z80 cpu test circuit

Posted on Feb 4, 2014

GND and VCC are perpendicular to the rest of the pins in the circuit diagrams, whereas the real Z80 is a DIP with no pins at all in these locations. I find that this makes the circuit diagram harder to read. I have to map between the real physical pins and the pins in the diagram. In regard to your second question. When we draw symbols for processors on schematics

z80 cpu test circuit
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we often rearrange the pins to make the schematic easy to draw and read. You will soon get used to it. Those 470 ohm resistors place a hex 0x00 on the data lines so that the uP always executes a `NOP` instruction. They arent needed if you later connect an EPROM to the data and address lines, which you will have to do at some point. A good memory mapping technique for a small computer (very small) like this is to use A15 for the RAM/ROM select bit. When A15 is low (a `0`) it selects ROM, and when high (a `1`) it selects RAM. This maps into 32k of ROM and 32k of RAM but you can use less of each and still use that bit. Once you get some code running in ROM you`ll be able to program service calls for various devices like keypads and LEDs and whatever else you want to drive with it like relays, etc. You might find it easiest to interface with a Serial type LCD display rather than an actual monitor at first, as any display type you use should have it`s own storage because the on board storage will be so small. You can also map more memory in as pages but that requires more hardware. In the end you probably wont be able to use this for too much other than small stuff. You can build a small word processor or spreadsheet or whatever but it wont be as good as stuff on the modern PC. You`ll probably end up putting it aside at some point but if all you want is some learning experience then it may not be a total loss. After all, the...

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