HDMI over Pmod using the Arty Spartan 7 FPGA board

tl;dr: This post shows that driving DVI-D over an HDMI cable, directly connected to the High Speed Pmod connector of Digilents Arty S7 board, is very much possible- even at high resolution.

I’ve been working away on my RISC-V FPGA based computer ‘kit’, which is based on my VHDL CPU: ported to RISC-V. I wanted to get a new development board with faster ram, and found it hard to find boards with DDR3 memory, a large enough FPGA, SD card interface, and HDMI out.

The SD card was not really a problem – it’s low speed, you can just connect it with slow SPI I/O. HDMI is certainly considered high speed – bandwidth across the 4 serial channels top 4.4Gbps – but is driving HDMI though basic I/O interfaces possible?

It most certainly is!

A caveat, though: There is no protection circuitry. Do this at your own risk 🙂

The Digilent Arty S7 board can be had for sub £100, and my XC7S50 variant cost £119 shipped. It has DDR3, but no on-board HDMI. However, it turns out you can get perfectly acceptable (for my needs, anyway) output from the high-speed Pmod I/O ports. This will go very nicely with the small USB powered HDMI screens you can find, which are very handy. Image showing development board conencted via breadboard wires to an HDMI connector breakout board, which is driving a small HDMI display. I have put a 720p output test on github. It should load in Xilinx Vivado out of the box. You need to connect an HDMI cable to the Pmod, by either splicing a cable, or getting something like the Adafruit breakout board (seen above). It connects to Pmod JA, circled.

Image showing what I/O port to use.In the constraints file for the project, the Pmod pins are specified to use the TMDS_33 standard. The pinout is defined as follows:

A diagram showing what Pmod pins are for what HDMI purpose.The VHDL code in the example uses a simplified 1280x720x60 pixel clock of 75MHz – not the required 74.25MHz as per the standard. Due to this, some TVs/monitors may not accept the signal. It runs fine on my Acer XF240H and Samsung LU28E590DS using a 1m cable. I have not tried a television – they tend to be more picky. You can get much closer to 74.25MHz by chaining clock generators, but I have not done that in this instance. The refresh rate reports as 61Hz with this clock, likely due to it being out of spec.

If you want to learn more about how my example project generates a HDMI/DVI-D signal, you can find details here, which is part of my Designing a CPU in VHDL blog series.

Picture showing a monitor displaying a 720p signal.A video of it in action, using timings applicable for my 800×480 module.

You can change the pixel clock and the vga timings (sync starts, ends, porches ) in the code to generate different resolutions. For 1080p60Hz the pixel rate is supposed to be 148.5MHz, but again my monitor will accept a rate of 150MHz, and show 61Hz.

Even 1440p30 is possible. 1440p60 did not work, but I didn’t try hard to get a more accurate pixel clock in that instance. When you get to higher clocks, you can start to get timing constraint issues. At 1080p and 1440p I had some timing failures listed in the implementation report, but they did run. If you were using this in a real system, you’d have to fix those timing issues. That’s out of the scope of this blog, though 🙂

So there you have it. You really can bodge HDMI/DVI-D output directly through a Pmod!

Thanks for reading. Let me know what you think on twitter @domipheus.