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AM335x built-in bandgap sensor

Ryuuichi machida
Guru 12075 points
Other Parts Discussed in Thread: AM3358

Hi expert,

I am checking temperature sensor which is implemented in AM335x.

According to following wiki site, the sensor can measure temperature from -50 degree to 150 degree w/ 8-bit ADC.

 http://processors.wiki.ti.com/index.php/AM335x_Thermal_Considerations

(Please see "Measuring Case Temperature")

Could you please confirm following questions about above 8-bit ADC and give me feedback ?

Q1.  According to above wiki site,

"8-bit ADC gives the sensor a poor accuracy of ±10.8°C."

Is mentioned above correct information ? Is it mistake of  "±1.08°C" ?

If "±10.8°C" is true, when user set thermal shutdown, is the range which thermal shutdown event will occur from 136.2°C to  157.8°C ?

Q2. Could you show me the table which is desribed about relation b/w ADC code and temperature ?

Best Regards,

Machida

  • 0
    TI__Guru**** 393215 points

    Hi Machida-san,

    Yes, the wiki is correct about the ADC accuracy. The AM335X built-in bandgap sensor has no use for end customers, it's used in factory and test measurements.

  • 0 in reply to Biser Gatchev-XID
    Guru 12075 points
    Hi Biser-san,

    Thank you for your reply.
    I have two questions for your answer.

    "The AM335X built-in bandgap sensor has no use for end customers"
    1. Does it also mean that user can NOT use thermal shutdown ?
    2. If user want to check tempature of AM335x
    2-1. Is there no way to measure junction tempature (tj) only by using AM335x ?
    2-2. As alternative solution, Should user measure ambient tempature (ta) and based on this result, caluculate junction tempature ? (Need to additional external temp sensor to measure ambient tempature.)

    Best regards,
    Machida
  • 0 in reply to Ryuuichi machida
    TI__Guru**** 393215 points

    Ryuuichi machida said:
    1. Does it also mean that user can NOT use thermal shutdown ?

    Linux does not have this feature, I'm afraid.

    Ryuuichi machida said:
    2. If user want to check tempature of AM335x
    2-1. Is there no way to measure junction tempature (tj) only by using AM335x ?
    2-2. As alternative solution, Should user measure ambient tempature (ta) and based on this result, caluculate junction tempature ? (Need to additional external temp sensor to measure ambient tempature.)

    External sensor will be needed for measurement and junction temperature can be calculated.

  • 0 in reply to Biser Gatchev-XID
    Guru 12075 points
    Biser-san,

    >Linux does not have this feature, I'm afraid
    You mean if customer ask us about thermal shutdown, we should tell that this function is NOT supported on AM335x ??

    BR,
    Machida
  • 0 in reply to Ryuuichi machida
    TI__Guru**** 393215 points

    Hi Machida-san,

    I have checked with the factory team and they confirmed the answers I gave you above. Here is what they said:

    "If a customer really needs thermal shutdown feature with AM335x, an external temperature sensor can be added on board for monitoring board temperature and estimating junction temperature. Linux thermal driver needs to be modified for using a thermal sensor used on the board."

  • 0 in reply to Biser Gatchev-XID
    TI__Guru 60475 points

    Please refer to Application Report, SPRA953, for a description how the junction temperature can be determined.

    http://www.ti.com/lit/an/spra953b/spra953b.pdf

    The Thermal Resistance Characteristics for AM335x devices can be found in the data sheet.

    http://www.ti.com/lit/gpn/am3358

    Regards,
    Paul

  • 0 in reply to peaves
    Guru 12075 points

    Paul-san and Biser-san,

    Thank you for reply. But I have one additional question about method to estimate junction temperature.

    * According to datasheet, there is following description about thermal resistance characteristics.

    " Table5-12 provides simulation data and may not represent actual use-case values"

    So, the question is,

    Q1. Can we use these parameter to estimate junction temperature on customer's system board ?

    Best Regards,

    Machida

  • 0 in reply to Ryuuichi machida
    TI__Guru 60475 points
    Measuring the temperature on top of the AM335x package and using these thermal resistance parameters to calculate the junction temperature will give better results than trying to use an ambient temp sensor to determine junction temperature.

    Regards,
    Paul