Jens Andersen wrote:
Hi Garst and list,
I have played a little with the fet140_fll2.c example from the
TI fet tools package.
Using a freqency counter the DCO seems rock solid, but my
old Tek 210 dso don't like it, i suppose this has to with jitter.
the DCOLCK signal does have a jitter by design.
it is mixing two frequencies within 32 clocks. one is higher, the other
lower than the target freq, but the mean over 32 clock cycles is very
accurate.
chris
As my power requirements arent that low, im using a HF crystal on XT2.
I hope this may help you a little.
Best regards,
Jens
----- Original Message -----
From: "Garst R. Reese" <[email protected]>
To: "mspgcc-users" <[email protected]>
Sent: Wednesday, January 26, 2005 8:27 AM
Subject: [Mspgcc-users] [RFC] Precision DCO for F161x
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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