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Topic: Questions about Level shifter for RFM12B (Read 853 times) previous topic - next topic

bratan

Mar 13, 2013, 03:53 pm Last Edit: Mar 13, 2013, 05:36 pm by bratan Reason: 1
Hi,

I'm integrating RFM12B module to my design and a little over my head, so I would love some advice.
ATMega644p chip is powered by 5V, but I also have 3V supply for SD card (and RFM12B).
I know I need to shift 5V logic from ATMega to 3.3V for RFM12B chip. For this I'm following Adafruit's WaveShield design (SD card part) which uses 74AHC125 level shifter.
Question 1:
Can you tell me if I have i wired correctly (in the lower right corner). I have SS, SCK and MOSI going thru shifter, and then MISO is coming out directly into ATmega chip.  Does this make sense? :)  
Quesiton 2 (not very important):
Is there an alternative shifter I can use that will work with both SD card and RFM12B? 74AHC125 is not carried by TaydaElectronics where I prefer to get my parts from :)
Question3:
Adafruit's SD Card design has 10K pullup Resistor on SS line, I don't think I really need one for RFM12B (other SPI designs never mentioned it), am I correct?


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laadams85

You could always slow the clock speed and run the atmega at 3.3V.
For the logic level shifter sparkfun sells these
https://www.sparkfun.com/products/8745
and you can see the schematic here
http://www.sparkfun.com/datasheets/BreakoutBoards/Level-Converter-v10.pdf

Retroplayer

CD4050 and 74HC244 are other options.

The way you describe your wiring is correct. You are level shifting outputs from the Arduino to your device, but not the output from the other device since that level (3.3V HIGH) will be less than Arduino (5V) already.

bratan


CD4050 and 74HC244 are other options.

The way you describe your wiring is correct. You are level shifting outputs from the Arduino to your device, but not the output from the other device since that level (3.3V HIGH) will be less than Arduino (5V) already.

Awesome, thank you! :)
Xronos Clock - A talking arduino based alarm clock is now available. Check out xronosclock.com for pictures, source code, schematics, and purchasing info :)

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