Logic Buffer for SPI level Shifter 74LVC126

Question

I am using 5V logic microcontroller and I want connect the Hardware SPI bus to SD CARD and Ethernet chip(W5500). Using below circuit I am not able to initialization SD card.

Below is my schematic:

Base on 74LVC126AS14-13 data sheet this chip should be able to convert between 5V and 3.3V.

I even lower my SPI speed (Division of 4,16) and still same problem. (Clock speed is 7.37Mhz), I even by pass MISO (Direct) same problem.

Below is my questions

1. What is wrong with this circuit or even can I use Logic buffer on SPI bus
2. If I can not use logic buffer can I use something like TXU0304
3. Since I have two slave do I suppose to connected differently.

Answer 1

The bus is not always SPI, because SD cards must be switched into SPI mode by sending native-mode commands. So you have bidirectional open-drain signals.

There are specialized SD card translators like the TXS0206, but they do not handle 5 V.

It might be possible to use a general-purpose open-drain bidirectional translator like the TXS0108E. (Bidirectional translators that are not open-drain, like the TXB0108, will not work for SD cards.)

Answer 2

1. The problem is that generally the LVC should be used with 3.3V supply and it can do 5V to 3.3V conversion. LVC is not intended for 5V operation, and even if it works at 5V supply, it may not work properly with 3.3V input when powered with 5V supply.

2. Maybe, depends on what you want to do with it (see #3)

3. Maybe, as it is unknown how you connected it, but the rule is that you cannot connect two outputs together if they are active at the same time, so if you did so it will not work no matter what chip you use.