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[DPRG] webcam robot vision limitations
Subject: [DPRG] webcam robot vision limitations
From: Zac Wheeler
zac at poor-robot.com
Date: Thu Jun 28 12:56:39 CDT 2007
> I/O bandwidth to mass storage (SD flash card in my case) is the main limiting factor I've encountered. The PC104 computer I am using does not have DMA for the SD flash. Writes use PIO. There is device buffering in RAM which then can freeze the computer for a few seconds when flushed to SD flash. This reminds me of some digital cameras.
>
> So a word for others who may try this - mass storage DMA bandwidth is the most critical feature to look for. CPU speed, RAM, device I/O bus speeds are not as important as fast disk or flash storage. If you see a SBC that uses SD/MMC or CF flash, make sure DMA is used.
>
Do you have the option to use a CF card on your SBC instead of MMC/SD?
As it is, your CPU will offload the data but if the transfer is locking
your CPU for on the order of seconds, you're going to be only slightly
less screwed when it locks your DMA channel for a similar period of
time. Have you made sure that your SD card complies with SD 1.01? It
probably doesn't if it's an off brand. It sounds awfully slow for a QVGA
image write time.
You can buy high data rate SD cards, and my digital camera actually
requires them to use certain features. CF is a parallel interface and
some of the newer cards can hit 60 MB/s+ vs. the 10MB/s of a good SD
card. If you buy one of the CF cards with DMA supported you can top 100
MB/s.
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