LCDDMA_CTRL double-buffering Question

Dean Wedel

On the BeagleBone Black (AM335x) I am driving the LCD output via DMA. I see that in register field LCDDMA_CTRL that you can specify a "Frame Mode" using 2 frame-buffers. And also the LCDDMA_FB0_BASE/CEILING and LCDDMA_FB1_BASE/CEILING configurations.

My question is: How do I set which Frame-Buffer (FB0 or FB1) that I want the LCDDMA to use while I'm hand-drawing the other buffer? I want to set it to use FB0 while I render the image to FM1 for example.

Thanks in advance,
Dean

over 11 years ago

0 Tyler Berryhill over 11 years ago

TI__Intellectual 1980 points

Dean,

The LCD Controller will always start with FB0 when using two frame buffers.

-Tyler

0 Dean Wedel over 11 years ago in reply to Tyler Berryhill

Intellectual 760 points

Tyler:

Thanks for that info.

Now, after I'm finished with FB1 and it's ready to be displayed how do I tell the LCD Controller to switch to FB1? I haven't been able to understand which LCD registry to use to switch buffers.

Sincerely,

Dean

0 Tyler Berryhill over 11 years ago in reply to Dean Wedel

TI__Intellectual 1980 points

Dean,

The idea behind the ping-pong buffer approach is that the LCDC performs the switch between FB0 and FB1 automatically. Ideally here you could start with FB0 blank and allow the DMA to transfer a black screen while you're drawing FB1. This makes a smooth transition into display since the user won't see a half drawn FB0 initially.

If you can't draw your frame buffer in less time than it takes to transmit one frame I would suggest using the LCDC in single buffer mode, allocate another frame buffer in memory, then when you're done drawing that frame you wait for the Frame Done interrupt and update the FB0_BASE and FB0_CEILING address. This essentially the same concept as using the double buffer DMA mode but you're in control of when the frame buffer switch happens.

-Tyler

0 Dean Wedel over 11 years ago in reply to Tyler Berryhill

Intellectual 760 points

Tyler:

Yes!! That's exactly the effect I was looking for. Thanks to your help my code is working nicely now.

Thanks again!

Dean

0 Tyler Berryhill over 11 years ago in reply to Dean Wedel

TI__Intellectual 1980 points

I'm glad to hear it's working Dean. Good luck on your project.

-Tyler

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LCDDMA_CTRL double-buffering Question