• ADDRESING BOARD BUILD PROGRESS

    Yago • 11/10/2022 at 18:21 • 0 comments

    So this is the progress in the soldering of the incompleted addressing board:

    The upper row of circuits (the upper row of leds and such) is the HL (memory address) register, the lower row is the IP (instruction pointer) register. The square in the middle right is the multiplexer, which contains AND & OR gates, and finally, the incompleted circuit on the right are the output inverters. Terminal blocks will be placed on the top of the board to allow connection to other boards. In theory, there will be 4 other boards like this one.

  • BASIC CIRCUITS

    Yago • 05/14/2022 at 13:47 • 0 comments

    I've already shown the NAND gate:

    Note that the pull-up output resistor can either be 680Ω or 340Ω (two parallel 680Ω resistors) depending on the current requirement of the input it's connected to.

    Most other computers I've seen use D-latches or something similar for registers. 4 NAND gates per latch is quite a lot considering that the addresing space is 15 bit. The solution I've found is to use SR latches and design the control logic so that it resets registers before writting to them. This is the circuit for a SR latch with an enable input:

    We also have the multiplexer circuit. Muxes made out of nand gates require 3 gates, but using a combination of AND/OR/NANDgates we can reduce the component count:

    Note that if no input is enabled, the output will default to 1.

    Finally, the inverted buffer. This one allows to drive large amounts of OR gates, as in registers' enable signal. It should output up to 100mA in theory:

    More circuits will probably appear as I build the computer, I'll post them in the corresponding logs.

  • Gates & ring oscillator speed tests

    Yago • 05/02/2022 at 09:36 • 0 comments

    The design I'm using is based around LTL logic by Tim. It has the following circuit diagram:

    The noise marging should be quite good due to the difference in forward voltage between the input diodes and the led. As for speed, I've built the following 5 stage ring oscillator:

    I don't own a oscilloscope (too expensive for my budget...), so I used an Arduino with the amazing freqcount library. The entire oscillator produced around 4 MHz when testing 2n2222a transistors, which is around 20 MHz per gate and 5 MHz when testing the BC547C, which is 25MHz per gate. Other people have recommended me using the 2n3904 but I don't have access to them right now. I will probably use the BC547. It is worth noting that the capacitors add a big speed improvement.