convert from 3v to 5v

[Josephselim]

Hi all,
i need a driver to convert from 3v to 5v and vice versa,

actually, i've two circuits, full duplex,
the first cct deals with 3 v( zero logic 0~0.2 and one logic from 2.8 ~3 ), and the second cct contains a microcontroller which deals with 5 v

please some one help me

 

[Oznog]

It's called a "boost regulator". There are off-the-shelf ones but it's not something you find at Radio Shack.

Are you sure you need 5v for the uC? It's common that they're spec'ed to operate down to 3v, although you may need to operate it at a reduced frequency.

 

[bmcculla]

3V signals will usually work fine with 5V inputs - check the two datasheets to make sure that the Logic low and logic High voltages are compatible.

5V -> 3V can be a bit more complicated. If the 3V device has 5V tolerant inputs (this will usualy be listed in the features section of the datasheet) then you can connect them directly. Otherwise a simple resistive divider will drop the 5V signal down to 3V.

 

[Josephselim]

at first i would like to thank you Oznog and bmcculla
Oznog you have said

Are you sure you need 5v for the uC? It's common that they're spec'ed to operate down to 3v, although you may need to operate it at a reduced frequency

i am using atmel 89c51 , and i think it's working with 5v

the all story is, i have two circuits connected each other by serial bus ( full duplex)
the first circuit is a module from Motorola company
the data sheet of it says : the serial ports are not 5v logic compliant
for input signals, minimum input high voltage is 2v, and the maximum input high voltage is 3v. minumum input low voltage is 0v, and the maximum input low voltage is 0.8v,,
For output signals, minimum output high voltage is 2.4 and the maximum output low voltage is 0.5v

THE SECOND CIRCUIT IS ATMEL 89C51
so, this is all the story,
i can't risk to connect them now , i must make sure first that they are compatible each other ...

:?: :!: :!:

 

[bmcculla]

As I said before connecting a 3V output to a 5V input shouldn't be a problem - it will work.

Since the 3V part isn't 5V tolerant connecting your 5V output to the 3V input might damage one of the two parts and shouldn't be done. BUT if you put two resistors in as a divider - divide the 5V signal down to 3V - then you should be able to connect the two parts.

 

[Josephselim]

Hey bmcculla,
thanks for your reply
but you said

connecting a 3V output to a 5V input shouldn't be a problem

this is completely right, but not in my case
because i am use full duplex system
when the signal coming from 5v goin' through the voltage divider it be 3v, it's okay, but what would happen if the signal coming from 3v to 5v ?!!!!!!!
the 3v will be 1 or 2v after the voltage divider!!!!!!
:roll:

 

[Nigel Goodwin]

Hey bmcculla,
thanks for your reply
but you said

connecting a 3V output to a 5V input shouldn't be a problem

this is completely right, but not in my case
because i am use full duplex system
when the signal coming from 5v goin' through the voltage divider it be 3v, it's okay, but what would happen if the signal coming from 3v to 5v ?!!!!!!!
the 3v will be 1 or 2v after the voltage divider!!!!!!
:roll:

What?, you're doing full duplex on one wire?.

 

[bmcculla]

By definition full duplex means you can receive and send at the same time. Without some really weird circuitry you can't do full duplex with one wire.

It sounds like you have a half duplex setup using only one wire to send and receive, switching between sending and receiving to get bidirectional communication. I found the perfect part to do this; it's a Maxim part that does bidirectional level shifting using a sort of analog switch setup. I don't have the part number here but I'll post it when I get to work.

 

[bmcculla]

The MAX3370/MAX3371 are bidirectional level shifters that will work at up to 2Mbps. You can buy then through digikey.com. The SC70 is a tiny package so it might take a bit of practice to solder it.

 

[Josephselim]

Thanks bmcculla
The MAX3370/MAX3371, what i've been looking for
thanks again for your help