I was wondering, that if You use DCOCTL modulator ( first bits), you
already stay withing i.e. +-5% boundary (best case). And if you try baud
calculator from mspgcc.sourceforge.net,frequencies +5% and -5% have
different U0BRx and U0MCTL.
And when You tune DCOCTL without modulator (upper 3 bits). then You only
approximate target value i.e. worst case is around +-5%.(Yes I think
DCOCTL doesn't change USART precision much) as opposite to timing.)
So maybe You just succeeded with your particular chip (luck!)?
Would it be better to use 4 external resistors (i.e. 100k,2k7,2k7,2k7)
which could be switched in series, from unused outputs. Then tuning DCO
without modulator, and fine tune with resistors. You get accuracy +-
1.5%, and then set fixed U0BRx and U0MCTL modulator value
David Brown wrote:
And what about baud modulator UMCTL0. Is it precalculated fixed value?
Yes - it was calculated from the target frequency. It would also be quite
possible to use the regulator to vary the baud modulator (or even the baud
divisor registers). That could probably get you an even more accurate baud
rate. However, I wanted other timing functions to be improved as well, and
you don't gain anything much by aiming for a uart accuracy of more than
about 1%.
David Brown wrote:
On one system I made, I had similar requirements - I needed to generate
a
baud clock (uarts tolerate up to 5% difference between the sender and
the
transmitter), and had a 32kHz crystal on the timer pins as a reference.
I
made a very simple regulator in software. I aimed to drive the DCO at a
particular frequency (4.8 MHz, or something like that), and set up a
timer
interrupt that should occur regularly (based on the crystal timer). I
ran
timer B off the DCO frequency, and on the timer interrupt I compared the
measured timerB counts to the expected one and adjusted the DCO
accordingly.
It stabalised to within a fraction of a percent accuracy after a few
tenths
of a second, and the program then re-used timerB for other purposes.
Every
now and again, it would redo the calibration to take into account
temperature or voltage variations.
David
Background:
Steve has pretty much convinced me of the power savings advantages of
DCO, but I worried about providing a reliable and sufficiently accurate
clock for a 115200 serial line. For this purpose, I was using a
3.684MHz crystal. Although Fredic reports reports success with DCO and
the FLL algorithm (not applicable to the F161x parts), I am still
concerned about part variation.
The solution I am considering is to use one of the DACs to drive Rosc.
The initial setting of DAC would be 2.6V, midway beween 2.2 and 3V. Rosc
would be either 200k or 100k depending on frequency requirements of the
application. These values are available at 25ppm from KOA Speer as 0805
parts (RN32ALTD1003B25, RN32ALTD2003B25). The 100k resistor would allow
me to go to 7.368MHz.
In my application, I am also using a DS1390 RTC with battery backup,
which outputs a 32768Hz signal connected to TB0.
The general idea is to use RSELx and DCOx to get into the ballpark, and
then use the DAC to fine tune the frequency.
Is this stupid, overkill, or what?
Flames welcome.
Garst
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