Address decoder logic

There are a number of connections that cannot just be connected to each other. They will require additional circuitry to work. In my 6502 computer, some of the address connections will require such circuitry, I will use 74 series logic chips to accomplish this.

The RAM (62256), ROM (28256) and VIA (6522) have chip select pins. These pins need to be activated at the right point, this will be achieved with the address decoder logic.

Between the a15 from the CPU the chip select of the ROM needs to be inverted. Therefore I will need a NOT gate. The chip select of the RAM needs to be at logic 0 (since it is active low) when the clock signal is low and a15 from the CPU is 0. A NAND gate fulfils this requirement.

You may wonder if 4000 series chips could be used instead, since they have many 74 series equivalents! However, I would not recommend it. The two families have various positives and negatives and one negative of the 4000 series is propagation delay. With regards to the logic gates that I wish to use, the propagation delay is the length of time taken for a signal at a gate’s input to have an effect on the output of the gate. There are many circuits where this propagation delay would not be a problem. However, high-speed digital circuits are not one of them! Even though I have settled for the 74 series as my choice for logic chips. there is a further consideration to make, which family? There are TTL, CMOS and BI-CMOS versions, which can then be further broken down into TTL (74, 74L, 74H, 74S, 74LS, 74AS, 74ALS, 74F), CMOS (74C, 74HC, 74HCT, 74AC, 74ACT, 74ACQ, 74AHC, 74ALVC, 74ALVT, 74AUC, 74AUP, 74AVC, 74AXC, 74FC, 74FCT, 74LCX, 74LV, 74LVC, 74LV-A, 74LVT, 74LVQ, 74LVX, 74VHC) and BI-CMOS (74BCT and 74ABT).

I chose CMOS as this contain a few possibilities but my choice was the HC series. The propagation delays are lower than most of the other choices. The HC series is an improvement on its predecessors. The improvements that we are interested in are the input and output rise and fall times. There are others, that in theory would be even better choice but are harder to obtain.

At low clock speeds the propagation delay will probably not be an issue. However, with increases in clock speed, eventually, the propagation delay would likely be a limiting factor to how fast a clock speed I could use for the computer.