ECL logic ramblings

ECL logic, or emitter coupled logic is a family of very high speed logic. with ECL logic, a transistor is never completely on or off, meaning the transistor is never saturated, making it very fast. It also makes it weird and quirky, interesting enough to have a look at. First of all, ECL logic generally works on -5V instead of 5V, though later it was also available using a more normal 5V power. Second of all, the logic level is not 0V and -5V, but roughly -0.8V and -1.6V. Third, most ECL logic has 2 outputs, a normal and an inverting one. This can be used to make logic design easier or to have an differential pair output for noise immunity. ECL was used in multiple well known designs, for example the Cray-1 supercomputer used ECL.

All in all, interesting devices. The easiest way to experiment with them is by buying some logic devices. ON semiconductors still makes them, the NC10EP01 for example is a single gate OR/NOR that can be used with 5V or -5V as power supply. It’s a very fast device, reaching over 3Ghz switching speeds. The datasheet can be viewed here.

But where is the fun in that, it’s much more fun to make a few gates. The schematic for a NOR gate is easy to find, it’s even listed on the Wikipedia page about ECL. I changed the schematic a bit to make a 4 input NOR. I also made an SR latch based on the design found here. The schematic for the NOR gate is:

And for the Latch:

A few small PCB’s where made for this, both with the same pinout as they both have 4 inputs and 2 outputs. Each PCB has 2 headers for the power supply to make routing easier when using multiple boards.

As ECL has a -0.8V to -1.6V logic level, interfacing with it is a bit challenging. To help with this a small circuit is needed to turn ECL to 0-5V and back, so interfacing with it is easier. The circuit I used for this is as follows:

To turn ECL to 0-5V, a comparator with an external transistor is used. Any comparator that can be used with +5/-5V supply is usable, the LM339 is a cheap, but slow, option. As ECL outputs are differential, no extra circuitry is needed and it can be directly fed into the comparator. The external transistor might not always be needed, but for driving an LED it can be. To turn 0-5V to ECL, a similar circuit is used, with 1 input connected to 2.5V and the other to the 0-5V signal. The external transistor has a -1.8V and -0.6V supply, with losses this translates to roughly -0.8V to -1.6V, but a few tenths of a volt different is no problem. A circuit board was made to fit a few ECL NOR/LATCH PCB’s, with some LEDs and switches to make testing easier.

Overall, both the NOR and the LATCH worked as expected and are both reasonably fast, easy in the dozens of Mhz range. A far cry from commercial ECL devices, but interesting nonetheless. The rise and fall time of the NOR gate are as follows, blue being the input and yellow the output, especially on risetime the NOR is faster then the input:

The Kicad designs can be downloaded here.


2 Comments

  • Reply phil g |

    Great job!
    I spent quite a few years in the 1970’s and 80’s designing ECL integrated circuits for a super computer company (Cray Research). ECL can be quite fast as you noted.. One good use for it is to create a high speed oscillator. You can connect the an inverting output to an input and it will oscillate. If you bias it correctly (with two resistors picked so that the voltage at the input is set to the midpoint of the swing) you can connect most crystals between the input and output and it will lock the frequency of the oscillation to the crystal frequency.
    Phil

    • Reply riktw |

      Huh, cool trick, I know that an uneven number of NOT gates in series will create a ring oscillator, but the trick with a crystal is rather cool.

So, what do you think ?