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Updated Proposed Clock Circut and Simplified Halt

A project log for The UFS4057ES - A 4 bit ALU / CPU Build

Building a small computer with an EEPROM brain.

Dave CollinsDave Collins 01/14/2022 at 20:310 Comments

Here is the updated clock schematic based on suggestions from the Facebook group Z80 DIY/Homebrew Computers & Projects,  I have switched the capacitance values to much lower in order to be able to use larger value timing resistors.  I don't think I mentioned the formula for calculating the frequency is 1.2 / R*C. The main issue with the last schematic is I forgot about the input current limits of the 74HC14N which are around 20mA.  Using a minimum value timing resistor of 275 ohms, is pretty much required to limit the current to the input to a safe state below the current threshold of the input.  Another suggestion was to use a ceramic cap for the R-C networks, since they are less susceptible to temperature change; and with the value much lower a better choice since they are far and away better performing caps, in terms of reliability.  Another change I made was to tie the oscillation low using a transistor. this will halt the clock in full clock mode but leave the ability to single step, which will be useful for the state machine test bed program (you can do a single instruction jump to the next instruction which can augment the halt flag. Still not feeling up to building this up on the bread board, I've ordered 100K trimmers which won't arrive until at least Sunday.  Hope everyone's weekend goes well. 

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