MSP430G2553: MSP430G2553 Clock Speed

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Abu Bakar

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Part Number: MSP430G2553

Hi all:

I am having trouble figuring out this problem. Maybe someone here can give me some ideas. I am using the TI MSP430 Launchpad. 

I have fixed  the MSP430G2553 clock speed at 8 MHz in the code, and according to the datasheet, the clock speed should increase with increasing voltage. However, when I measure it using DCO calibration technique, I get a decreasing clock speed with increasing voltage. The variation is not too much; it varies between 7.92 to 8.16MHz. But shouldn't it be fixed at 8MHz, or maybe increase? I see nothing in the datasheet that explains this anomaly. 

Any ideas?

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Abu Bakar

• May 30, 2017 9:44 AM
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6 Replies

• Guru 68575 points

  

  Clemens Ladisch

  

  • May 30, 2017 1:01 PM

  Where does the datasheet say that higher voltage results in higher DCO frequency?

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• Prodigy 60 points

  

  Abu Bakar

  

  • Jun 2, 2017 6:29 PM

  In reply to Clemens Ladisch:

  

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• Guru 68575 points

  

  Clemens Ladisch

  

  • Jun 2, 2017 6:37 PM

  In reply to Abu Bakar:

  This figure shows allowed frequencies. For example, when V_CC is 2.7 V, then you must not use any frequency higher than 12 MHz.

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• Prodigy 60 points

  

  Abu Bakar

  

  • Jun 2, 2017 6:56 PM

  In reply to Clemens Ladisch:

  Thanks Clemens.
  Can you please answer the other half of my question i.e., why is the clock speed decreasing with increase in voltage? I am using following code to set the DCO frequency to 8MHz
  
  BCSCTL1 = CALBC1_8MHZ;
  DCOCTL = CALDCO_8MHZ;
  
  I am getting these frequency values
  
  Voltage(V) Clock Speed(MHz)
  2.3 8.2
  2.5 8.1
  3.0 8.0
  3.5 7.9

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• Prodigy 60 points

  

  Abu Bakar

  

  • Jun 2, 2017 6:58 PM

  In reply to Abu Bakar:

  Voltage(V) --------Clock Speed(MHz)
  2.3------------------ 8.2
  2.5------------------ 8.1
  3.0------------------ 8.0
  3.5------------------ 7.9

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• Intellectual 965 points

  

  Dubnet

  

  • Jun 2, 2017 7:30 PM

  In reply to Abu Bakar:

  If I read the information on page 30 of the MSP430G2553 data sheet correctly, that actually falls into the specification for power supply rejection error of the DCO at +/-3% from 2.2 to 3.6V

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