5 volt picaxe to 3v module

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russbow

Senior Member
I am hoping to interface a picaxe 18m2 ( at 5 volts ), to a HMC 5883L module ( at 3.3 volts ), using I2C for communication.
Are there any " gotcha's " to look out for, or must both device run off the same voltage.
If different supply voltages are OK, should I tie the I2C data and clock lines to the higher or lower voltage.
 
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inglewoodpete

Senior Member
There was quite a thorough discussion on 3.3v to 5v interfacing in this thread.

westaust55 could probably provide quite a bit of help in this interfacing area but it's the middle of the night here. He'll surface in a few hourzzzz.
 
Hemi345

Hemi345

Senior Member
If you can solder surface mount, there are simple level translators for i2c that make this very easy, such as the PCA9306
 
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westaust55

Moderator
as per my post in the thread IWP provided a link to, when just dealing with i2c comms, I use the single transistor scheme as per the Philisp/NXP i2c specification document found here:
http://www.cs.unc.edu/Research/stc/FAQs/Interfaces/I2C-BusSpec-V2.1.pdf.
See Figure 45 in section 18.1. Figure only attached here for ease of reference since I have ofter referred folks to the i2c specification.

I have always used 2N7000 (FET type) transistors for those required. Note that there are pull-up resistors on both sides of the transistors.
 

Attachments

PieM

PieM

Senior Member
I use Infrared Thermometer - MLX90614 3.3V on a 14M2 5V with data and clock lines tied to 3.3V by 4.7k
Scl and Sda are open collector and high level seem to be OK for Picaxe!
 
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russbow

Senior Member
Thank you all for your comments. Pretty obvious really :eek:
Been so used to " 3 fresh AA cells ", never thought of running Picaxe from lower voltage. The serout to 5 volt display works fine too.

@Westy. Thanks for the bi-directional circuit. Duly added to the grimoir:)
 
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