Description
This project shows how to use a SD-Card with FAT16 file system. It is based on the example MMC/SD-Card driver (mmcsd.c) and FAT file system driver (fat.c) delivered with the CCS C compiler. Unfortunately, the example didn’t work properly for me. So I searched the web and found some required changes to make it work. All modifications are described in the following article.
Hardware
SD-cards can be interfaced to a microcontroller via SPI-Bus, hence only 4 I/O pins (SDI, SDO, SCK, CS) are required to connect the SD-card to the PIC.
SD-cards operate at 2.7V .. 3.6V while the 18F PICs usually operate at 5V. Hence a voltage level translation is required for the SD-card inpus (SDI, SCK, CS). There are a lot of examples available in the web where a voltage divider with resistors is used to translate the 5V signals to 3.3V. This will work, but at higher SPI speeds this might cause problems. Due to that, I used a 74AC125 buffer to do the level translation which works more reliable than the voltage divider.
SD-Card Module
Here the schematic of the SD-Card module with the voltage level translation. The pins on the connector SV1 need to be connected to the microcontroller pins.
Important: SDO and SDI are the pin names from the SD-card point of view, i.e. SDO needs to be connected to SDI on the microcontroller and SDI needs to be connected to SDO in the microcontroller!
Software
As mentioned in the description for this article, I used the ex_fat.c example which is delivered with the CCS C compiler.
Unforunately, when I run the demo and created some files with the demo, I was not able to see the files when I pluged the SD-card into my PC. After some investigations I found a solution for this problem in the CCS forum which I will summarize here.
Read
Modifications in mmcsd.c
Search for
uint32_t g_mmcsdBufferAddress;
and add under it:
uint32_t g_mmcsdPartitionOffset;
Next, just before the “mmcsd_init” function add a new function:
void mmcsd_check_part(uint16_t off) { if (g_mmcsd_buffer[off + 0] == 0x80) { // active partition uint8_t t; t = g_mmcsd_buffer[off + 4]; if (t == 0x04 || t == 0x06 || t == 0x0B) { // FAT16 or FAT32 partition g_mmcsdPartitionOffset = make32( g_mmcsd_buffer[off + 11], g_mmcsd_buffer[off + 10], g_mmcsd_buffer[off + 9], g_mmcsd_buffer[off + 8]) * MMCSD_MAX_BLOCK_SIZE; } } }