SparkFun Forums

[MostlyHarmless]

Hey, all-
I've been trying to confirm if pull-up resistors are needed on the spi signals to a microSD card. The pqi datasheet shows them for the SD interface (http://www.pqi.com.tw/upload/download/micro%20SD(2).pdf) and seems to be part of the SD spec. Regular SD card applications show them on all comm signals (http://www.altera.com/products/ip/ampp/ ... i-core.pdf), or just on the ~CS (http://focus.ti.com/lit/an/slaa281b/slaa281b.pdf).

And the schematic for the "Breakout Board for DOSonCHIP FAT16 FAT32 uSD Module" here on Sparkfun doesn't show pull-ups.

Given Sparkfun's excellent stuff, my bet is that they are not required!

Anyone have a definitive answer/reference? Thanks for the help!
Cheers,
MH

[frank26080115]

Any signals that can be floated should be pulled low or high properly via a resister. This is a generic design rule on MOS devices. Because DI and DO are normally high, they should be pulled-up. According to SDC/MMC specs, from 50k to 100k ohms is recommended to the value of pull-up registers. However the clock signal is not mentioned in the SDC/MMC specs because it is always driven by host controller. When there is a possibility of floating, it should be pulled to the normal state, low.

from http://elm-chan.org/docs/mmc/mmc_e.html

and

And the schematic for the "Breakout Board for DOSonCHIP FAT16 FAT32 uSD Module" here on Sparkfun doesn't show pull-ups.
MH

remember that microcontrollers can have internal pull-up resistors

[TheDirty]

I've never used them and never had any problems. If I was marketing a product I would probably go by the letter of the SD spec, but technically SPI interfaces do not need pullups.

I'm not certain when it's possible to float the DI and DO lines as these are also driven by the uC or the SD card itself.

[MostlyHarmless]

Thanks! Especially for that great reference, frank. I will leave the pull-ups in the design. I'll be sure to post a follow up if I run into any issues.
Cheers,
MH

[ohararp]

I have to use pullups for the library that I currently use. I only have them on the SDI and SDO lines though.

[MostlyHarmless]

ohararp-
Interesting. Other comments here indicate that these are required for floating pins or for the SD card (the SD specs). This seems to be a software issue then? What about your library causes those PU's to be required? Thanks.
Cheers,
MH