed mode questiions |
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| Quote: | | Unfortunately this answer has added more confusion. Does a trigger only result in 256 samples being collected or 256 samples * frame count? This was not clear from the answer. |
Each trigger event will initiate the acquisition of 256 * frame count samples. For instance, if frame count were loaded with 10, when the trigger fires 2560 samples will be collected.
| Quote: | | Also is the power of 2 restriction in the code incorrect? |
The frame count may be any integer number between 1 and 65565. It is not restricted to powers-of-two. The total number of samples acquired for each trigger event will be this number * 256.
| Quote: | | Lastly Where is the method Framed() in the Help file? |
Apparently, it was inadvertently omitted. This will be corrected in the next release.
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Jim |
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