[J-core] HDMI

Rob Landley rob at landley.net
Wed Oct 19 02:15:23 EDT 2016

On 10/18/2016 06:57 PM, Rich Felker wrote:
> On Tue, Oct 18, 2016 at 05:59:04PM -0500, BGB wrote:
>>>> my thought here is that the 250MHz needed to pull off HDMI output may be out
>>>> of reach for lower end FPGA's, but I am not sure.
>>> Why do you think we need 250Mhz ? For the lower pixels format, aka
>>> 640*480 @60Hz, we only seems to need 25Mhz. Is there something I
>>> missed ? Also the turtle will only come with an HDMI output, like the
>>> RPi do.
>> 25MHz is the pixel-rate, but the pixels need to be sent as a serial
>> stream of bits; this pushes the required bitrate up *considerably*,
>> and you would end up needing to drive 250MHz on the output pins (and
>> for serial parts of the interface).
>> contrast with the analog interfaces, one doesn't need to send nearly
>> as many bits, and the bits don't need to get through reliably; it is
>> more just the faster the DPWM is, the higher the output image
>> quality can be (one might only need a few bits per pixel to get
>> "acceptable" output).
> I don't think there's any need for concern here. My understanding is
> that Jeff has already tested HDMI output via a simple pattern
> generator bitstream and it works fine. Scanning out a framebuffer
> should not be any harder to meet timing for; AFAIK the plan is to have
> a small local buffer that fills via DMA, and output bits from this
> buffer at the HDMI video clock.

Jeff posted the example code he used, attached to this message:



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