And what about baud modulator UMCTL0. Is it precalculated fixed value?
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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