When Difference Matters: Differential Signaling

March 29, 2016 by Bil Herd 21 Comments

We have talked about a whole slew of logic and interconnect technologies including TTL, CMOS and assorted low voltage versions. All of these technologies have in common the fact that they are single-ended, i.e. the signal is measured as a “high” or “low” level above ground.

This is great for simple uses. But when you start talking about speed, distance, or both, the single ended solutions don’t look so good. To step in and carry the torch we have Differential Signalling. This is the “DS” in LVDS, just one of the common standards throughout industry. Let’s take a look at how differential signaling is different from single ended, and what that means for engineers and for users.

Single Ended

Single Ended: TTL, CMOS, LVTTL, Etc.

Single Ended and Sources of Noise

Collectively, standards like TTL, CMOS, and LVTTL are known as Single Ended technologies and they have in common some undesirable attributes, namely that ground noise directly affects the noise margin (the budget for how much noise is tolerable) as well as any induced noise measured to ground directly adds to the overall noise as well.

By making the voltage swing to greater voltages we can make the noise look smaller in proportion but at the expense of speed as it takes more time to make larger voltage swings, especially with the kind of capacitance and inductance we sometimes see.

Differential

Enter Differential Signaling where we use two conductor instead of one. A differential transmitter produces an inverted version of the signal and a non-inverted version and we measure the desired signal strictly between the two instead of to ground. Now ground noise doesn’t count (mostly) and noise induced onto both signal lines gets canceled as we only amplify the difference between the two, we do not amplify anything that is in common such as the noise.

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Hackaday Links: January 10, 2016

January 10, 2016 by Brian Benchoff 11 Comments

Everybody loves cheap stuff, and we hate telling everyone about coupon codes. That said, TI has a new LaunchPad development board they’re promoting. It’s based on the MSP432, the ARM extension of their MSP430 line. The MSP432 is an ARM Cortex M4F, low power, and planned for production later this year.

Here’s your daily CES garbage post. Through a collaboration between Sony and Nissan, a car has become a video game controller controller. A controller plugs into the ODB II port, reads throttle, brake, and steering wheel positions (and buttons on the dash/steering wheel, I guess), and translates that into controller input for a PlayStation 4. What games do they play with a car? You would think Gran Turismo, Rocket League, or other games with cars in them. Nope. Football.

Dangerous Prototypes is a legal Chinese company! [Ian] didn’t say anything about the process about becoming a legal Chinese company because he wrote a blog post, not a book. Shenzhen Dangerous Prototypes Electronics Technology Limited allows them to have an office in the Shenzhen electronics market, hire local and foreign hackers, host Hacker Camp Shenzhen, and allow people to apply for ‘Authorized Authority’ visa letters for the people who need them. Great news for a great company.

The Forge hackerspace in Greensboro, NC is growing. In just over a year they have 160 members and they’ve already outgrown their 3,400 square foot space. Now they’re moving to a larger space that’s twice the size and they’re looking for donations.

People have been taking old iPad screens and turning them into HDMI displays for years now. [Dave] got his mitts on a panel from a Macbook Pro 17″, and turned it into a monitor. It required a $50 LVDS adapter, but the end result is great – a 1920×1200 panel that looks pretty good.

Video FPGA With No External A/D

September 9, 2015 by Al Williams 23 Comments

You have an old PC with a nonstandard RGB video out and you need to bring it to a modern PAL TV set. That’s the problem [svofski] had, so he decided to use an Altera-based DE1 board to do the conversion. Normally, you’d expect reading an RGB video signal would take an analog to digital converter, which is not typically present on an FPGA. Instead of adding an external device, [svofski] used a trick to hijack the FPGA’s LVDS receivers and use them as comparators.

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Using LVDS Laptop Displays

April 9, 2015 by Brian Benchoff 17 Comments

No doubt anyone reading this has access to a few ancient laptops and the displays contained within. While those laptops are probably still stuck with a Gig of RAM and Windows ME, the display panels are probably still good. They don’t have HDMI, DVI, or VGA, though, which means those panels will need a converter.

[Jared] had a different idea. Instead of reusing laptop displays with a converter, why not connect them to an LVDS connector on some modern hardware? He had a RIoT board with a native LVDS connector, and with some clever reverse engineering and PCB fabrication he can put those old displays to work.

[Jared] had a very cool sunlight-readable ‘transflective’ LCD from on old Portege R500 laptop. If he was going to take apart one laptop to use with modern hardware, this was the one. Opening up the display he found a tiny connector but no obvious markings of what pins did what. The datasheet was also not to be found. By shorting two pins together, he could figure out what the pins were: shorting the clock freezes the screen, shorting the HSYNC and VSYNC means the screen loses sync. Blues, reds, and greens can be found the same way.

With the pins identified, a breakout board was in order. This is just a small board to break out the very small wires to solderable pads and a driver for the backlight. With that, and the RIoT with an LVDS output, [Jared] was able to use new hardware with this old but still serviceable display.

Hackaday Links: Sunday, April 21st, 2013

April 21, 2013 by Mike Szczys 10 Comments

Regular reader and master hacker [Bill Porter] got married. Congratulations [Bill] and [Mara]! The two of them just couldn’t leave their soldering irons at home. The actually swore their vows by soldering together a circuit during the ceremony (blinky wedding dress, el wire tuxedo, and all).

[Kevin] sent in a link to [Red Fathom’s] hacked Wacom tablet. It’s the screen from a Wacom-enabled laptop brought back to life with a Teensy and an LVDS interface module.

The Neato XV-11 is able to find its charging station when the batteries run low. [Derek] figured out that you can make a second station using some reflective tape.

If you use your drill a lot you’ll eventually break the rubber thing that holds the key to the chuck. Here’s a way to 3D print a replacement.

[Torxe] put eight floppy drives to use as a polyphonic Arduino-controlled MIDI player. And while we’re on the subject of Arduino controlled projects you should take a look at this web-interface to tell you if the foosball table is being used.

And finally [Th3 Bad Wolf] sent in this link to a milling machine built out of LEGO. It is able to mill floral foam and uses a lathe-like setup for one of the table axes.

LVDS On An FPGA Could Make It Possible To Reuse Laptops LCDs And The Like

October 3, 2012 by Mike Szczys 41 Comments

Search around the Internet and you’ll find a landfill of forum threads asking how to drive the LCD screen from a dead laptop. The answer is always that there is just no way to do it. That’s because most of them use a Low-Voltage Differential Signalling protocol that just isn’t available through the hardware used in hobby projects. But the appearance of this board could signal that things are about to change. We don’t want to get your hopes up too much. This isn’t an open source project, but it is a piece of hardware that can make LVDS available for the 8, 16, and 32-bit microcontrollers you’re used to working with.

It’s a derivative of a project [Thomas Jespersen] worked on for a customer. It uses an FPGA to implement the LVDS standard used by high-pixel-count LCD displays. It contains enough memory for a full frame-buffer, and includes a Motorola-8080 communication standard. [Thomas] gives a full description of how the setup works in the video after the break. Demonstrations start about 7:30 into the video with an STM32 F4 Discovery board driving the display.

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Putting Laptop LCDs To Use With An FPGA

September 9, 2011 by Brian Benchoff 29 Comments

We’re always impressed with the number of laptop displays we’re able to pick out of the trash. Most of the time the computer is borked beyond repair so we end up with a lot of functional but unusable LCD panels. As a service to us all, [EiNSTeiN_] figured out how to control an LCD panel using a cheap homebrew FPGA board.

LCD panels don’t use a simple protocol like VGA for turning pixels on and off. Instead, the very high-speed LVDS is used. LVDS is beyond the capabilities of simple microprocessors, so [EiNSTeiN_] built himself a clone of an XuLA FPGA prototyping board and set to work. After figuring out the signal lines to the panel, [EiNSTeiN_] pored over the timing diagrams for the LVDS controller and the LCD panel. From the data sheets, he figured out data is usually sent to the panel at about 500 MHz. The homebrew FPGA board couldn’t manage that speed so [EiNSTeiN_] cut the FPGA clock in half.

While LCD’s 60 fps refresh rate was reduced to 30 fps, [EiNSTeiN_] says there’s only a little flicker. Not bad for something that could have easily been trashed.