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MSP430F5438A - Is Vref- required if using internal Vref+ as external reference?

Bernhard Boehmer
Expert 1210 points

Other Parts Discussed in Thread: MSP430F5438A

In order to maximise the number of analogue inputs on the MSP430F5438A, would the following be possible/advisable:

1.) use P5.0 as Vref+ reference output for external circuitry (e.g. 2.5V), but also

2.) use P5.1 as an analogue input (A9) (in order to maximise the number of analogue inputs). i.e. NOT to have to use P5.1 as Vref- (lower reference) output, but to use AVSS instead, as the intention is to supply 2.5Vnom from the MSP as an external reference.

If possible, are there any disadvantages? It seems rather wasteful for P5.1 to HAVE to act as lower reference if this is just going to switch AVSS anyway. With careful (placement) layout, there should not have to be any performance penalties.

  • 0
    Guru 227245 points

    The ADC12 allows you to pick either Vref- or AVSS as negative reference.

    It makes no sense picking Vref- when it is tied to AVSS. The main purpose is to allow offset voltages to be eliminated (e.g. measure signals between 1V and 2.5V while having the resolution of 1.5V/4095)

    Bernhard Bohmer said:
    With careful (placement) layout, there should not have to be any performance penalties.


    AVSS is already there as separate signal from DVSS, so you can properly route it to the supply without any digital current ripple. As I said, Vref- is onyl there to provide a different negative reference than AVss.

  • 0 in reply to Jens-Michael Gross
    Expert 1210 points

    Thank you for the reply, Jens-Michael.  The following aspects of the documentation contribute greatly to the confusion:

    1) Pin names: If "VREF+" refers to the use of pin P5.0 as internal reference (i.e. P5.0 = internal reference +ve OUTPUT), and "VeREF+" refers to the use of pin P5.0 as external reference (i.e. P5.0 = external reference +ve INPUT), then the datasheet (slas655c) naming for P5.1 as "P5.1/A9/VREF-/VeREF-" would imply that P5.1 can be used for its VREF- function (i.e. P5.1 = internal reference -ve OUTPUT). However, this does not seem to be possible. slau208m Figure 28-1 does not provide routing or registers for this. Why have the "VREF-" in the name then?

    2) The datasheet (slas655c page 7) pin description for P5.1 says i.a. "Negative terminal for the ADC's reference voltage for both sources, the internal reference voltage, or an external applied reference voltage" (emphasis mine). However, this "internal reference voltage" (i.e. OUTPUT) function does not seem to be possible (although from the pin description it might even seem to be mandatory) - see (1) above. Should the pin in the datasheet not just be termed "P5.1/A9/VeREF-" instead and the description updated to "(Optional) Negative terminal for the external applied reference voltage to the ADC"?

    3) The slau208m Figure 28-1 block diagram does not refer to the INCHx=1001 option as "A9" anywhere. Should the -ve reference input in this block diagram not be termed "A9/VeREF-" instead of "VREF-/VeREF-" (and similarly for A8/VeREF+)?

  • 0 in reply to Bernhard Boehmer
    Guru 227245 points

    If you look at the possible reference selections in ADC12MCTLx, you'll see that you can chose to use the internal reference against Vss or against Vref-. So Vref- is input if used as negative reference in conjunciton with the internal reference, and if is used as Veref- when used together wiht external reference.

    I agree that it wouldn't make sense to have this as Vref- output, as in this case AVss can be used as well.

    Bernhard Bohmer said:
    3) The slau208m Figure 28-1 block diagram does not refer to the INCHx=1001 option as "A9" anywhere.

    Indeed. And I agree that this might lead to some (temporary9 confusion. But A9 is jsut a name. And the schematics in the users guide shows that the Veref+ and Veref- pins are connected to the multiplexer at input 8 and 9. The nomenclature used inside the ADC12 registers is INCH8/INCH9, not A8/A9. In theory, each of the A0 to A15 signals could have any name. But for convenience, those without any other meaning have been named Ax.

    Don't stick on names, orient yourself on meanings. Too often, names are misleading or ambiguous when jsut being interpreted by themselves. And your understanding what a name could/should mean is not everyone's.

    To quote Humpty Dumpty: "When I use a word, it means just what I choose it to mean - neither more nor less". Same is true for datasheets. And advertisings.

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