5vdc mcu to 3.3vdc mcu serial comm

Thread Starter

bewter

Joined Dec 29, 2008
2
Greetings All, Simply, I have an atmega128 5vdc chip I need to serial comm to a 3.3 wifi module. I have been told I need something like a max3232 chip to do the conversion. I've read the data sheet on this chip and am not certain it meets my needs nor what would be required wiring it in. Can anyone point me in the right direction, maybe a schematic of a proper hook up?
 

leftyretro

Joined Nov 25, 2008
395
Greetings All, Simply, I have an atmega128 5vdc chip I need to serial comm to a 3.3 wifi module. I have been told I need something like a max3232 chip to do the conversion. I've read the data sheet on this chip and am not certain it meets my needs nor what would be required wiring it in. Can anyone point me in the right direction, maybe a schematic of a proper hook up?
Well first you need to look over the specs on the wifi module if you can. Some 3.3v devices can accept 5vdc signals and are called "5 volt input tolerant". Those that can't, can usually be handled by either a series current limiting resistor or a two resistor voltage divider between a 5volt logic output signal pin to a 3.3volt logic input signal pin. A 3.3 volt digital output signal pin can usually wire directly to a 5 volt digital input signal pin as 3.3 volts is usually above what a 5vdc system will consider a logical one.

The Max3232 sounds like a RS-232 to TTL level converter and is not applicable to hooking 5 volt systems to 3.3 volt systems.

Lefty
 

John Luciani

Joined Apr 3, 2007
477
Greetings All, Simply, I have an atmega128 5vdc chip I need to serial comm to a 3.3 wifi module. I have been told I need something like a max3232 chip to do the conversion. I've read the data sheet on this chip and am not certain it meets my needs nor what would be required wiring it in. Can anyone point me in the right direction, maybe a schematic of a proper hook up?
You probably do not want a MAX3232. If your WIFI module does not have 5V tolerant
inputs I would use a buffer like the 74AHC14 which does have 5V tolerant inputs.
Power the 74AHC14 from the 3.3V supply.

I have a schematic that shows a 3.3V level translator for an FTDI USB to RS232
cable that uses the 74AHC14. See the "FTDI Level Translator" application hint at
http://tinyurl.com/5rfmrz

(* jcl *)
 

Alberto

Joined Nov 7, 2008
169
Connect 3 10K resistors in series and connect one end to ground and the other end to the micro (5V device) then connect the 3 V device to the 2nd 10K resistor so that you should have 20K from ground for the 3 V device and 30K for the 5 V device. this devider works pritty well and is what I use normally to interface Xbee module to pics.

You will use max3232 only if you need to connect your 3.3 volts device to your RS232 PC port.

Alberto
 
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