Linux_BD wrapper for arty z7-20 hdmi issue

[pisoiu]

Hi all,

I recently purchased an arty z7-20 board to play with it. I have extensive experience with microcontroller design and programming, mostly asm on 8051, but I am new to FPGA and ARM.

For now I am playing with linux_bd project from git, I successfully imported it into vivado 2017.2.1 and I try to understand what is in the schematic from block design. When I boot the z7-20 board from sd card with the built images of petalinux and I connect the board to the hdmi tv, the tv shows me it is in 720p/50hz video mode. What I do not fully understand it is how this image data come from the framebuffer in ddr3. Looking at the schematic, I see hdmi output comes from rdb2dvi, that one is fed with rgb data by an axi4 stream to video out, and this one is fed by axi-vdma through an axi subset converter. By my calculation, image data is 1280x720pixels x 24bits/pixel x 50 hz =~ 1.1 gigabit/second. How is this chain transferring data so fast? What am I missing? Because the memory performance for DDR3 declared in zynq datasheet, for -1 grade devices, is around 1.066 gigabit/second. And that memory have to do something else from time to time besides showing video data.

Thank you.

[sbobrowicz]

Since the memory bus on the Arty Z7 is actually 16-bits wide, the memory throughput is:

1050 * 16 = 16.800 Gb/s = 2.1 GB/s

Note the Arty Z7 bus only operates at 1050 MT/s, because of the oscillator used. 

Your calculations regarding video bandwidth are correct, you just forgot to take into account the DDR bus width when calculating the max theoretical bandwidth.

[pisoiu]

Thank you. In the xilinx doc, the unit for performance indicator is Mb/s, there I made a wrong assumption. I see the light now