Problems with Teensy 4.1 and level shifting using a 74LVC

Guillaumeb28

New member
Hi,

I need to connect multiple devices than run at 5V to a Teensy 4.1 through an SPI bus (slave devices) but I encountered some problems with level shifting.

I use an SN74AHCT1G125 to shift the CLK line from 3,3V to 5V and an SN74LVC1G125 to shift the DATA line from 5V to 3,3V.

SN74AHCT1G125 : https://www.ti.com/lit/ds/symlink/sn74ahct1g125.pdf
SN74LVC1G125 : https://www.ti.com/lit/ds/symlink/sn74lvc1g125.pdf

On the SN74AHCT1G125, my wiring is the following :
OE grounded
A (in) connected to the Teensy CLK pin
Y (out) connected to device CLK pin
VCC connected to a 5VDC voltage source, also used to power the device.

On the SN74LVC1G125 :
OE grounded
A (in) connected to the device DATA pin
Y (out) connected to the Teensy DATA pin
VCC connected to 3,3V Teensy pin.

Capture d’écran.jpg

The SPI interface doesn't work.

I ran some basic test :
- When I set the CLK pin high or low, the output on the Y pin of the 74AHCT is correct (high 5V/low).
- But when I connect the 5V source directly to the A pin of the 74LVC, the output on the Y pin is not at 3,3V.

However, according to the datasheet of the 74LVC, when OE is low and A is high, Y should be HIGH.

Is there anything that I am missing in my wiring ? Does anyone already used this setup ?

Thanks !
 
Last edited:
Just to be sure: are all your grounds [Teensy GND, 74AHCT GND, 74LVC GND and external +5V supply GND] hooked up together?
Is the external 5V supply also connected to Teensy pin VIN? Or is the Teensy powered over USB?

Paul
 
Just to be sure: are all your grounds [Teensy GND, 74AHCT GND, 74LVC GND and external +5V supply GND] hooked up together?
Is the external 5V supply also connected to Teensy pin VIN? Or is the Teensy powered over USB?

Paul

All the ground are connected together.
The Teensy is powered over USB.
The Teensy pin VIN is not connected to anything.
 
Is it possible to disconnect the 74LVC Y pin from Teensy pin 12? Just to make sure that pin 12 is not pulling the Y signal to GND?
Otherwise I can only conclude that the 74LVC has passed away.

Paul
 
One last thought: looking at the SMD packages of the 74LVC, which are pretty small, could it be that Y pin accidentally solder-bridged to the GND pin?

Paul
 
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