SD card benchmark and bus speed problem

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by wlamers on Thu May 16 07:29:14 MST 2013

I have done some extensive benchmarking of the SD interface on a LPC1778 and found some intersting things.


First of all the specs:

- crystal: 12 Mhz

- CPU clock 96 MHz (set to be able to come as close as possible to 50 MHz for the SD card (96/2 = 48 MHz)

- SD card: Apacer industrial SD SLC chips memory 4 GB


I have done both RAW multi-sector writes (64x512 bytes at a time) and FAT32 (Chan FatFs) writes.

First I see that when I do writes on the start of the card's adresses (sector 0) during RAW writes I get a considerable lower speed than at an offset of say 1 GB. The results are for 1 MB writes 2.81 MB/s vs 14.96 MB/s. This is with a bus speed off 48 Mhz.

But this is the problem. I can not do any FAT writes on this bus speed. Things go wrong when checking the FAT table during opening of a new file. All goes well at a 20 MHz bus speed.


Anyone seen this before?


BTW speeds are comparable at 20 MHz for RAW vs FAT writes (multi-sector writes, cluster aligned, and writing 1MB or more at a time).