Replies to This Discussion

Permalink Reply by 13brv3 on October 11, 2009 at 8:55am

Hi Bill,

Thanks for mentioning the GPS. I hadn't noticed that the EM406 was only rated down to 4.5V, so that will certainly be the bottom end.

I'm currently looking at two different regulators, and will probably order a few of each, then start with the 4.5V version.

4.5V - http://www.national.com/ds/LP/LP2985.pdf
4.75V - http://www.micrel.com/_PDF/mic5252.pdf

I could certainly test the VCC with a variable supply, but it's more trouble than just replacing the chip, and the 4.5V "should" work. I'll try the new chip first, and if I have any problems, I'll go to the variable supply.

Cheers,
Rusty

Permalink Reply by Dedadz on October 9, 2009 at 9:40pm

Bill,

I'm wondering if the PIC CMOS input levels are also required for the 406 GPS on the serial channel. According to the 406 spec, the outputs are TTL (0-2.85V). This may be a systematic problem with both the receivers and the GPS. I'll be digging out the scope tomorrow to see what they really are. I think we should be concerned about making the device interfaces compatible and reliable before blessing one over the other just because they may work in a few instances under specific conditions.

Thoughts? Dedadz

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Permalink Reply by Dedadz on October 9, 2009 at 9:57pm

BTW, the UBlox also has TTL outputs. I'm wondering if this has caused some of the "losing GPS lock" problems with the Arduino folks?
-Dedadz

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Permalink Reply by William Premerlani on October 10, 2009 at 3:57am

Dedadz,

This could possibly be the "losing GPS lock" problem some of the Arduino folks. On paper, the Arduino processor is not compatible with the UBlox. There was a thread where someone raised the issue to Jordi, but he didn't seem to think it was a problem.

Best regards,
Bill

Permalink Reply by William Premerlani on October 10, 2009 at 3:54am

Dedadz,

Regarding the interface between the PIC and the GPS, there are level shifters on the red board to bring the 2.85 volts coming out of the GPS up to the full supply voltage of the PIC.

Also, I agree with you that we should make the device interfaces compatible.

When I started out, I was using PICs in the 18F series, they had TTL inputs for the PWM, and CMOS for the serial port, so we used shifters for the serial port, and did not have to worry about the interface to the radio.

When we moved to the dsPIC30F, the 5 volt pulses provided by my radio fooled me into thinking that since radios had 5 volt supplies, they would provide 5 volt pulses.

It looks like the threshold on the dsPIC30F is right around .6 of the supply voltage. So, when the receiver supplies 3 volt pulses, the supply has to be under 5 volts.

If I had to do it all over again, I would have provided level shifters on the PWM input channels.

Best regards,
Bill

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Permalink Reply by 13brv3 on October 11, 2009 at 7:10am

Hi Rana,

I only tested the UAVX very briefly, since I have 3 fully equipped Mikrokopters, and too many other projects to spend time on. I'll get back to it one of these days.

Rusty

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Permalink Reply by William Premerlani on October 11, 2009 at 7:22am

Rana,

It seems that you have very good luck. First, with your epic voyage, and now with your decision to place a diode in series with the the battery. That was a stroke of genius!

Sooner or later, I will deal with the voltage compatibility issue.

Radios that supply pulses with peaks that are larger than about 3.1 volts work just fine. For example, the radios that I use supply 3.4 volts, they work just great.

Radios that supply only 3 volt pulses are on the very edge of working.

Also, it depends on what supply voltage you are using. Lower supply voltages work better if you have a radio with low pulse voltages.

Best regards,

Billu

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Permalink Reply by William Premerlani on October 11, 2009 at 11:00am

Rana,

Most receivers work just fine with the redboard. It all depends on the voltage level of the pulses that they put out.

Some receivers produce pulses at the supply voltage of the receiver, typically 5 to 6 volts. That is more than enough.

Some receivers produce pulses with about 3.3 volts height. This is also quite adequate.

However, some receivers produce pulses in the 2.8 to 3.0 volt range. In those cases, whether the receiver works with the red board then depends on the supply voltage. If the supply voltage is low enough, those receivers will work too, but if the supply voltage is is too high, the pulses will not be high enough to trigger the Schmidt triggers in the dsPIC.

Best regards,
Billu

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Permalink Reply by William Premerlani on October 11, 2009 at 3:53pm

Hi Rana,

You asked:

"This means we have to use level converters in case the pulse height is 2.8 or less.

Can these level converters be like a single npn transistor with 1kohm resistor on base and collector ?"

If you have a radio with pulse height less than 2.8 volts, I would strongly suggest using a different radio, there are plenty of models that have more than 3 volts.

There are several ways you could make your own level shifter, including integrated circuits that are made just for that, or npn transistor and resistors, as you suggest (although they would invert the polarity of the pulses), or with MOSFET transistors, such as what is in the design of the red board for shifting the voltage level coming out of the GPS radio.

But I don't think any of those techniques are worth the trouble, if the voltage is that low, just get a different radio.

Best regards,
Bill

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Permalink Reply by William Premerlani on October 9, 2009 at 1:47pm

Andrea,

By the way, I forgot to ask, what is the model number of your receiver?

I want to start lists of radios that are compatible with the red board, and those that are not.

Best regards,
Bill

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Permalink Reply by William Premerlani on October 9, 2009 at 8:12pm

Andrea,

I am almost certain that the problem is that the voltage level of the pulses produced by your radio is not high enough to toggle the ratiometric CMOS Schmidt triggers in the dsPIC input channels.

However, depending on what the voltage level is, you might be able to get everything working ok by lowering the supply voltage a little bit to the board and radio.

I suspect that your ESC is supplying more than 5 volts. If you use an ESC that produces 5 volts, there is a good chance that everything will be ok.

The other choices are to use a 4.8 volt battery, or to buy the Castle Creations Berg Microstamp radio receiver.

Best regards,
Bill

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Permalink Reply by smguest on October 9, 2009 at 2:31pm

Bill,

FYI - I don't use channel 3 but have had no problems with the channel 4 input for selecting the mode. I use a Futaba R607FS (2.4GHz) and a Dimensions Engineering SportBEC to power everything. I've flown with this configuration and haven't noticed any problems so far.

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