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confusing clock settings (DCOCTL) of MSP430G2553


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Hello all, 

         I'm getting confused with the configuration of the clock in my G2553 device. I tried to find in the forum regarding this post but couldnt clear my doubt.

 

1.The DCOCTL has DCOx and MODx fields but there is no table in the user guide mentioning the values to be given to these fields to get the required DCO Freq.

 

2. To get 1Mhz DCO clock I used

    DCOCTL = CALDCO_1MHZ;
    BCSCTL1 = CALBC1_1MHZ;
and I got 1MHz DCO.
 
3.When I check the DCOCTL and BCSCTL1 in the debugger the values are  
DCOCTL = 0xBC
BCSCTL1= 0x86
 
4. It means that there is modulation. I dont know why this modulation. 
About the calibration, is it to get the operation in the required range of temperature as I found in Data sheet?
 
5. I'm confortable with  FR series where the clock settings is so much easier to understand.
 
Please give your views in clearing this confusion!
Thanks in advance.
 
~ vin43
 
 
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There's a chart with frequency ranges for DCO / RSEL settings on page 278 in the family user guide: http://www.ti.com/lit/ug/slau144j/slau144j.pdf

A table with actual numbers was probably omitted due to the large variability of the internal oscillator.

 

As far as I understand it, MODx basically allows to smooth out the frequency steps you see in that chart, i.e. gradually push up the frequency set by DCO to just under DCO+1 for MODx = 31. E.g. MODx=16 will set the frequency at half-way between two DCO steps.

 

Taking the calibrated 1MHz as an example:

DCO=5 (101), MOD=28 (11100), RSEL=6 (0110)

Looking at the chart, DCO=5 and RSEL=6 brings us into the 1MHz ballpark, and I guess MODx was used to tweak that value during calibration.

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