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mrtele
Joined: 26 Jun 2026
Posts: 2
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| Help with SD Card and PIC24 |
 Posted: Fri Jun 26, 2026 7:47 am |
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Hello everyone,
I've been trying without much success to communicate with an sd card on a new project. I'm using a PIC24FJ512GB606, connected to the SD Card with the standard pinout for SPI communication.
Looking through the forum, I saw the pinned post "mmcsd.c and fat.c modified and fully functional", though unfortunately I was not able to use it as of yet, as my code is still using the setup_spi() and spi_read/write functions rather than the #use spi and spi_xfer option.
I've had some partial success with the driver from this page (https://simple-circuit.com/ccs-c-fat-library-for-mmc-sd-cards/), after modifying it, mostly to make sure all the variables are explicitly unsigned. I've made a few more changes to the init routine, according to the init sequences I've found online:
| Code: | MMCSD_err mmcsd_init()
{
uint8_t i, r1, r3[4];
g_CRC_enabled = TRUE;
g_mmcsdBufferAddress = 0;
output_drive(MMCSD_PIN_SELECT);
to_spi_m0v();
mmcsd_deselect();
delay_ms(250);
for(i = 0; i < 10; i++) // Send 80 cycles
MMCSD_SPI_XFER(0xFF);
/* begin initialization */
i = 0;
do
{
delay_ms(1);
mmcsd_select();
r1 = mmcsd_go_idle_state();
mmcsd_deselect();
i++;
if(i == 0xFF) {
if (r1 == 0)
return 1;
else
return r1;
}
} while(r1 != MMCSD_IDLE);
i = 0;
do
{
delay_ms(1);
mmcsd_select();
//! r1 = mmcsd_send_op_cond();
r1 = mmcsd_send_if_cond(r3);
mmcsd_deselect();
i++;
} while((r1 & MMCSD_IDLE) && i != 0xFF);
/* an mmc will return an 0x04 here */
if(r1 == 0x04)
g_card_type = MMC;
else {
g_card_type = SDSC;
i = 0;
do {
r1=mmcsd_app_cmd();
r1=mmcsd_sd_send_op_cond();
} while (r1 == 0x01 && i != 0xFF);
if (r1 == 0x01) {
return r1;
}
mmcsd_select();
r1 = mmcsd_read_ocr(r3);
mmcsd_deselect();
if(r1 != MMCSD_ILLEGAL_CMD) {
r1 = r3[3];
if(bit_test(r1, 6)) // If bit 30 of the OCR register is 1 (CCS is 1) ==> SDHC type
g_card_type = SDHC;
}
}
/* set block length to 512 bytes */
mmcsd_select();
r1 = mmcsd_set_blocklen(MMCSD_MAX_BLOCK_SIZE);
mmcsd_deselect();
if(r1 != MMCSD_GOODEC)
return r1;
/// this would be a good time to set a higher clock speed, 20MHz
to_spi_m0f();
// Turn OFF CRC check, some card return 0 (MMCSD_GOODEC) and some others return 0x04 (MMCSD_ILLEGAL_CMD)
mmcsd_select();
r1 = mmcsd_crc_on_off(FALSE);
mmcsd_deselect();
r1 = mmcsd_load_buffer();
return MMCSD_GOODEC;
} |
Through this section I get the expected responses, and the card is recognised as SDHC (it's a 4GB SD Card, which I tested on a Windows PC and seems to work fine; it was formatted to FAT32 using the "official" SD Card Formatter utility).
However, right after the call to mmcsd_init in fat_init,, there's a sequence of mmcsd_read_data calls that appear to work but read no data (all the variables pointed to remain at zero). The final one in the sequence (ec += mmcsd_read_data(offset_ + 0x3EC, 4, &Next_Free_Clust);) returns a 5, ending fat_init prematurely. As far as I can tell, this 5 originates as a response from a READ_SINGLE_BLOCK command deep within the read_data call.
Does anyone have any insight into the possible cause? It's my first time interfacing with an SD Card, so I might be missing some obvious stuff. |
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asmallri
Joined: 12 Aug 2004
Posts: 1670
Location: Perth, Australia
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| Posted: Mon Jun 29, 2026 8:22 pm |
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If this is your own hardware then the first step is to validate the hardware configuration. The most obvious are, do you have the correct pull-up resistors - (e.g. 10K pull-up on the cards SDO output and the CS line)?
Do you have a low ESR capacitor (Ceramic or tantalum capacitor around 10uF) as close as practical to the power pins of the SD card socket?
_________________
Regards, Andrew
http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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mrtele
Joined: 26 Jun 2026
Posts: 2
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| Posted: Tue Jun 30, 2026 7:54 am |
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I don't think we have one actually, we'll be sure to add it for better stability.
In any case, I ended up testing the drivers from the pinned post. I had to make a number of modifications, but it seems to be working now (making directories, reading and writing some simple txt files). I'll post the files once I finish with some more tests. |
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