Basic Gates in LTL

A project log for LCPU - A CPU in LED-Transistor-Logic (LTL)

This projects tracks my efforts to develop discrete LED-Transistor logic building blocks and designing a CPU from them.

TimTim 02/16/2020 at 19:090 Comments

After successful validation of the LTL concept in real hardware I started to build up a library of common gates in LTspice as a foundation for the design of a CPU.

The basic gate of LTL is the NAND2 gate. Symbol and circuit shown above. In my final design I used gates with different threshold level so I added a "G" to a high threshold device with a green LED.

Every gate was tested in a simple testbench in LTspice. I arbitrarily chose a fan out (FO) of 7 for the test case, although this does not occur in the real design.

Test waveforms for the NAND2 gate are shown above - nothing peculiar. There is a little crosstalk between the inputs of the gates if one input is high and the other one is pulled low. This is due to the extremely high slew-rate of the falling edge on the output. In a physical implementation this will hopefully be reduced a bit by additional parasitic capacitances to the power plane.

Next is the NOR2 gate. This can be easily realized by a wired AND of two LTL inverters. A minor but very important detail: If a gate uses a wired AND at the output, neither of the LEDs will be representative of the output signal. In practice this means that additonal indicator-LEDs may have to be added to monitor certain nodes.

Last one is the AOI2 gate (AND OR INVERT). You may not be familiar with this kind of gate, but it is a very useful building block due to it's simple implementation. For example, it can be used as a multiplexer or as part of an ALU.

Finally, a list of part counts for each gate. Since my intention is to build a CPU with a minimal amount of discretes, it is important to keep track of this.

Not too exciting, so let's get to the more special building blocks next...