SD/SDHC Card Interfacing with ATmega8 /32 (FAT32 implementation)

Hi friends,Here is my project on interfacing of SD Card (microSD). microSD cards are available very cheap nowadays, a great option for having a huge memory in any embedded system project. It is compatible with SPI bus, so the interfacing is easy. SD card adapters are also easily available in market, one can easily make a bread-board adapter by soldering few pins on it. Following figures show the SD card pin-out & the bread-board adapter design by soldering 7-pins of a breakout header on the microSD adapter (Click on images for larger view).

I had started this project with 1GB microSD card from SanDisk (later on tested with transcend cards also). The microcontroller is AVR ATmega8 or ATmega32 running at 8Mhz internal clock. MAX232 is used to interface the circuit with PC for monitoring the data. A 3.3v supply is used for powering the mega8, microSD and max232 (though the specified supply for max232 is 5v, it works comfortably at 3.3v).7 pins of the microSD are used here, shown in the figure of pin-out.

Schematic for ATmega8 is shown here (updated on 10 May 2010, SD series resistors are removed, as they were limiting the speed of SPI bus. 51k pullups are added on CMD/DAT lines. This gives better stability with different cards. Also, two 3.6v zeners are added to protect SD in case when the ISP programmer voltage levels are of 5v. these diodes are not required if your programmer has settings for 3.3v output)(Note: VCC & GND pins of MAX232 are not shown in the schematic, but they must be connected in the actual hardware)

Following is the schematic for ATmega32, without RTC (updated on 10 May 2010):

Following is the schematic for ATmega32, with RTC (added on 17 May 2010). Here two supply voltages are used, 3.3v for SD & 5v for remaining ICs.

The aim of this project was to learn interfacing of SD card and to understand the data transfer in raw format as well as in FAT32 format. I started with raw data transfer, sending some data to any block of the microSD, reading a block of it, reading and writing multiple blocks, erasing multiple blocks. All this in raw format. I used RS232 for viewing the data read by microcontroller from SD card. The uc sends the data to HyperTerminal. Similarly, to write data to card, the data was fed thru HyperTerminal, by typing some text.

Once raw data transfer achieved, I formatted the card with windowsXP (FAT32) and loaded it with some text files, directories and other files (all stored in root directory of the card). After that I wrote the FAT32 routines to read files, get file list (using HyperTerminal again), finding the total/free memory of card. All this data is sent to HyperTerminal by the uc.

Following is the HyperTerminal window showing different options:

Options 0 to 4 are low level functions dealing with raw data. If you use option 0, 1 or 3, you may have to reformat the card before using the FAT32 routines.0: Erases selected number of blocks strating from selected block1: Writes data to specified SD block address. Data to be entered in HyperTerminal using PC keyboard2: Readss data of specified SD block address. Data is displayed on HyperTerminal window3. Writes selected number of blocks strating from selected block4. Reads selected number of blocks strating from selected block

Here, the multiple-block functions related to options 3 & 4 are disabled due to memory constraint as that time mega8 was used for testing and these functions are not required for FAT32 testing. While testing with mega32, options 3 & 4 can be enabled by removing a macro (#define FAT_TESTING_ONLY) defined in SD_routines.h.

Options 5 to 9 are related to FAT32 . Only short file names are supported right now, 8byte name+3bytes extension. If you store a long name file in SD, it will be displayed by these routines in short name format only.For testing these options, format the card with FAT32 file system and store some directories and text files (because text files can be read & checked thru HyperTerminal).

5: Displays list of available directories and files with size (in the root directory of the card)6: Reads a specified file and displays the file contents on HyperTerminal7: Create/Append file with specified name, enter text from HyperTerminal8: Deletes any existing file with specified name9: Displays total & free memory of the card (using FSinfo sector of the SD card)

Following figures show the HyperTerminal window when options 5 & 9 are selected:(These figures show menu from Ver2.3 or earlier. Menu style is changed from Ver_2.4 onwards, which is shown in the update history)

Note: HyperTerminal is used here at 19200 baudrate, No parity, Flow Control 'none'.

This project needs very few components and can be done easily at home. Try it out!

Update History-----------------------------------------------------------------------------

Version 2.4:- Real Time Clock circuit support is added for time & date entries in the files. Now the current date of file creation and file update will be entered in the FAT table (can be viewed by checking file 'properties' using a PC)(The RTC will also be useful in data-logging with time-stamp)- Three more options added in the Hyper Terminal menu for displaying or updating RTC date & time. New menu is shown in the above figure.

(Note: Version 2.2, 2.3 & 2.4 are tested on ATmega32, but they can be adopted to any controller having RAM >= 1KB and Flash >= 16KB)Current memory usage (Ver_2.4): Flash: 12908 Bytes; RAM: 700 Bytes (appx.);Version 2.3:

- Support for SDHC cards added (tested with SanDisk & Transcend microSD and microSDHC cards). The initialization sequence and command formats are modified.- A bug which was causing the program flow to go into infinite loop if the character number 512 in a sector was a CR (Carriage Return, '\r'), in the writeFile function. Thanks to David & Piotr M. who pointed it out in the comments.- Code is also tested successfully at 16MHz clock (8MHz SPI clock) with for SD/SDHC cards.

Follwing are the Hyper Terminal windows showing card detection (One window shows baudrate as 38400, that was while testing for higher clock speeds, current code still uses 19200 baud and 8MHz internal clock of Mega32).

Version 2.2:

- Append file feature added. 'createFile' function replaced with writeFile, which looks for the filename first, if the given file name doesn't exist then it creates new file and writes data, but if the file already exists, then it opens it and appends the entered data.- A bug removed which was giving error related to use of 'LONG'- The FAT32.c & .h files are updated on 13 Sep 09 to remove a bug which was limiting file size to 32MB (Thanks to Kun-Szabo Marton who pointed it out)

Data transfer rate: 1 raw data block (512 bytes) takes 4.15ms for reading or writing (123.37 KBytes/s) at current 4 MHz SPI clock rate. If you have flash more than 8k, you can declare the SPI_receive() and SPI_transmit() functions as 'inline' functions. This will increase the transfer rate to 140 KBytes/s. These transfer rates can be further increased by using a 16MHz crystal (8 MHz SPI clock). FAT32 file reading is done at 78 to 91 KBytes/sec.

Version 2.1:- A bug removed which stopped creating new files after 32*8 files in the root directory- The root directory was unnecessarily getting expanded by one cluster whenever a file was created. Fixed in the new version- Also, the fixed cluster size of 8 sectors is removed, this version will support other cluster sizes as well

Version 2.0:- Support added for SD cards having first sector as MBR rather than the boot sector- createFile and deleteFile functions added- A bug fixed in reading files stord at far locations in memory also correction made to accept 8+3 char file name (by mistake, it was taking 7+3 earlier)- FSinfo sector used for storing total free cluster count &next free cluster number for faster file access- Instant freeMemory display (earlier it was taking more than 30secs) using FSinfo sector. FSinfo sector is updated now whenever a file is created or deleted- File memory size display in decimal, like windows (earlier it was in hex)- Raw SD functions multiple block read and write, which are not required for FAT32, are disabled using FAT32_TESTING_ONLY macro for getting extra space required by createFile & deleteFile changes (you can activate it if you have more than 8k flash) Right now flash is 99.9% full- Clock speed raised from 1Mhz to 8 MHz, new SPI speed (after SD initialization): 4MHz instead of 500K & baudrate: 19200 (for HyperTerminal) instead of 4800-------------------------------------------------------------------------------

Download/view source code files V2.1 (for ATmega8):Following files are compiled using winAVR inside AVRStudio. This Version does not support SDHC cards. Also, append file feature is not available.1. SD_main.c2. SD_routines.c & SD_routines.h3. FAT32.c & FAT32.h (Ver 2.1, last updated-13 Sep 09)4. SPI_routines.c & SPI_routines.h5. UART_routines.c & UART_routines.h6. Makefile7. HEX file (Ver 2.1, last updated-13 Sep 09)