SD/MMC clock control

mbro131 · Joined: 22 May 2006 Posts: 9

I am using a PIC18F2550 to interface with a SD/MMC card. My system needs to be able to use high capacity cards (1GB+) and must also allow them to be interfaced with a Windows system.
The FAT16 file system can reference a maximum of 32MB if 512byte block sizes are used, however if larger block sizes are used then this increases. Is it possible to use 16kB block sizes (to allow for 1GB cards) but only transfer this from the PIC 512bytes at a time?

According to the Simplified Physical Layer Specification:

Douglas Kennedy · Joined: 07 Sep 2003 Posts: 755 Location: Florida

Most of this you will know already.
The MMC/SD is the physical layer and has restrictions as to commands and things like blocksize (Ex 512 bytes). The physical MMC layer hasn't any knowledge whatsoever of how it is logicaly formated. The logic layer can be any organization you want that is consistant with the physical layer constraints. Microsoft fully owns the FAT design in any of its forms and on any platform ( Ex PIC micro). Until recently it was felt that FAT16 could be used on a PIC platform without Microsofts consent. This was convenient since a PC could be used to do the heavy lift work of formating the cards and the PIC would work within this formatted file structure. A FAT design employs a table of file names and pointers into a cluster table that in turn references physical storage in cluster size increments. You are right the cluster size can be a large multiple of the smallest physically writable block (EX 512bytes). In practice Microsoft formatting software is the main constriant..it will choose FAT12 below a certain disc capacity FAT16 for the next size and FAT32 for larger sizes. Now if you are writing your own format routine it might be wise to create your own file system design and avoid the legal and other constraints that come with a Microsoft patented design.
As to the physical block size it has relevance to performace of the card but is only marginally relevant to the logical format you choose. A cluster size of 32k is still a multiple of 512 bytes ( the least amount of data you can write to a MMC) so 64 consecutive reads will fully access the logical cluster..the issue arises as to temporary storage... larger clusters often need larger temporary storage to keep the code simple.