rwgast
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 11:32 am

Ok so the PI has many uses and im sure people from all walks of life will be using the thing. But as of right now who are we kidding the people in these forums are mostly hackers, coders, and hardware buffs. This bare board is really bringing out my DIY spirit, as im sure it is all of you. My problem is im not a hardware hacker, im an ex-electrician who knows a bit of DC theory and how to solve circuts. Im also a coder and aspiring ARM assembly language based os developer thanks to the PI. I would love to learn more electronics and i plan on it once i have a PI with all of its gpio glory!

Im in a situation right now. I spent a little more than i intended and am unemployed, so all I have for a display is a 15 inch; wide screen LCD, problem is its VGA only, ill never understand why these older lcd's are like that. The only way I have to connect it at the moment is a composite RCA to VGA adapter, which is just going to suck and run at a horrible resolution. Unless I solve this issue before i get the PI im gonging to be using ssh/vnc, for a bit which sucks because it ties up my main development system.

So my plan was to purchase this HDMI to VGA for 40 after shipping, and hope it works fine and doesnt break..

But thinking about it id like to get a start on DIY hardware asap! Im willing to spend about the same amount of money $40 is reasonable although I'm sure not necessary! Does anyone out there have the knowledge to build an HDMI to VGA converter? I guess theres and LCD header on the board but since none of us have a PI yet its probaby out of the question to design a VGA connector that works with it, but HDMI is standard. Anyways someone built something like this for the Beagle Board right here. It uses a resistor ladder which is probably not going to be the most accurate way to do this but something is better than nothing right?? Maybe someone out there know how to do the with a serial DAC chip? Anyone got some skills to design this or at least help me design it?? Im just itching to bust out that old Radio Shack bread board i paid way to much for.

spurious
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 11:42 am

ASDA do a HDMI TV for £50

The problem with HDMI - VGA is that it's not just swap wires about as HDMI is a digital protocol and VGA is an analog protocol, so you need a very complex bit of kit that does the work. This is basically a chip and so the cost is the chip, not wires and packaging.

But... you still have a composite port, which you can use to phono or wire up to a scart connectors.

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Chromatix
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 11:56 am

The R-2R ladder DAC is sufficiently accurate for 8-bpc pixel output.  There might be a trick with the format of DVI on the wire though, and I haven't investigated this.  R-Pi doesn't use HDCP encryption, fortunately, so the HDMI signals will be identical to DVI.

If you connect the DDC lines through correctly, you shouldn't have to manually configure anything on the board.

Bear in mind however that you're dealing with multi-MHz signals and will need to pay attention to details accordingly.
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Jongoleur
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 11:57 am

Some points:

This subject has been discussed to death in other topics. Have a look around, there may be some suggestions.

HDMI-VGA is possible, but proper solutions are expensive and cheap solutions are untried and untrustworthy (until someone buys them and does some testing!). As you note, there is a home-constructed solution for Beagleboards, but I've heard that its not entirely reliable.

To be honest, you'd be better served by doing as spurious suggests above and getting a cheap 16" TV with HDMI, or putting up with an old TV with SCART/Composite video input until funds become available.

btw - this might be better presented in the Projects and collaboration forum, so instead of locking it off now, I'm moving it there.

Edit for extra content!
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HeadCase
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 2:09 pm

I know this topic probably needs a stake through its heart, but here is my guesstimate of what would be involved in making a HDMI --> VGA adapter.  This is based on the the absence of a cheap single chip solution that can be dropped onto a small PCB. That would be a nice simple little  project.

Note that its NOT just a pcb with a few wires soldered on we are talking about here. The HDMI spec uses high speed differential serial data streams as well as a i2c control bus to talk to the display device. VGA needs 3 analog colour signals and H and V sync signals all derived from the HDMI signals.  VGA also has a serial control bus - which probably needs translating. HDMI allows +5 at 50mA to power the converter circuit, which should be sufficient.

A possible design would look like -

o HDMI socket with termination for the diff signals

o VGA socket with possibly some termination

o 68-100 pin tqfp PLD (Atmel or Xylinx or Lattice)  to do all the serial decoding and drive the DACs and make the V sync H sync signals

o Triple video DAC  like - http://www.analog.com/en/digit.....oduct.html  - cheap, low power and looks like it can drive the VGA cable directly

o small microcontroller (PIC or Atmel) to detect the VGA monitor connected and talk to HDMI and control the operation of the PLD. Also, this micro can probably reflash the PLD firmware if required. Using a microcontroller would help make this easier as an OS project.

The parts cost would be maybe $15 - $20 each for a few hundred.

This is not a trivial project. Lots of pdf reading of HDMI and VGA specs would be required.  By the time it is done the cost difference between a cheap VGA monitor and a HDMI monitor may be less than the adapter.  On the other hand it may be a nice project for some engineering students who wish to learn about electronics and video standards.

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johnbeetem
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Feb 26, 2012 3:40 pm

It's not a DIY project — you're dealing with high-speed differential signals that require good PCB design and impedance matching, and fine-pitch devices that are very hard to solder.  There's a list of HDMI to VGA adapters that "should work" at the RasPi wiki: http://elinux.org/Rpi_Screens .  Once people have actually tested some of these with actual RasPi boards it should be clear which work and which do not.

I've never designed one of these myself, but I think the most obvious way to make an HDMI to VGA adapter is to use an HDMI/DVI receiver such as the TI TFP403 "Panelbus Digital Receiver" to extract the RGB pixels followed by a Video DAC such as the TI THS8135.  You can get data sheets on these at http://www.ti.com .  These chips cost USD5.45 (Q1K) and USD4.15 (Q100).  Once you add in connectors and PC board, you're better off with an off-the-shelf module — assuming it works.

sawbones
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Tue Jul 14, 2015 12:29 am

I was searching for this info so that I could use one of my Android tv mini PC's with a spare VGA monitor. I have a pi, too, so I read this thread.

I joined the forum so that I could share this for the benefit of other diy-ers.

Here is a solution from Analog Devices:
http://www.analog.com/en/design-center/ ... d-overview

You could potentially build this yourself for free or at an extremely low cost.

If their site is still like it used to be. It is very simple to register your business and request samples. I believe that you can normally request from 1-5 or maybe more of just about every semiconductor which they manufacture. From transistors to their best microcontrollers and everything in between.

The hard part would be the board layout and soldering. I can tell you from experience with similar projects in the past that trying to solder small wires to chips which are designed for surface mount can be very challenging (if not impossible).

Luckily A.D. provides the layout files which you can then use to have a company produce the PCB for you. Actually this can be done fairly cheaply if you don't mind waiting on shipping from China. Another problem is if the microcontroller has a BGA pinout. Usually they offer them in more than one package (different pin configurations).

I think that they even provide the source for programming the microcontroller.

Probably there are other solutions as well using different microcontrollers and or other DACs which are cheap and commonly used by hobbyists. There are potentially other solutions which can be hand soldered (such as a microcontroller in a DIP package). That would make things much simpler.

Anyway, this is one way to do it. Possibly made entirely out of free parts(not counting PCB etching, or breadboard).

Cheers,
-sb
Last edited by sawbones on Thu Jul 16, 2015 1:09 pm, edited 2 times in total.

johndough
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Tue Jul 14, 2015 8:38 am

Hi

Maybe one of these is within budget to use, and perhaps one to open up and examine?

http://www.walmart.com/c/kp/hdmi-to-vga-video-adapter

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RaTTuS
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Tue Jul 14, 2015 8:53 am

johndough wrote:Hi

Maybe one of these is within budget to use, and perhaps one to open up and examine?

http://www.walmart.com/c/kp/hdmi-to-vga-video-adapter
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sawbones
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Thu Jul 16, 2015 2:46 pm

Just a quick update. Very cheap Chinese adapters can be had for under $5 on eBay, if you don't mind waiting on shipping from China, and just above that price point if you are US based and order from a US reseller.

I'm not sure about the quality, but am ordering now and will post back with results after arrival from reseller.

I decided to buy one which accepts a male HDMI input (rather than one having a male connector) because I have several devices which output micro HDMI, and it will be more convenient to use the cable or adapter which the device provides. Then I also retain the ability to use normal HDMI with a full sized cable.

I will post back with results. Maybe even with pics. I'm sure it's easy to open.

It will be interesting to see what sort of chips they are using in these ultra cheap models and if they include some sort of microcontroller to set EDID and so forth.

Many of the models look identical to the $19 adafruit one. I wouldn't be surprised if they are identical (maybe worse solder job?).

Cheers!
-SB

abc123mewot
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sat Nov 19, 2016 4:13 pm

I was looking for the same thing, I want to reverse engineer one of these converters because I believe they are just pulling 10-15 pins off the HDMI connector and connecting them directly to the VGA connector my reasoning behind this is because of how damn small a converter can be made (as seen here https://goo.gl/BaSpbg) and I don't believe you could fit a chip or and passive components in that kind of package but i could definitely be wrong sadly I cant find any good schematics online and I don't own one myself and if I did it would be connected to my raspberry pi asap

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mahjongg
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sat Nov 19, 2016 4:41 pm

abc123mewot wrote:I was looking for the same thing, I want to reverse engineer one of these converters because I believe they are just pulling 10-15 pins off the HDMI connector and connecting them directly to the VGA connector my reasoning behind this is because of how damn small a converter can be made (as seen here https://goo.gl/BaSpbg) and I don't believe you could fit a chip or and passive components in that kind of package but i could definitely be wrong sadly I cant find any good schematics online and I don't own one myself and if I did it would be connected to my raspberry pi asap
no, its certainly not true that these are just adapters that directly connect HDMI pins directly to the VGA connector, (with the sole exception of GND, and the I2C connections used to address the eeprom with CEC data inside the monitor) all the video signals have to be converted from the high speed digital data stream and converted into analog RGB and brightness signals and synchronisation signal. To do that you do need a complex chip. Its certainly NOT something you can hope to build yourself.

There are also well designed HDMI to VGA and badly designed (hardly working) HDMI to VGA converters. Some also output stereo sound, some don't, but NO! simple adapter cables will ever work, these cables do exist, but they exist only because some cheap video cards do not have extra VGA connectors, but put out VGA signals onto their existing HDMI connectors to save costs, silicon is cheap, but connectors cost money. A very BAD non-standard idea that has caused immense confusion to this day.

HDMI to VGA converter chips themselves can be very tiny, just a few mm by a few mm, and a mm high, so they certainly fit into the tiniest of devices.

There is however a way to add VGA to a Raspberry PI with little cost, because the raspberry PI chip actually incorporates the hardware needed to create VGA signals with little more than three resistor ladder Digital To Analog converters (DACs). Our in house genius, Gert Van Loo ( a fellow Dutchman of me, and a member of the raspberry PI foundation, and actually a broadcom chip designer who worked on the broadcom chip the PI is based on) has found out how to do this, using the GPIO header. He designed the "GERT VGA666" VGA adapter for the PI, named after the fact that six bits are used for each RGB color, so 2^6 or 64 levels per colour or 64x64x64 = 26.066.944 colors can be reproduced (18 bit color depth).

more info here: viewtopic.php?f=100&t=86658

you can buy a GERT VGA666 kit here: https://www.pi-supply.com/product/gert- ... pberry-pi/

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Burngate
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sat Nov 19, 2016 5:58 pm

abc123mewot wrote:... and I don't believe you could fit a chip or and passive components in that kind of package ...
Why not?
$10 will get you an education as to how they do it.

steveb4pi
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Nov 27, 2016 9:16 am

The HDMI-> VGA converters found on eBay for less than £5 work 100% just fine (I know, I have used half-a-dozen in my various PhotoFrame projects). You can get standard HDMI or mini-HDMI 'flying leads' (to suit Pi Zero). These are ALL made in China, so if you use a UK seller it's just double the price for the same device

Still less than £10, you can get version with (stereo) sound output, however these typically come with a tiny 5v power input socket (and a USB cable) because they could overload the HDMI socket max. current spec. (50mA ?)

For the DIY die-hard, a bit of Google with give you a circuit using 20 i/o pins to generate 6bit RGB (18 pins) + H & V sync from the Pi ... as others have pointed out the GPU handles all the 'bit banging', all you have to do is install the driver ...

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rpdom
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Re: Any electronics buffs? HDMI to VGA DIY!!??

Sun Nov 27, 2016 11:27 am

steveb4pi wrote:For the DIY die-hard, a bit of Google with give you a circuit using 20 i/o pins to generate 6bit RGB (18 pins) + H & V sync from the Pi ... as others have pointed out the GPU handles all the 'bit banging', all you have to do is install the driver ...
You don't even need to install a driver. Just change a few lines in /boot/config.txt.

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