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Electronics => Electronics => Topic started by: TrickyNekro on May 19, 2008, 10:31:58 AM

Title: 5V micro with 3.3V module...
Post by: TrickyNekro on May 19, 2008, 10:31:58 AM: First of all thanks in advance for any help provided...

I'm using atmega8535/16 for a single project and I want to use a CYWM6935 from cypress semi...
The problem is that I'm using 5V to power the micro and 3.3V for the module...

I know I can regulate power and such with a resistor and a zener but I have two basic questions...
First... On the command lines on the module... I put a 3.3V zener with what resistor... Think 4K7 is good????

And secondly... will AVR recognize the +3V3 as logic high or I need a transistor for amplification at 5V...
And if I use a transistor noise and transistor switching speed shouldn't be considered????

Thanks again,
Lefteris
Title: Re: 5V micro with 3.3V module...
Post by: Admin on May 24, 2008, 08:22:01 AM: You can just get away with ~2 diodes in series (each diode dropping ~.7V), no resistor needed.

Quote
And secondly... will AVR recognize the +3V3 as logic high or I need a transistor for amplification at 5V...
Yeap it will recognize it (the datasheet says exactly what voltage is considered high).
Title: Re: 5V micro with 3.3V module...
Post by: Steel_monkey on May 25, 2008, 02:49:03 AM: CYWUSB6935 datasheet says that it is possible to put more than Vcc on input while current is limited beyond 1 mA. So, you need at least 2 kOhms between mega output and CYW input.
For atmega, "logic high" is 0.6Vcc (read datasheet), that is 3 V, so no additional parts required. However, for additional stability conventional NPN can be used.
   +5
      |
      _|_
      | |
      | | 4K7
      |__|
      |
      |----------------------------------- to ATmega input
      | collector
4K7 | /
CYW output   ______ base | /
-------------------|______|---------------------||
      |    | \
      |   | _\| emmiter
      _|_      |
   | | 10-20 K    |
   | |       |
   --|--    |
      | |
   --- GND    --- GND
OOps, I said justwhat Admin has already said.
Title: Re: 5V micro with 3.3V module...
Post by: TrickyNekro on May 25, 2008, 02:10:42 PM: Thanks for the replies guys!

I have some questions here...
I've been doing some search on my own and...

Is a 74HC04 fast enough for this type of communication... It said it's speed it's nearly TTL levelers and it's rated fast speed...
The bad thing is that it inverts the signal but by using two inverters you get the correct one...

Steel_monkey:
Thanks for the schematics... but doesn't this really inverts the signal?!?! If not I guess it's perfect...
And thanks for the search you've done for me...
I have some details I want to share... I'll post later...

Anyways,
Thank you both!

Lefteris
Title: Re: 5V micro with 3.3V module...
Post by: TrickyNekro on May 25, 2008, 02:56:34 PM: I founded it!!!

It's on Elektor September 2007!!!

It's only one question here...
Are the signals from a 5V feed micro equal level to a parallel port...

If yes then the only thing needed is some resistors on the channels and some pull down...
I'll post schematics... no copyright here... the guy who did this project offers the schematics for free at his site...
He made a spectrum analyzer at 2.4GHz with the module and he is using the PC's parallel port...

Thanks again,
Lefteris
Title: Re: 5V micro with 3.3V module...
Post by: Steel_monkey on May 26, 2008, 04:27:32 AM: Yes, my schematic really inverts signal, I have forgotten this "little problem" :'(