SD (or microSD) module with/without line drivers (thoughts)

Hi,

I'm looking to buy a bunch of SD card modules to SPI interface to Nano's. Before I go and get a good number of units, I wanted to know if there is any downside/restrictions of using the versions without the 4 channel line driver incorporated (is it just there for belt&braces?)

Cheers

I'm not sure what you mean by line drivers. Typical microSD modules have level-shifting I/Cs that allow them to be used with 5V processors such as the Nano (SD cards are 3.3V devices). Unfortunately, typical full-size SD card modules do not have those chips, and I believe they should not be used with 5V processors at all, only with 3.3V processors. I've outlined my reasoning here:

If you have a choice, I think the best option is microSD modules with level-shifting chips, typically the 74LVC125A.

MartinPrescott:
if there is any downside/restrictions of using the versions without the 4 channel line driver incorporated (is it just there for belt&braces?)

If by 'line driver' you actually mean the level converters to allow the 3.3V logic SD cards to work with a 5V logic Nano, then the downside of not using the level converters is that you will likley fry the SD cards.

Ah sorry, I'd overlooked the 3.3V logic level requirement..DOH! (given that most of the SD card modules without the 74LVC125A still have the 3.3V converter.

Many thanks for you help.

I had assumed that the 74LVC125A was maybe added for impedance matching of the SPI lines as it is described as a buffer/line driver in the data sheet (see below).

"74LVC125A
Quad buffer/line driver with 5 V tolerant input/outputs; 3-state
Rev. 7 — 11 April 2013 Product data sheet.
General description
The 74LVC125A consists of four non-inverting buffers/line drivers with 3-state outputs
(nY) that are controlled by the output enable input (nOE). A HIGH at nOE causes the
outputs to assume a high-impedance OFF-state.
Inputs can be driven from either 3.3 V or 5 V devices. When disabled, up to 5.5 V can be
applied to the outputs."

I think that the basic idea behind a buffer is to "tidy up" the signal downstream of the buffer (maybe for higher-speed performance) if the upstream tracks are of widely different lengths etc.?? Also to buffer-isolate the downstream device from noise on the signals (if they are coming from a more active area of the board)??

It can also be used to level shift....so there you have it.

There might be some reason why the 74LVC125A is used instead of something like the TXS0104E or TXB0104 (4 channel Bi-directional logic level voltage shifter) that are used on the level shifter BOB's (it might be a slightly cheaper device given that it is just 4 omni-directional buffers wheras the TXS0104E chip schematic is more sophisticated).

I would like to stick to SD ideally, purely for physical size as the card will be pulled in-out regularly for data-offload.

I have run prototypes with the non 74LVC125A SD modules straight from Nano's, so guessing that they might fry the card over extended time as you say.

I have found an SD module with the 74LVC125A on board (slightly more expensive), so will stick with that one

Weird that the vast majority of the SD modules don't have the 74LVC125A, but most of the the micro-SD versions do!

Cheers

MartinPrescott:
Ah sorry, I'd overlooked the 3.3V logic level requirement..DOH! (given that most of the SD card modules without the 74LVC125A still have the 3.3V converter.

I think you mean they still have the 3.3V voltage regulator. That's only for the SD card's Vcc.

I have run prototypes with the non 74LVC125A SD modules straight from Nano's, so guessing that they might fry the card over extended time as you say.

Yes, it seems that some SD cards are quite robust. But even those may not last long.

I have found an SD module with the 74LVC125A on board (slightly more expensive), so will stick with that one

I would very much appreciate a link to that module. I was unable to find one.

Iv'e seen the same module from a few different sources. The one below is the same as the one above, but is branded Catalex

http://www.xelcore.com/product/arduino-spi-sd-tf-card-adapter-v1-0-module-3-3v-5v-compatible-with-rpi-stm32/

Thanks very much. I couldn't get the xelcore link to come up. And I couldn't find this on Ebay. But at least we now have the dx.com source.