Re: Low cost GPS receiver?

[Hal Murray]

> Chris, one side of the level shifter is 3.3V, the other is 15V.  If I am
> powering it by the VCC at 3.3 or 5V, how does this work? 

Look up the data sheet for the MAX232 or any of a zillion similar chips.

The general idea is that the chip includes a couple of flying capicators 
charge pumps.  It's neat when you see it.  Start by connecting a cap from pwr 
to gnd.  Now disconnect both ends and connect the bottom of that cap to pwr.  
Now the top of the cap is 2x pwr.  Connect that to another cap that goes from 
hi-pwr to gnd.  That will dump some of the charge from the flying cap into the 
hi-pwr cap.  Flip the switches back and forth and you will charge up the 
hi-pwr cap.

Now do the same (well, upside wown) from hi-pwr to neg-pwr.  If you start with 
5V, that gives you roughly 10V and -10V.  If you start with 3.3V you get 6V 
and -6V.

The 15V spec is probably the max input voltage on the RS232 to TTL direction.


chris.kuethe@gmail.com said:
> If you genuinely do need real EIA232 levels the MAX232 and similar parts have
> built in charge pumps to generate the required voltages. I'm betting that a
> pc with a 5v level shifter with work just fine with a GPS.


MAX232 and friends include an inverter as well as the level shifting.  Most 
serial I/O is actually upside down from RS232, expecting an inverter in your 
level conversion.

That works if you connect chip level I/O to other chip level I/O, bypassing 
both I/O-RS232 conversions and both inversions.

If you need to go from 3V to 5V, make sure it doesn't invert.  (Or that it 
does if your 5 end is not upside down expecting you to connect it to RS232 
without a level shitfer.)

-- 
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