# The Clock

The clock is a six-digit HH:MM:SS timepiece using only DDL gates (1N4148/1N4007 diodes) for the logic and display functions (the LEDs are also diodes, of course). The power supplies and currently the crystal-oscillator timebase use more traditional circuits, including transistors and ICs. You can see the clock working in this brief video:

IMPORTANT: I've added a Builder's Notes log section for details and errata about the project for those experimenting with DDL or building their own clock. Please read before you begin.

# Diode-Diode Logic (DDL)

DDL is an idea for building complex logic systems, like digital clocks or computers, using only solid-state diodes as switching elements. In the current version of DDL, a 1N4007 diode forms an inverter by switching radio-frequency current based on logic-level signals. Combined with a 1N4148-based OR gate, this makes a NOR, from which any combinational or sequential logic function can be constructed. Modular circuit boards have been designed and proven, so that working with DDL can be (almost) as easy as using TTL or CMOS logic. Just grab the DDL01 datasheet and start designing!

If you want to build something now, check out the simple demo circuit. You probably have the parts sitting on your workbench.

Like building contemporary logic circuits from individual TTL gates, discrete transistors, or relays, DDL is at its heart a perverse technological pursuit: an artificial challenge in an age of effortless gadgetry. So, have fun with it...

# Goals

The ultimate goal of DDL is to build a simple computer using only diodes as logic elements.

The Diode Clock project is a first large-scale test to see if the idea scales to "real" systems. It does. 276 diode NOR gates are combined to make the clock, providing reasonable proof that a simple computer could be assembled. The reality is that a clock built with DDL is more useful than a computer that could practically be built with the current iteration of the "technology."

We live in the transistor-computer age. At some point, our current computing technology will join the vacuum tube, the electro-mechanical, and the mechanical computing devices in the technological trash bin. DDL isn't the next computing technology (I can assure you of that), but it's time to start thinking about what will be.

# DDL Devices

I've designed several PCBs enabling any digital logic circuit to be constructed using diode logic. These are all released under the CC-BY-SA license. After I've finished documenting it all, I'll release everything in one central GitHub repository. For now, grab files here:

I'll be adding documentation and design files for these boards over the next few weeks. Look for updates in the project log.

# Background

When I was a kid, my parents bought me a copy of Steve Ciarcia's "Build Your Own Z80 Computer" - it was around 1983 or so. I remember seeing a diagram of diode ROMs and simple diode AND/OR gates (perhaps in another book) and wondering if you could build an entire computer using just diodes. Of course, I soon found that there were two things that made this impossible. First, simple diode gates didn't exhibit any gain (they have an effective fan-out of less than one), so signals would get weaker with each successive stage; this rules out sequential logic entirely. Second, there is no way to construct an inverter, and hence a whole class of combinational logic functions (e.g. NOR, NAND, XOR) are not possible. This bothered me. For many years.

In late 2002, I was experimenting with some RF direction-finding circuits that used PIN diodes to switch RF signals using DC current. They work like a variable RF resistor that's controlled by the amount of DC current passing through the diode. I also remember reading in the 2000 ARRL Handbook that common rectifier diodes like the 1N4007 can be pressed into service as crude RF switches similar to PIN diodes. I thought that if the switched RF signals were detected (rectified) by faster diodes (e.g. 1N4148), the resulting DC current might be used to switch the next stage of gates. A little bench time, and I was convinced it could work. This topology solves both problems with diode-only logic circuits. Since the 1N4007 can switch enough RF power to provide a gain greater than one, an arbitrary number of logic stages can be used, and sequential logic like latches and flip-flops become possible. Further, since a shunt PIN diode switch turns off RF current with applied DC bias, it naturally forms an inverter. Combined with a "traditional" diode-OR, this creates a NOR gate, from which any arbitrary logic function can be realized. After building the first prototype gates, I christened the logic family "DDL" or "Diode-Diode Logic" in accordance with the convention for early bipolar logic (RTL, DTL, TTL).

# Going Forward

The current iteration of DDL was designed with common diodes (IN4148/IN4007) because they're cheap, widely available, and frankly, overlooked and underestimated as components. However, the 1N4007, while functioning as a crude PIN diode when pressed into such service, is far from an RF-optimized part. Likewise, the 1N4148, despite littering the floor of countless workshops, just doesn't compare to the performance of dedicated Schottky RF detectors. Using these common diodes naturally limits the performance of the logic. The DDL01 Hex NOR board, when wired as a divide-by-two D flip-flop, can hit clock speeds of 3.3kHz consistently. That's KILO Hz. Pretty slow for a logic family, but respectable considering that the 1N4007's are probably most commonly used in 50/60Hz AC rectifiers. In a follow-on project, I'm going to push the limits of DDL performance, using "real" RF PIN diodes, Schottky detectors, and good SMT RF PCB design to see just how fast these crazy circuits can go.

# DISCLAIMER

"If you wish to make an apple pie from scratch, you first must invent the universe." - Carl Sagan

In many places in this project, I refer to DDL and the Diode Clock as being "diode only". This is not strictly true, and I'd like to be the first one to address it. In the DDL Demo Board and the DDL02 power supply, "conventional" transistorized circuits (74AC14 inverters, bipolar transistors, and power MOSFETs) are used to create the required RF power to run the logic circuits. I don't think this disqualifies DDL as a diode-only technology: such circuitry is confined to the power supplies, and all logic functions are implemented just with diodes. In fact, I think with some extra work (maybe a lot of extra work), you could replace the power supplies with diode-only versions made from GUNN or IMPATT diode oscillators, or go in a completely retro-mechanical direction with spark gaps or Alexanderson alternators for generating the required RF energy. This is left as an exercise for the reader.

Likewise, the timebase for the Diode Clock uses a PIC microcontroller and a TCXO oscillator to produce the 1 pulse-per-second input for accurate timing. I have experimented with DDL-only relaxation oscillators, and although they can produce a waveform that will drive the clock, achieving accurate timekeeping has proven difficult for a number of reasons. I'll document some of these experiments here.

Carl Sagan said it best. I'm starting with apples. Or maybe seeds.