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Circular Indirect Addressing Mode with DMAC instruction

MarkoR
Prodigy 50 points

Hi,

DSP: TMS320C28346ZFET

I need to know if it is possible to use circular indirect addressing mode with DMAC instruction.

From the document SPRU430E page 6-86 I know the DMAC syntax is DMAC ACC:P,loc32,*XAR7++ and the implementation with circular indirect addressing should look: DMAC ACC:P, *+XAR6[AR1%++], *XAR7++ (SPRU430E page 5-24). So one pointer is set with buffer begin address every time when end of the buffer size is reached.

My problem is that the circular buffer index increments only by one but I need to be able to define my own circular buffer pointer increment. For an example the circular buffer address is incremented by four meaning that every forth 16-bit value is read by DMAC instruction. Is it possible to define circular buffer pointer increment step value for DMAC instruction?

I need to implement simple DFT calculation that uses one reference sine waveform table (contains one sine period) that is used for every DFT frequency point calculation. In our application I can not use CPU time to recalculate reference sine tables.

Thank you.

  • 0
    TI__Mastermind 27200 points
    Marko,
    Unfortunately there is no addressing mode which does exactly what you want. The offset pointer increment is always one. The best I can suggest, since you are using a device with FPU, is to make use of the zero overhead RPTB instruction. That will allow you to increment the offset pointer by any value you want. For example, if you know the data size is always a multiple of four you could use something like:
    RPTB LABEL, #34
    ADDB XAR1, #4
    DMAC ACC:P, *+XAR6[AR1], *XAR7++
    ADDB XAR1, #4
    DMAC ACC:P, *+XAR6[AR1], *XAR7++
    ADDB XAR1, #4
    DMAC ACC:P, *+XAR6[AR1], *XAR7++
    ADDB XAR1, #4
    DMAC ACC:P, *+XAR6[AR1], *XAR7++
    LABEL:
    This won't implement circular addressing so you'll have to reset the buffer address each time in the code. Hoping this helps.
    Regards,
    Richard
  • 0 in reply to Richard Poley
    Prodigy 50 points

    Hi Richard,

    Thank you for the idea. Using the RPTB instruction is good idea although this method is not as efficient the DMAC instruction with RPT is in sense of used CPU cycles.

    I decided that I will use the DMAC with circular indirect addressing for the reference table that contains single sine/cosine period for each frequency. So the only limitation I will have is that I can make DFT calculations for the frequency values that divide well with buffer length - to meet the requirement where reference sine/cosine has repeating amplitude values in every period.

    My current tested implementation:

            RPT     AR5
         || DMAC    P, *+XAR6[AR1%++], *XAR7++

    Still it will be nice if we can configure AR1 increment step - perhaps new idea for C2000 designers ;)