[J-core] HDMI

Cedric BAIL cedric.bail at free.fr
Wed Oct 19 12:34:33 EDT 2016

On Tue, Oct 18, 2016 at 11:15 PM, Rob Landley <rob at landley.net> wrote:
> 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:
> http://lists.j-core.org/pipermail/j-core/2016-August/000341.html

Oh thanks ! I missed that.
Cedric BAIL

More information about the J-core mailing list