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MSP430G2553: Capacitance to Digital Converter

Renan Santos Adriano
Mastermind 26505 points

Part Number: MSP430G2553
Other Parts Discussed in Thread: MSP430FR2676,

Hello Team,

Good day.

My Customer is working for MSP430G2553 and MSP430FR2676. They would like to understand deeply the functioning of the block Capacitance to Digital Converter and what kind of conversion it carries out.

To be more clear this is an excerpt from their code (the scope is to do capacitive sensing):

TI_CAPT_Custom(&one_button,&dCnt);
if(TI_CAPT_Button(&one_button))
{
P1OUT |= BIT0; // Turn on center LED
}
else
{
P1OUT &= ~BIT0; // Turn off center LED
}
Focusing on the function: TI_CAPT_Custom (const struct Sensor *groupOfElements, uint16_t *deltaCnt), which measures the change in capacitance of the sensor. Then, they display on CCS the tool graph setting its address to &dCnt, so to see on this graph the behaviour of the sensing pin. In output, it gives unsigned digital values such as 8000, 7000 and so on, so their question is if there a chance to know how many farads (pF, uF, etc.) correspond to these unsigned values?

Regards,

Renan

  • 0
    TI__Mastermind 49692 points

    Hello,

    You've listed two different devices here with different modules. I'm assuming you're referring to the MSP430FR2676. Also, I'm assuming "Capacitance to Digital Converter" means the CapTIvate module.

    Are they using the CapTIvate Design Center? I would recommend using that for CapTIvate development (e.g. tuning) rather than CCS. Have they referenced the CapTIvate Technology Guide? It contains a wealth of information about many aspects of CapTIvate evaluation and development.

    Regards,

    James

  • 0 in reply to James Evans
    TI__Mastermind 26505 points

    Dear James,

    Thank you for this response.

    The suggestion is already followed by my customer. The tuning of the sensor is not their real problem. They are using both pin oscillator technology with msp430g2553 (with CCS) and captivate with msp430fr2676 (with the captivate design center). Considering the code they used to program the old "g2553" they have the variable "deltaCounts" which is basically the same thing as "Delta" in the CapTivate Design Center using the new "FR2676" where Delta can be described as it follows: "When the sensor experiences a rapid change in capacitance, like a finger touching the sensor, the count value will be higher or lower than the LTA and the difference is the delta".

    The problem with this is that the Delta value is a digital unsigned number, but they would like to understand the correspective capacitance value, or at least its order of magnitude (pF, nF, etc.).

    Regards,


    Renan

  • 0 in reply to Renan Santos Adriano
    TI__Mastermind 49692 points

    Hi Renan,

    Thanks for the clarification.

    According to the Best Practices section in the CapTIvate Technology Guide:

    Sensitivity is measured in capacitance per counts. In the context of capacitive touch detection, the magnitude of change introduced by a touch is on the order of picofarads or hundreds of femtofarads. It is not uncommon for a solution to see a touch introduce 1 pF of change and be measured as 300 counts. And while the sensitivity might be 3.3 fF/count, this should not be considered until the noise is factored in. Refer to the section on Noise below.

    Walter also shared some details in the following thread that may be helpful as well.

    MSP430FR2633: CapTIvate Electrical Characteristics: Counts to Capacitance?

    Regards,

    James

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