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Re: [avr-gcc-list] compiler not generating IO

From: Steve Franks
Subject: Re: [avr-gcc-list] compiler not generating IO
Date: Fri, 27 Jan 2006 09:24:42 -0700

Just to relate my experience -

I've personally never had a problem with a device that had an 8MHz
internal oscilator to run at 38.4kbps (UBRR=12).

That said, if I set OSCCAL to the recommended value, it usually kills
the UART.  I think this is due to the fact that UBRR=12 on a 'real'
8MHz oscilator gets you 39 or 40 kbps, and all the devices I've used
tend to have a slightly slower clock when uncalibrated (or maybe
vice-versa).  Either that or I need to re-read the OSCCAL doc's. 
Anyway, I've never needed a calibrated clock, or more than 38.4kbps,
so it hasn't been an issue for me.  Occasionally some spec will call
for 115k or moderately reliable timer/counters, so I use the 7.3828MHz
crystal with good results.

I've been thinking lately about using a 16MHz xtal with a silabs.com
CP2102 uart <-> usb bridge - that device can 'spoof' the windows XP
baud rate settings, so it will really run at 1,000kpbs when an app
calls for 115k (that's the biggest number I've seen everything
support).  Then you can get approx 1,000,000 * 8/10 / 8 =
100k-bytes/second transfers for doing light-weight
data-acquisiton-type stuff with an avr.  Of course, that data rate
doesn't leave many clock cycles left over for processing data, but I
figure one could get a 24-bit adc to run into the computer at maybe
25kHz for doing some high-res FFT's.  Everyone needs one of those on
their benchtop, no?  Anyone know of a good pre-existing app for
win32/linux to do the calc & FFT display?  Most I've seen use a
soundcard, but maybe there's an open-souce app around that I could
replace the hardware interface to?  Probably, I should just buy one of
those $100 audiophile firewire 'soundcard's instead.

Ah well, sorry for the novel ;)


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