[J-core] HDMI

[Rich Felker]

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.

Rich