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PIC A COMPACTFLASH CARD

by Mark Samuels

Start ý A Choice PIC ý Getting Connected ý Address Decoding ý The Identify Drive Command ý Laying the Foundation ý No Limits ý Sources and PDF

THE IDENTIFY DRIVE COMMAND

Almost all CompactFlash cards on the market today have both sector and buffer sizes of 512 bytes. To ensure thatýs true for a particular card, use the identify drive command, which can be sent to the card without loading any of the other registers. When the value 0xEC (identify drive) is strobed into the command register, the buffer is immediately filled with 512 bytes of useful information, including sector size, buffer size, model numbers, serial numbers, number of heads, cylinders, tracks, and too many other goodies to be listed here. However, they are well documented in the CompactFlash Association Specification.

The sample source code I have provided for downloading shows a simple application of reading from and writing to a CompactFlash card. The program essentially uses two bytes on a card to store a counter, which keeps track of how many times the card has been inserted into the socket. This is accomplished by waiting for the card detect signal (CD1) to go low, indicating that a card is present.

An identify drive command is executed to get the sector size (word 5) and buffer size (word 21) of the particular card that has been inserted. The identify drive command doesnýt require any parameters, so none of the other registers have to be loaded with any data. Simply load the command register and strobe ýWE.

The CompactFlash Association Specification states details about each command and the results, including the breakdown of the 512 bytes of information dispensed by this particular command. To get to a specific piece of information, simply strobe ýOE enough times to get to that data in the sequence. For example, the number of bytes per sector is stored in the sixth word (word 5), so strobing ýOE eleven times will make the first byte of that particular data field appear on the data bus.

Most of the information on the card is stored as 16 bits, so the subroutine in the code (CF_READ) actually strobes ýOE twice, storing the first byte read in DATA_LO and the second byte read in DATA_HI. The program then uses these values to know how many times to write to the buffer before that information is actually transferred from the buffer to the card memory. Next, the program reads a sector at a particular location but only reads the first two bytes from the buffer. These two bytes are the counter, which is incremented and written back to that same location on the card.

One note of caution, running this program on a CF card that has been formatted and has data already stored on it may corrupt that data and make the card unreadable until it is reformatted, because this program will overwrite a specific sector on the card. That read and written location is clearly annotated in the source code, so feel free to play around with the target location and amount of data read or written.

There is a useful shareware application that I used to read the raw hex data from the card to verify the actions of the PIC. The program is called WinHex and is available from CNET and various other shareware archives on the Internet. As long as you have the means to access a CompactFlash card as some sort of disk on your PC, you can use WinHex to view each individual byte stored on the card.

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