A little help please..... 5V to 3.3V level converter

Seth Wilson

Greetings all.

I am designing a project and I wish to use the basic stamp 2 as the mcu. The device I want to interface with the BS2 will communicate serially, however the levels it wants to communicate with are 3.3V instead of the 5V that the BS2 pins output.

My question is this. Is there a chip I can put in between to convert the levels, or can I use other components to achieve the same effect, or does it even matter? I have done a search and found the MAX line of chips, but from what I can see they have chips to convert 5V (or 3.3V) to RS-232 levels, which isn't exactly what i need.

Thanks in advance for any advice given... it is greatly appreciated.

Seth.

Newzed

Use a 3.3V regulator.· The IRU1206-33CY is a good choice.· $1.00 at Digikey.



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Sid Weaver
Do you have a Stamp Tester yet?
http://hometown.aol.com/newzed/index.html

·

Jon Williams

If it's a straight digital connection, it seems like you could do this:



This circuit should work in either direction.


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Jon Williams
Applications Engineer, Parallax

Chris Savage

Seth,

·· The IC listed at the link below is a bi-directional 5V/3.3V interface/Buffer.· It is designed to do just what you need.· Even has a bypass cap on-board.· 24-pin DIP format module.

http://www.sjjmicro.com/SJJEMS1000.html

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Beau Schwabe

If this connection is from·the Stamp to a 3.3V device or from a 3.3V device to the Stamp, then Jon's circuit will work just fine with the stamp
voltage thresholds.· Otherwise Chris's suggestion is appropriate.· If you find that the 3.3V device requires more drive current than 333uA
(5.1K 10K configuration), you can try other resistor combinations.

5.1K     10K     333uA     5V <--> 3.3V (Jon's suggestion)
2.2K     3.9K    822uA     5V <--> 3.2V
1K       1.5K    2mA       5V <--> 3.0V
220      390     8.2mA     5V <--> 3.2V

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Paul Baker

Don't forget 1.8k and 3.3k @ 0.98mA 5.0V<-->3.3V [noparse]:)[/noparse], I use this combo because both values are easily had (got strings of 20 each in the radioshack resistor bag) and produces a happy medium of current carrying, and is near 1mA simplifying total current·calculations.

BTW, if you are feeding an output of the 3.3V to an input of 5.0V device (ie not a bi-directional line, one way traffic from 3.3V device to 5V device) feed the line straight through, no resistors needed. To see an example of this look at·http://www.captain.at/electronics/pic-mmc/ which shows connecting a 5V·PIC to a 3.3V MMC card. The interface is SPI which has no bidirectional lines, the signals from the 5V PIC to the 3.3V MMC go through one of the resistor dividers, the signals from the 3.3V MMC to the 5V PIC go straight through. If you are confused at all by this description, use a resistor divider on all lines because while it doesn't help in 3.3V->5.0V, it doesn't hurt and you cant make a mistake of directly connecting a 5V output to a 3.3V input (which is bad).

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·1+1=10

Seth Wilson

Wow, thanks for all the great replies.

I thought about using resistors, given Ohms Law, but was not sure and didn't want to damage the device I am connecting it to.

I'll give the voltage divider a try and report back.

Thanks again, the support from Parallax is unrivaled!

Seth.