SD card mounted causes a lot of interference, perturbing the ADC

[jean_prieur]

Posted on June 18, 2015 at 13:29

Hello everybody,

I use a SD card (SDIO).

- When the SD card is inserted but not mounted, my analog inputs convert voltages with a very good precision.

- When I mount the SD card, my analog inputs starts to convert voltages with a very poor precision.

- When I unmount the SD card, same problem. 

- When I unplug the SD card, my analog inputs convert voltages with a very good precision.

- When I re-plug the SD card, same problem of precision.

I think it's because the SD card plugged and mounted causes a lot of interference? I tried to analyze the electromagnetic field near the SD card and it's true, when the SD is mounted there is a lot more interference.

The fact is, in my board design, the analog inputs tracks are routed near the SD card. There is a high probability that the analog inputs capture the noise of the SD, do you ever experienced this problem?

Do you think there is any workaround? I can't change my electronic design... For example, can I fully deactivate the SD after the unmount? Because the unmount is not enough and there is still interference.

Thanks a lot for your help, I hope there is solution!

[Tesla DeLorean]

Posted on June 18, 2015 at 20:24

Does the unmount turn off the SDIO peripheral and deinit the pins?

Do you see a clock signal?

[jean_prieur]

Posted on June 19, 2015 at 11:26

Hello Clive,

I only use the fatfs library to unmount the SD card,

/*-----------------------------------------------------------------------*/
/* Mount/Unmount a Logical Drive */
/*-----------------------------------------------------------------------*/
FRESULT f_mount (
BYTE

vol,

/* Logical drive number to be mounted/unmounted */

FATFS *fs

/* Pointer to new file system object (NULL for unmount)*/

)
{
FATFS *rfs;
if

(vol >= _VOLUMES)

/* Check if the drive number is valid */

return

FR_INVALID_DRIVE;
rfs = FatFs[vol];

/* Get current fs object */

if

(rfs) {
#if _FS_LOCK
clear_lock(rfs);
#endif
#if _FS_REENTRANT /* Discard sync object of the current volume */
if

(!ff_del_syncobj(rfs->sobj))

return

FR_INT_ERR;
#endif
rfs->fs_type = 0;

/* Clear old fs object */

}
if

(fs) {
fs->fs_type = 0;

/* Clear new fs object */

#if _FS_REENTRANT /* Create sync object for the new volume */
if

(!ff_cre_syncobj(vol, &fs->sobj))

return

FR_INT_ERR;
#endif
}
FatFs[vol] = fs;

/* Register new fs object */

return

FR_OK;
}

I do not deinit the SDIO pins after the unmount, is it safe to init the pins each time I want to read/write the card?

[Tesla DeLorean]

Posted on June 19, 2015 at 12:17

I do not deinit the SDIO pins after the unmount, is it safe to init the pins each time I want to read/write the card? 

I have no idea, but it's probably more practical then removing and reinserting the card. Perhaps you can writing enough data in a burst that the card goes into sleep mode between them.

[jean_prieur]

Posted on June 19, 2015 at 12:45

I try to deinit the SD card and power off it after each communication and now it's working well! I use the SD card only for some save/load (with lot of datas, no real time) so I think it might be not a problem to init/deninit each time.

I can see that there is no interference any more. It's a good point for EMC standards I think :)