8M Mini SD Interface Questions

[adevries]

Hello Community,

I noticed that the 8M Mini DevBoard does not have any external pull-up resistors on its SD card interface. I thought that the SD specification requires that the data lines are pulled up to 1.8 or 3.3V? Are the data lines being pulled-up internally? And if so, how can the SD data lines be internally pulled up when the SD interface operates at 3.3V? It was my understanding that the i.MX 8M Mini cannot use internally pull-ups/pull-downs when the interface is running at 3.3V (due to Errata e50080).

Is it possible to use internal pull-up resistors when interfacing an 8M Mini with an SD card?
If I instead use external pull-ups, should the SD data lines be pulled up to NVCC_SD2?

Lastly, when the SD interface does a hardware reset (when the voltage to the SD card is turned off), are all the data lines, clock line, and command line also held low? Or do they remain high or clocking?

Thanks.

[igorpadykov · Accepted solution]

Hi  adevries

baseboard schematic SPF-31407 on p.10 has comments

IO internal pull up/down is not supported in 3.3V mode, must disable the internal pull up/down
via software and use external pull up/down resistors instead.

>If I instead use external pull-ups, should the SD data lines be pulled up to NVCC_SD2?

yes

> when the SD interface does a hardware reset (when the voltage to the SD card is turned off),

>are all the data lines, clock line, and command line also held low?

yes held low

Best regards
igor
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[adevries]

Hi Igor,

Thanks for the response. That makes sense and is consistent with everything else I've been reading about this issue on the 8M Mini. 

Since the 8M Mini evaluation board does not come with external pull-up resistors on the SD card interface, should I expect that interface to not function until external pull-ups are added? One of my coworkers was doing some tests with the EVK, and I'm pretty sure he was loading software through the SD card. 

Without external pull-up resistors, is it possible the SD card interface will work sometimes?

P.S. Do you have any recommendations on what strength pull-up resistor to use? Would 10k ohms be sufficient?

Thanks

[igorpadykov]

Hi adevries

>Without external pull-up resistors, is it possible the SD card interface will work sometimes?

one can try Demo Images from below link (it use internal pull-ups) which work fine:

i.MX Software | NXP 

> any recommendations on what strength pull-up resistor to use? Would 10k ohms be sufficient?

yes 10k ohms will be sufficient.

Best regards
igor

[adevries]

Hi Igor,

>Without external pull-up resistors, is it possible the SD card interface will work sometimes?

one can try Demo Images from below link (it use internal pull-ups) which work fine:

i.MX Software | NXP

I'm confused by your recommendation.The 8M Mini does not support IO internal pull up/down in 3.3V mode, one must use external pull up/down resistors instead. The SD card interface operates at 3.3V, and the 8M Mini eval kit does not have external pull up resistors on the SD card lines. So how can the demo image use internal pull-ups and still work fine? I thought you can't use internal pull-ups at 3.3V?

[igorpadykov]

Hi adevries

"does not support" means degradation over time, please check below part of errata

i.MX 8M Mini Mask Set Errata for Mask 0N87W

so this will affect impedance behaviour only after some time.

Best regards
igor

[StevenNg]

Hi, Igor, 

Errata 50080 stated that "The impedance of these pads changes over time from 20K to upwards of 92K ohm.".

1. So if i use it for purely pullup or pulldown (3.3V), will it valid over a long time?

2. Is there maximium value of degraded resistance?  will it degrades too big till it is "open circuit" after a long time?  

thanks.

-steven-

[adevries]

Hi Igor,

Okay, I think I understand now. When operating at 3.3V, the internal pull up and pull down resistors may work initially, but over time, they may degrade to an unacceptable level. So while the SD interface on the eval board may be working initially, it's not guaranteed to work as time goes on. Is that correct?

Thanks

[igorpadykov]

Hi adevries

yes this is correct.

Best regards
igor

[adevries]

Hi Igor,

Great, thanks for confirming!