MSP430F67751 - FLL Frequency tolerance

Sivasuryan Ramachandran83

Other Parts Discussed in Thread: MSP430F67751

Hi,

In our application, we are using MSP430F67751. We had configured the MCLK pin for 16MHz using XT1CLK (32768 Hz Crystal) signal and checked fequency using counter.

We had configured as follows for testing.

1. FLLREFDIV = 0

2. D = 0

3. N = 488

4. DCORSEL = 5

Hence I need to get signal of frequency, at Fdcoclk = (D * (N+1)*(Ffllrefclk /1)) = 0 * (488+1) * (32768/1)=16023552 Hz

But we are getting the MCLK frequency of about 16035866 Hz

Kindly let me know any frequency tolerance is there for FLL inside the MSP controller.

Thanks in Advance,

R.Sivasuryan

over 7 years ago

0 Clemens Ladisch over 7 years ago

Guru 243090 points

16023552 Hz is the target frequency. What you actually get is the nearest frequency that can be reached by one of the DCO and modulator settings.

0 Sivasuryan Ramachandran83 over 7 years ago in reply to Clemens Ladisch

Prodigy 30 points

Kindly let me know the frequency tolerance for that particular DCO and modulator settings in number.

0 Clemens Ladisch over 7 years ago in reply to Sivasuryan Ramachandran83

Guru 243090 points

See table "DCO Frequency" on p. 68 of the datasheet. For any one particular setting of the DCO, the tolerance is huge; for example, RSEL=5 and DCO=0 can result in any frequency between 2.5 and 6 MHz; that's 140 %.

Due to these huge tolerances, the FLL searches for some setting that is nearest the desired frequency.

The step between two DCO settings (S_DCO) is at most a ratio of 1.12. The modulator then can choose 32 steps between these two DCO settings, so the step between two modulator settings can be at most a ratio of about 1.0037, i.e., an error of about 0.37 %.

(And if you rely on the clock to be stable, it's likely that you will get hurt by the jitter introduced by the modulator.)

0 Sivasuryan Ramachandran83 over 7 years ago in reply to Clemens Ladisch

Prodigy 30 points

Thanks Ladisch for your valuable information

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MSP430F67751 - FLL Frequency tolerance