On Thu, Sep 29, 2011 at 4:06 PM, Herbert Graf wrote: > > You may have to try a few cards. > Yep, that's what I plan to do. > While 50MHz is the physical clock rate, you won't get anywhere near that > throughput. > > The reason is protocol overhead (which isn't the big) and write time > (which is HUGE). You'll have to hunt for a card with as short and as > consistent a write time as possible (IIRC SD cards have a write block of > 512 bytes). > I think modern SD cards have improved quite a lot. On the computer, I get consistent 80Mb/s+ writes. > > Even bit banging, I'd notice some cards would take many queries of "are > you done" before they were done writing. > > For example, here is a web site where someone measured the performance > of their 2GB SD card: > > http://forum.xda-developers.com/archive/index.php/t-242802.html > > Details: > Card tested "Storage Card" > Min. read time: 0.25ms > Max read time: 18.00 ms > Avg. read time 1.87ms > Sector/block: variable 4-64 > Min write time: 0.50ms > Max write time: 16.00 ms > Avg. write time: 4.02ms > Read/write ratio: 0.46 > Sector size: 512 bytes > Start Sector: 0 > End Sector: 3910656 > Total Sectors read: 2504 > Total data read: 1.00MB > Total read time: 4673 ms > Total sectors written: 2504 > Total data written: 1.00MB > Total write time: 10057ms > > Notice the HUGE variability of read and write times? That's the killer > for apps the need consistent write performance. > > I'd assume newer cards are better, but I'd love to hear back what stats > you end up getting? > I'll test and let you know. On the pc, using a super cheap micro sd card an= d super cheap USB card reader, I'm getting consistent 48Mbit/s writes. If I can even score half that on my PIC32, that'll be more than enough, especially with SPI running at close to 50MHz via DMA. Since the nature of the application is sequential writes, SD should work perfeclty. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .