This is a library for dataflash memory. Dataflash is essentially external FLASH memory on a chip; this library lets you make the Arudino talk to that external memory chip and store values or data there, for applications like datalogging.

The code is from the atmel butterfly, ported to gcc by Martin Thomas. I changed it to c++ so it can be used as a library for arduino. You can download the library here. Put the dataflash directory in your arduino-0008/lib/targets/libraries/ directory and (re)start arduino. You can ask your question in this topic in the forum.

I have tested the library with AT45DB041B. Others should work but I don't have them available. This chip is not available in a DIL package so you have solder wires to it or put it on a small pcb, like this. Connect it like this:

dataflash  arduino
pin 1 -- pin 11
pin 2 -- pin 13
pin 3 -- pin  8
pin 4 -- pin 10
pin 5 -- pin  7
pin 6 -- V (2.5-3.6V)
pin 7 -- ground
pin 8 -- pin 12

It is 5 volt tolerant but it needs a lower voltage. I haven't tried to power it with 5 volt. I used a zetex voltage regulator. Very easy, doesn't need extra components. If you have an Arduino BT you can also get 3.3 v from there. The other connections for SPI are directly connected. This should be possible according to the manual and so far it works. I have attached a small example with some comments. Copy the dataflash_intro from the directory from the zip file to your arduino sketch directory and it should work. The code in the library is commented but for convenience I copied it below.

 
Function name : Page_To_Buffer
Returns :	None
Parameters :	BufferNo -> Decides usage of either buffer 1 or 2
		PageAdr	-> Address of page to be transferred to buffer
Purpose :	Transfers a page from flash to Dataflash SRAM buffer

Function name : Buffer_Read_Byte
Returns :	One read byte (any value)
Parameters :	BufferNo -> Decides usage of either buffer 1 or 2
		IntPageAdr -> Internal page address
Purpose :	Reads one byte from one of the Dataflash internal SRAM buffers

Function name : Buffer_Read_Str
Returns :	None
Parameters :	BufferNo -> Decides usage of either buffer 1 or 2
		IntPageAdr -> Internal page address No_of_bytes	-> Number of bytes 
to be read 
		*BufferPtr -> address of buffer to be used for read bytes
Purpose :	Reads one or more bytes from one of the Dataflash internal SRAM buffers,
and puts read bytes into buffer pointed to by *BufferPtr

Function name : Buffer_Write_Enable
Returns :	None
Parameters :	IntPageAdr -> Internal page address to start writing from
		BufferAdr -> Decides usage of either buffer 1 or 2
Purpose :	Enables continous write functionality to one of the Dataflash buffers
buffers. NOTE : User must ensure that CS goes high to terminate this mode before 
accessing other Dataflash functionalities

Function name : Buffer_Write_Byte
Returns :	None
Parameters :	IntPageAdr -> Internal page address to write byte to
		BufferAdr	->	Decides usage of either buffer 1 or 2
		Data -> Data byte to be written
Purpose :	Writes one byte to one of the Dataflash internal SRAM buffers

Function name : Buffer_Write_Str
Returns :	None
Parameters :	BufferNo -> Decides usage of either buffer 1 or 2
		IntPageAdr -> Internal page address
		No_of_bytes -> Number of bytes to be written
		*BufferPtr -> address of buffer to be used for copy of bytes from AVR
buffer to Dataflash buffer 1 (or 2)
Purpose :	Copies one or more bytes to one of the Dataflash internal SRAM buffers
from AVR SRAM buffer pointed to by *BufferPtr

Function name : Buffer_To_Page
Returns :	None
Parameters :	BufferAdr -> Decides usage of either buffer 1 or 2
		PageAdr	 -> Address of flash page to be programmed
Purpose :	Transfers a page from Dataflash SRAM buffer to flash

Function name : Cont_Flash_Read_Enable
Returns :	None
Parameters :	PageAdr	-> Address of flash page where cont.read starts from
		IntPageAdr -> Internal page address where cont.read starts from
Purpose :	Initiates a continuous read from a location in the DataFlash

Function name : Page_Buffer_Compare
Returns :	0 match, 1 if mismatch
Parameters :	BufferAdr -> Decides usage of either buffer 1 or 2
		PageAdr	-> Address of flash page to be compared with buffer
Purpose :	comparte Buffer with Flash-Page

Function name : Page_Erase
Returns :	None
Parameters :	PageAdr	-> Address of flash page to be erased
Purpose :	Sets all bits in the given page (all bytes are 0xff)