May 2006, Issue 190

ARM-Based Modern Answering Machine
Philips ARM Design Contest 2005 First Prize

FAT

The article “Portable FAT Library for MCU Applications,” by Ivan Sham, William Hue, and Pete Rizun (Circuit Cellar 176, March 2005), is a great starting point for learning how to implement the FAT16 on an SD/MMC card. Their implementation is a perfect match for this project because the goal here is to allow the TAM-TAM and a PC to exchange files. Therefore, the files and the way they’re stored must be compatible. The FAT16 file system is almost obsolete (at least from Microsoft’s point of view), but it’s still supported in Windows XP.

The code was designed for a different microcontroller family, but it was relatively easy to update for this project. I modified the hardware-dependent library to account for the LPC2138 SPI. An initial implementation on the Keil development boards was successful, and I was quickly able to read and write files.

I added a few functions to the original code, which doesn’t have an option for reading, modifying, or writing files. The original code enables you to read or write only. This is a problem when recording WAV files because information about the file length is located in the header at the beginning of the file. But this information isn’t available until the end of the recording. I addressed this problem by allowing the file to be open in Write Only mode to write the incoming WAV data. Then, it’s rewound to the beginning of the file to write the header information.

I also added a function to delete existing files. Without the delete function, there’s no way to remove an existing file from the file system. Lastly, I created a file rename function.