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ADC10OSC (10-Bit ADC Internal Oscillator) -- How to Control Frequency?

Thomas Foxcroft
Expert 2996 points

The G2231 spec for its internal 10-bit oscillator frequency ranges from 3.7MHz (min) to 6.3MHz (max).

  1. What supplies the ADC10OSC its signal?
  2. What register controls the ADC10OSC frequency?
  3. If you change the DCO frequency, does it have any effect on the ADC10OSC frequency?
  4. Is this the default clock if no clock is selected with the ADC 10 Control 1 register (ADC10CTL1)?

 

  • 0
    Guru 227245 points

    Thomas Foxcroft said:
    What supplies the ADC10OSC its signa

    Fromt eh specs I'd say a simple R/C oscillator.
    Thomas Foxcroft said:
    What register controls the ADC10OSC frequency?
    None. It's 'just there as it is'. somewhere in the specified range.
    Thomas Foxcroft said:
    If you change the DCO frequency, does it have any effect on the ADC10OSC frequency?
    No.
    Thomas Foxcroft said:
    Is this the default clock if no clock is selected with the ADC 10 Control 1 register (ADC10CTL1)
    technically no. There's always a clock source selected. The default selection is ADC10OSC. See the register description - below the bit diagram there is a notice tellign whether it is read or write or both, and what the default is. numbers without brackets are set to default on PUC whiel those in round brackets are set to default on BOR but will 'survive' a PUC.

    If you need a precise clocking of the ADC10, then you'll need to supply (and use) the required frequency through SMCLK etc.
    But if you want to lett it freely running and just want to read the last conversion result ata timer interrupt, or want to trigger each conversion by a timer, you can as well use this ADC10OSC. It consumes less current than an active SMCLK and if you enter low power modes with SMCLK deactivated, it will still be able to drive teh ADC10 while you trigger the timer (and the conversion start) by e.g. a low-frequency ACLK.

  • 0 in reply to Jens-Michael Gross
    Expert 2996 points

    Mr. Gross:

    As for your reply that a simple RC oscillator supplies the ADC10OSC, well, I'll agree with your assumption, but only because information is missing about the matter. For example, the G2231 data sheet only describes the frequency parameters for the ADC10OSC as a range from a minimum to a maximum (so it's probably affected only by temperature), nor does the data sheet and MSP430x2xx Family User's Guide provide information about configuring its frequency.

    As for your reply about the default values for the ADC 10 Control 1 register, I drove right by the "Register Bit Conventions" section at the begining of the guide that describes the register notation. Now I understand. And the additional optimization advice is very useful.

    Thank you very much for your help. You made a difference.

  • 0 in reply to Thomas Foxcroft
    Guru 227245 points

    Thomas Foxcroft said:
    For example, the G2231 data sheet only describes the frequency parameters for the ADC10OSC as a range from a minimum to a maximum (so it's probably affected only by temperature),

    Or manufacturing tolerances.

    Thomas Foxcroft said:
    nor does the data sheet and MSP430x2xx Family User's Guide provide information about configuring its frequency.

    No, it is provided as an oscillator just as REFO or VLO on other devices. There is no register anywheer where you can influence it. Well, if you could, there is not much need to provide additional access to ACLK or SMCLK. :)

    Actually the users guide gives some hints: in the diagram of the ADC10, the ADC10OSC is drawn as a blackbox with no controllign register bits or inputs. It is 'just there' providing a clock signal.
    Also there is the section abotu 'Conversion clock selection':

    The ADC10OSC, generated internally, is in the 5-MHz range, but varies with individual devices, supply voltage, and temperature. See the device-specific datasheet for the ADC10OSC specification.

    In conjunction with this, the datasheet has to be interpreted as 'can be as high as, but never lower than'. There are other parameters with a similar meaning, such as the DCO ranges for different RSEL settings, or the timings for the oscillator fault detection.

    Thomas Foxcroft said:
    I drove right by the "Register Bit Conventions" section at the begining of the guide that describes the register notation.

    It's easy to miss. Nobody looks there after all this 'blabla' stuff at the beginning of the 'read this first' chapter.
    Unfortunately, the legal stuff is considered more important and therefore presented more prominently that this actually really important information. And those skipping the legal blabla will skip this too.
    Happened to me too the first time. (hey, on the 5x devices, the register conventions get even more cases, as there is an additional BOR condition. (with curled brackets)

    Thomas Foxcroft said:
    Thank you very much for your help.

    You're welcome.

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