Connect a 3.3 V device with a 5 V device


Thread Starter

Alberto Rojas

I want to connect a 5 volt device (TTL) with a microcontroller that uses 3.3 Volts, Data is going to be flowing in Both directions (from micro to device and device to micro), I need a cheap 0.3 USD solution a low power comsumption.

Curt Wuollet

You might want to check further, many current 3.3V micros are 5V tolerant. If not, check Maxim's catalog, I believe I saw IF chips there. Thirty cents for bidirectional translation seems
a bit optimistic but, who knows anymore?



You may not need anything at all. Check out the
V(ih), V(il), V(oh), and V(ol) specifications for the microcontroller and the TTL device. These describe the output voltage levels of a HIGH and LOW signal, as well as the input voltages required to detect a HIGH or LOW signal for each device. For example, for a TTL input to detect a HIGH, it only needs greater than 2.0V, and the microcontroller probably outputs around 2.6V.

For a good tutorial on this, check out the definitions of fanout and noise-margin:


In most TTL-to-3.3V CMOS applications (if this is the case), the two devices will interface with no problem. Worst case, you might need to use a couple of pull-up or pull-down resistors.

Is it possible to just use one type of logic family in your application (e.g. a 3.3V supply microcontroller and 'HC CMOS devices)? This would be the easiest.

Good Luck,

I am using the MSP430f412 of mototrola that unfurtunatly can not tolerate 5 V I am considering to use the SN74CBTD3384DW of motorola to do the share the bus.
I will check in Maxims!
The 5 Volts Device "Has" a one when it get in the port 2.4 Volts, so hopefully I wont have to much problems to interconnect that lines but I am a little bit worry because it is close to the to have noise probles with a 3 Volts input, my 3.3 volts microcontroller max input is 3.6 Volts, I have thought to use this device SN74CBTD3384DW Do you have any comment on this item?