Using Low-Voltage Devices...

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LFLekx

Member
I don't know if this has been covered before... I can't find a reference when I search. :)

I'm thinking about interfacing a digital-compass module from Honeywell to a PICAxe system I've built, in order to give it orientation-seeking ability. The module is an I2C unit, but it runs on 3.3 volts.

Can I simply add a trio of diodes in series with the Vdd pin, to drop the voltage from 4.8 volts to 2.7v...? Will I have to add resistors into the I2C signal lines, in order to limit the current...?

Just curious... :D
 
W

westaust55

Moderator
Which particular compass are you considering?

Looking at the Honeywell's HMC6352 2-Axis Digital Integrated Compass Solution

The stated features are:
2-Axis Magnetic Sensors with Electronics and Microprocessor
Fully Integrated Compassing Solution
Integrated Compassing Algorithms and Calibration Routines
Miniature (6.5 by 6.5 by 1.4mm) 24-Pin LCC Package
2.7 to 5.2 volt Supply Range (Good for Low Power Battery Operation on OEM PCBs)
I2C Digital Interface
User Selectable Slave Address Permits Multiple Sensors on Bus Interface
Drop-in, plug and play feature Allows for more High volume production
Technology and Product Features
[/quote

so it will operate at 4.5V or 5V the same as your PICAXE and no need for any diodes or other voltage control
Click to expand...
 
hippy

hippy

Ex-Staff (retired)
If 3V3 operation is required, the easiest solution is to use a 3V3 LDO Voltage Regulator and to run the entire system on 3V3 if possible.

Uisng a 3V3 I2C device and 5V processor should be fairly simple as the I2C bus is specified as open collector, simply pull-up the lines to 3V3. Most micro's should read the I2C bus signal correctly. You'd only have potential problems if the micro used a hard-driven SCL line.
 
S

Steve Joblin

Member
The other approach is to use a voltage divider... a 1.8k and 3.3k are good values to go from 5V to 3.3V
 
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D

Dippy

Moderator
I haven't checked all PICAXEs ,as I haven't got time, but certainly the 18X (16F88) uses ST (Schmitt Trigger) for SDA and SCL....
 
L

LFLekx

Member
westaust55 said:
Which particular compass are you considering?

Looking at the Honeywell's HMC6352 2-Axis Digital Integrated Compass Solution
Click to expand...
The one I'm looking at is the Honeywell HMC6343. It's a 3-axis device with tilt adjustment - something I'm looking for, because I intend to use it in a flying rocket to correct its' heading and orientation.

Its' specs call for a Vdd range of 2.7 to 3.6 volts. This might not be a problem, if I didn't have to drive R/C servos from the same processor... which require a drive signal of 4.8 volts.
 
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ylp88

Senior Member
As Hippy said, it may be easier to try and run the whole thing from a 3.3V supply. And in accordance with Dippy, I've successfully run a 28X1 from a 3.3V supply using SPI and i2c, as well as a few other chips from 2xAA batteries with no problems. Just check the datasheet to see what type of input it has.

In addition, if the rest of the circuit will work on 3.3V, the relatively slow servo PWM signal should be easy enough to buffer to 5V if you need the higher drive voltage.

ylp88
 
B

BCJKiwi

Senior Member
Suggest you look at a MAX884 3.3V reg.
This will run from a supply as low as 4.3V so it can be fed from the 4.8/5 supply.
It only uses 11uA quiescent current (less in standby or shutdown) and is available in 8pin Dip or SO
Will deliver up to 200mA.

You would need to investigate running the PICAXE from 3v3 as well, as the i2c bus for the Honeywell device appears only to handle 3v3, not 5v, else the level shifting that Jurgen refers to would need to be employed.

There are 5V tolerant 3Axis accelerometers available (see Dimension Engineering's DE-ACCM3D for example) but these typically are much larger as they are built up modules on a small pcb with 3v3 regulator and pic on board so may not be suitable.
 
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Brietech

Senior Member
LFLekx said:
The one I'm looking at is the Honeywell HMC6343. It's a 3-axis device with tilt adjustment - something I'm looking for, because I intend to use it in a flying rocket to correct its' heading and orientation.
Click to expand...

You might want to watch out there! At least here in the states, I believe it is quite illegal to add "active" guidance to a rocket (as there is then very little difference between your rocket and a missile!). My rocket club had proposed adding actively controlled fins to the rocket, but determined legal roadblocks would probably prevent us.
 
L

LFLekx

Member
Brietech said:
You might want to watch out there! At least here in the states, I believe it is quite illegal to add "active" guidance to a rocket (as there is then very little difference between your rocket and a missile!). My rocket club had proposed adding actively controlled fins to the rocket, but determined legal roadblocks would probably prevent us.
Click to expand...
I've been giving that a lot of thought... and have come to the conclusion that there is a WORLD of difference between target-seeking and orientation-keeping. Since all I'm looking for is the ability to point the cameras to a certain bearing, while keeping the rocket vertical, I don't believe that it will be a problem.
 
M

MFB

Senior Member
Tilt compensation?

I am interested in your comments about using a tilt-compensated sensor, as these normally employ accelerometers to detect tilt. I have found that this approach only works for near static conditions and suspect that the rockets acceleration would invalidate this compensation technique.
 
L

LFLekx

Member
MFB said:
I am interested in your comments about using a tilt-compensated sensor, as these normally employ accelerometers to detect tilt. I have found that this approach only works for near static conditions and suspect that the rockets acceleration would invalidate this compensation technique.
Click to expand...
You could be right. The thing about the particular part I have in mind is that it can be mounted flat against the circuit board, which will be vertically-oriented for most of the flight. Other (2-axis) compass modules would have to be mounted perpendicular to the board... and I'm hoping to avoid putting stress on the connections that could potentially cause them to separate.
 
D

daisy8350z

New Member
HMC6343 and PICAXE

Hi everyone

I have been using the CMPS03 compass with the 28x1 PICAXE on an automatic robot and just changed to the HMC6343 compass instead. I am however having a problem reading any information from the new compass module.

Does anyone have any basic code they have used to interface with the HMC6343 on the i2c ? The device can provide a tilt corrected compass heading, roll information in 2 directions and seems particularly useful, if I can get it to work.

Thanks for your help

daisy

P.S. I powered the new HMC6343 with a 3.3v regulator (regulated from the 5v at the PICAXE) and then just connected up the i2c ports and a couple of pull up resistors.
 
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