# 3.3V -> 1.8V level shifter

#### bumclouds

Joined May 18, 2008
81
Hi everyone,

I'm trying to connect a device which uses 3.3V logic to the input pins of a microprocessor that can only tolerate 1.8V.

So I need to step down the signal from 3.3V to 1.8V.

Can anyone on this forum recommend a common and easy to find IC which does this job? DIP package is the only option for me here.

Thankyou!

- Andrew

#### kubeek

Joined Sep 20, 2005
5,796
What speed are we talking? A simple resistive divider should do the job for the 3.3->1.8v conversion. The other way, most devices nowadays accept anything greater than 0.8V as a logical 1, so you should be safe there, but check the datasheet.

#### crutschow

Joined Mar 14, 2008
34,681
.....................
The other way, most devices nowadays accept anything greater than 0.8V as a logical 1, so you should be safe there, but check the datasheet.
Not quite. The maximum logic 0 it often 0.8V or less but the minimum logic 1 is usually at least 2-3V. The voltage in between those two levels is a no-man's-land where the circuit may interpret it as either a 1 or a 0 depending upon the particular circuit tolerances.

But that still means that a 3.3 logic high will be correctly interpreted as a logic 1 by 5V logic.

#### kubeek

Joined Sep 20, 2005
5,796
You're right, I misinterpreted the datasheet I was reading, the actual thresholds for 3.3Vcc were <0.6V for 0 and >2V for 1.

#### steinar96

Joined Apr 18, 2009
239
A resistive divider may cause problems due to either too high input resistance (if its too stiff) or waste of power (too soft). There are many other ways to translate while maintaining proper conditions at the IO pins. These include diode clamping circuits.

#### crutschow

Joined Mar 14, 2008
34,681
If you can tell us the frequency and/or pulse width of the signals we can better recommend a solution.

#### bumclouds

Joined May 18, 2008
81
The device I am using is a PSX game controller which (i think) has a baud rate of 9600kbps.

The device I am using to read from the game controller is a Texas Instruments microprocessor whose input pins are not 3V tolerant (confirmed on the TI forums).

Will using a simple resistor divider for each pin do the trick?

#### Papabravo

Joined Feb 24, 2006
21,259
ON Semiconductor and Fairchild have a line of single gate parts in SOT-23 packages that work on any Vcc from 1.65 V to 5.5V and have asymmetrical "TTL like" thresholds. This is in contrast to "CMOS like" thresholds at Vcc/2

I can dig up links to actual datasheets if you are interested.
http://www.onsemi.com/PowerSolutions/product.do?id=MC74VHC1GT50

#### Brownout

Joined Jan 10, 2012
2,390
Will using a simple resistor divider for each pin do the trick?
I think you should be fine.

#### crutschow

Joined Mar 14, 2008
34,681
A voltage divider of about 5K and 5k (to ground) should work well.

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