SN74HCS125: Using for SPI 5V to 3.3V

Hi Clemens,

Thanks for the quick answer. Can you take look at my diagram and see if it is correct :

1. What VCC of buffer should be  for SCK,MOSI,CS  74AHC125 and what VCC for MISO (My uC is 5V and slaves are 3.3V) 
2. Termination resistor do I put after the buffer or before
3. Since I have two slaves should I use on buffer and keep OE always Enable or should I use two  buffer and connect each to their SPI select

Thanks

Hi Amir,

Your supply needs to be the required output voltage of the buffer. For all CMOS devices, the output will equal the supply.

If you want to input 3.3V and get out 5V, then you need a 5V supply, and a device that has reduced input thresholds (aka TTL-compatible inputs).

2. There are two separate traces, before and after the buffer. Both might need termination.

3. If the two slaves deactive the MISO outputs when not used, then they can be connected together.

Hi Emrys,

Thanks for quick answer.  I have two more questions :

1. If VCC is 5V and input logic is 3.3V how it does detect VinH or VinL. Datasheet 6.3: VCC = 5.5 VinH is 3.85V which is above 3.3V 

2. Can 74AHC125 due vise versa too. Uc logic is 5V as input and 3.3V output  and VCC be 3.3V

Thanks

Hi Emrys,

Thanks for detail answer. Unfortunately SN74LV4T125 is out of stock .  And if I want to use TXU0304 that 3 inputs   so I have to use two separate one for each slaves. Do you have any other alternative part number you can suggest

Thanks

Hi Amir,

Did you try looking on TI.com? For single supply up translation, all you need is a ttl-compatible CMOS input device. Here's a pre-filtered list: https://www.ti.com/logic-voltage-translation/buffers-drivers-transceivers/noninverting/products.html#p0max=4.42;18&p480=4;8&p776=TTL-Compatible%20CMOS&p1498=Automotive;Catalog&sort=p192max;asc