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TMS320C6742 power consumption estimation

Giuseppe
Expert 6375 points

Other Parts Discussed in Thread: TMS320C6742

Dear team,

 

We would like to have an better estimation of power consumption for the DSP TMS320C6742 at the following conditions:

 

- computational load 100M MAC’s

- Tj = 105°C

- data transport to/from external program memory (I suppose that this is necessary condition as there is no program store memory in DSP)

- no other external pin activity

 

There is an excel spreadsheet “C6748_46_42_PowerSpreadsheet_RevD.xls” for this calculation however we are not sure how to set the parameters to meet the conditions listed above.

 

Thanks for your help,

 

Kind regards,

Giuseppe

  • 0
    TI__Guru* 84110 points

    Giuseppe,

    Did you read through the associated Wiki app note and did not find what you wanted? It suggests how to interpret the important fields and how to determine accurate estimates.

    Giuseppe said:

    - computational load 100M MAC’s

    Enable the DSP and EMIF, then put the appropriate numbers in under the %Utilization column. You will have to build your code and look at the results in cycle count to make an accurate estimate of %Utilization for the DSP. Clock rate, MAC resolution, skill at optimization will all affect the %Utilization number, but it probably will not be very high.

    Giuseppe said:

    - Tj = 105°C

    There is a box in the spreadsheet that says "Junction temperature" and there is a box under it for the value. If setting this value is really one of your questions, you may have more problems with the other fields than I will be able to help with.

    Giuseppe said:

    - data transport to/from external program memory (I suppose that this is necessary condition as there is no program store memory in DSP)

    If the program fits in the internal memory and/or cache, the EMIF might not be used during normal operation.

    Giuseppe said:

    - no other external pin activity

    The spreadsheet defaults to everything being disabled, so all you have to do is enable what you need, the DSP and EMIF if the EMIF is really needed. You might make two estimates, one during boot time with the EMIF and another during operation without EMIF, if that would really be the case.

    Tj = 105C is a very hot environment for something that does not interact with the outside world in any way. No peripheral activity to collect input data means the MACs are being continuously run on stale data; no peripheral activity to output the results means that there is not much value in the calculations done by the DSP. This sounds unrealistic, which if true means the result of the power estimation will be inaccurate.

    Regards,
    RandyP

  • 0
    TI__Prodigy 370 points

    Giuseppe,

    The following Wiki link has a very good write-up on how to use the power estimation spreadsheet as well as a link to downloading it.

    http://processors.wiki.ti.com/index.php/C6748/46/42_Power_Consumption_Summary

    In terms of the usage conditions you mention, the C6742 is a 200Mhz DSP.  If the code is DSP code and well pipelined, the part will handle a MAC in a single cycle (there are cases where more than 1 in a cycle is possible).  So a good starting estimate is 100M MACs computational load is about 50% CPU utilization. Depending on the nature of the application, you might do better (or worse).

    There is a simple input parameter in the spreadsheet for Tj.  You will notice the default in the spreadsheet is 25C, but that is easy enough to change to your case.

    For modeling the memory usage, you need to determine what kind of memory will be used (mDDR or likely DDR2), and have some insight into the memory bandwidth needed from the application and comparing that to the theoretical max bandwidth of the interface.  There is also the ability to tune the % of time these are reads vs writes - the default assumes 50% reads and writes but if you know the application can be managed differently, you have the opportunity to change that.

    For your case, all other peripherals can be noted as zero activity.

    This should get you a good first pass.  The C6742 is a good low power DSP and this use case does not seem very demanding.  Good luck in pulling it together.

    john

  • 0 in reply to John Smrstik
    TI__Expert 6375 points
    Thanks John and Randy for your feedback, Giuseppe