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Jan561
Jan561
Visitor II
December 19, 2019
Solved

[DMA] Under which conditions will the NDTR register be reloaded when enabling the stream?

  • December 19, 2019
  • 1 reply
  • 1447 views

Hi,

This is what I found in the reference manual:

​

Once the transfer is completed, this [NDT] register can either stay at zero (when the stream is in normal mode) or be reloaded automatically with the previously programmed value in the following cases:

– when the stream is configured in circular mode.

– when the stream is enabled again by setting EN bit to '1'.

What I understand is, that in circular mode after each transfer the NDT register gets reloaded (which makes obviously sense).

​

But what happens in the following situations (for non-circular streams):

  • Stream automatically disabled after last transfer and enabled again
  • Stream automatically disabled, then configuring other values, then enabled again
  • Stream manually disabled and then enabled again without configuring other config values
  • Stream manually disabled and then enabled again with configuring other config values
  • Stream automatically disabled, then manually disabled, then enabled again
  • Stream crashed and enabled again

​

I hope this question isn't that stupid =)

Answers are really appreciated.

This topic has been closed for replies.
Best answer by waclawek.jan

NDTR is not one register but two: one into which you write, and a second one, which you read, let's denote them NDTR_W and NDTR_R.

When DMA is enabled, NDTR_W is copied into NDTR_R and then governs the rest of the DMA process. As long as you don't overwrite NDTR_W, every subsequent enable (or "recycle" in circular mode) uses the same, last-written, value from NDTR_W to copy into NDTR_R.

(Btw., the pointer registers are also doubled, but in a different way - there's a read-write register, which does not participate in DMA process except that its content is copied into its "invisible working" counterpart when the channel/stream is enabled/recycled.)

The net effective result is, that you cannot continue an aborted transfer without rewriting the registers, and that also means that you cannot abort/continue a circular transfer at all.

JW

1 reply

waclawek.jan
waclawek.janBest answer
Super User
December 20, 2019

NDTR is not one register but two: one into which you write, and a second one, which you read, let's denote them NDTR_W and NDTR_R.

When DMA is enabled, NDTR_W is copied into NDTR_R and then governs the rest of the DMA process. As long as you don't overwrite NDTR_W, every subsequent enable (or "recycle" in circular mode) uses the same, last-written, value from NDTR_W to copy into NDTR_R.

(Btw., the pointer registers are also doubled, but in a different way - there's a read-write register, which does not participate in DMA process except that its content is copied into its "invisible working" counterpart when the channel/stream is enabled/recycled.)

The net effective result is, that you cannot continue an aborted transfer without rewriting the registers, and that also means that you cannot abort/continue a circular transfer at all.

JW

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