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How to use the DMA module to work with arrays of 16 bit numb

 
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dmitrboristuk



Joined: 26 Sep 2020
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How to use the DMA module to work with arrays of 16 bit numb
PostPosted: Wed Apr 27, 2022 5:09 am     Reply with quote

Hello. Please tell me how to transfer 16 bit values from an array of numbers to the timer1 register. The trigger for the DMA module is an interrupt from timer0.
It is necessary to transfer the values from output[32] one by one each time an interrupt from Timer0 occurs. Thank you.
device PIC18F45K42
Code:

const unsigned int16 output[32] = {10, 100, 250, 580, 1005, 10, 10, 13, 45, 1000, 852, 10, 10, 10, 456, 10, 895, 12, 10, 25, 100, 200, 800, 2, 1000, 1000, 1000, 1000, 500, 850, 11, 10};
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Wed Apr 27, 2022 7:05 am     Reply with quote

I have to say, why on earth use DMA?. Just have an interrupt for timer0,
and transfer the two bytes. Increment the counter saying where they are
to come from on each transfer. Given these are internal registers, so no
delays needed etc., the transfer will probably take less time than would
be involved in setting up DMA....

For the 16bit values, this is what DMA_WORD is for.
dmitrboristuk



Joined: 26 Sep 2020
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PostPosted: Wed Apr 27, 2022 7:38 am     Reply with quote

This will be the correct entry.
Code:

dma_start ( 0, DMA_SOURCE_ADDR_IS_PFM | DMA_DEC_SOURCE_ADDR | DMA_DEST_ADDR_UNCHANGED | DMA_HW_TRIGGER_STARTS_XFER | DMA_ONE_SHOT_FROM_SOURCE_COUNT , &TMR1 , &output[31] , 64 , 2 );
temtronic



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PostPosted: Wed Apr 27, 2022 9:12 am     Reply with quote

I agree with Mr. T ,probably takes longer for DMA xfr than the 'normal' way.
It'd be interesting to see the listings to see how much more code(and time) DMA version would take.
Ttelmah



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PostPosted: Wed Apr 27, 2022 9:40 am     Reply with quote

No. You can't have two counts.

You need to just have 32 as the count, and set the DMA to do word transfers
(in the setup_dma).
dmitrboristuk



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PostPosted: Wed Apr 27, 2022 11:17 am     Reply with quote

Gentlemen, thank you very much. That is, the fifth parameter is not the number of bytes, but the number of trigger operations. And I agree that the classic way is much more practical and convenient. But when it comes to learning new products, why not give it a try.
Just in case, I will clarify whether it is correct to write the address (&), and not the name of the array element.
Ttelmah



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PostPosted: Wed Apr 27, 2022 11:45 am     Reply with quote

& is correct.

The array name (on it's own), is the address of the first element. For later
elements, you have to explicitly address the element. So use &.
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