level translator 3.3 to 5V on SPI output lines of NI8451

[SergeB]

Hello everyone,

So i am using a NI8451 to talk to my DUT via SPI. I notice that SPI's highest voltage drive level is 3.3V. However, my NI DAQ SPI lines are each connected to an inverter (74HC14) on my DUT and the output of my inverter is given me some weird readings. I'm thinking that the 3.3V signal generated by the 8451 is not always being interpreted as a TTL high signal by the inverter because it's falling in between the high and low level thresholds. Therefore, I want to implement a 3.3->5V level translator on my SCLK, MOSI and CS lines so that the TTL levels are not getting misread by the inverter. 

I have tried using a SN7407 (open collector driver) with a 10k pull-up resistor on the output to 5V on all 3 of my SPI lines. I don't know if this is the correct way to do it because the signals on my scope are not right (SCLK don't work) and don't know if the NI8451 I/O lines can support it. I have attached a schematic of my connections. If someone could please help me on how to achieve this, I would greatly appreciate it!!!

thanks again.

Serge

[Buechsenschuetz]

Maybe using a 74HCT14 instead of a 74HC14 could solve your problem. The inputs of the 74HCT14 are compatible with TTL levels, and in general any voltage level above 2.8V is interpreted as logic HIGH in the TTL world. 

[SergeB]

Hi Buechsenschuetz,

Thank you for your response. Unfortunately, I don't have the luxury of changing anything on my DUT but your suggestion is a good idea 🙂 

[SergeB · Accepted solution]

So i was missing resistors from the output of my DAQ to input of my level shifter. I'm guessing i needed this voltage drop on the SPI lines so that they would not be potentially exposed to receiving a 5V kickback.

And also I had a floating ground which i forgot to connect ><.

The circuit now works with emitting the SPI signals at the 5V level 🙂 

[Daniel.Parrott]

Glad to see you got it working, and thanks for posting the fix to your problem!