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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

ADDRESS DECODING

The functional procedure to access the CF card is also easy, after the 116-page CompactFlash Association Specification is dissected. The most important bit of information in the specification is the address decoding of the eight control registers within the controller portion of the card (see Table 1). Access to the full range of necessary registers can be achieved using only three address lines, so ýREG and A10ýA3 are hardwired to V_CC and GND, respectively.

To load one of these registers, the data sent is placed on the data lines to the card, the 3-bit address of the desired register is placed on the address lines, and ýWE is strobed low. Itýs that easy. By loading the various registers with sector addresses (either in logical block addressing or cylinder/head/sector addressing) and a command in the command register, commands can be sent to perform a read, write, or any number of other operations. The results of these operations most often go through the internal buffer, and sequential accesses to the data register incrementally access each byte.

For example, if a card has a sector size of 512 bytes, a buffer size of 512 bytes, and the appropriate addressing registers have been loaded followed by a read sector command, the buffer is then loaded with the contents of that sector. The first access to the data register (accomplished by putting the address of the data register on the address bus and strobing ýOE) will read the first byte in the buffer. The next strobe of ýOE will read the next byte in the buffer and so on.

The same would be true of a write sector operation. After the sector location has been loaded into the appropriate registers and the write sector command has been loaded into the command register, the first byte to be written is placed on the data bus. When ýWE is strobed, that byte is written to the first location in the buffer. The next byte to be written is then placed on the data bus, ýWE is strobed again, and so on. Because these operations go through the buffer, which is internally read from and written to a full sector at a time even if the desire is only to write one byte to the card, a full 512 bytes must be written to the buffer. The data in the buffer does not transfer to the card memory until the buffer is full.

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