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Status Register and Instruction Decoding Schematic

A project log for 8 Bit Breadboard CPU

A home-brew 8 bit Microprocessor built on a breadboard 64K Address Space, IRQ, and DMA 16 Bit Stack Pointer, 4 8 bit Registers or 2 16 bit

2-zons2-Zons 12/17/2018 at 22:102 Comments

Built the schematic for the status register / instruction counter and instruction decoder with microcode (EEPROMs).  This is more of a working design.  Currently it has inverters on all the decoder outputs but many of the control signals are active low and will not require interting.  When I build it I will optimize.  

https://upverter.com/2-Zons/ec2b22eca1358d33/8-Bit-CPU-Status-Register-and-Instruction-Decoder/

I am building all the microcode in a database so I can run queries to try and find all the possible groups of control signals that are exclusionary (never used at the same time) this way I can use more encoders to reduce the amount of EEPROM's.  Currently I need 5, but might be able to get it down to 3 or possibly 2.  There are 50 control signals.  If they were all exclusionary I could get by with 1 EEPROM (6 bits = 64 signals).   After I've designed all the microcode I may even find there are control signals that aren't used in any instruction.

I've also been working on some slight redesigns of the other schematics, including the ALU.  I found some LS181 and LS182's on ebay so I will be building my ALU out of those.  Although I like @roelh's ALU design, I can reduce the chip count significantly by using 2 181's and a 182.   I might just build @roelh's first as I won't be getting the 181's and 182 for a while.

I can't get to doing any actual building as I am still waiting on some key components to arrive, like wire and capacitors.  Unfortunately there are no decent electronics shops where I live (near Edmonton, Alberta).  There used to be an good one called Active Electronics but they shut down.  There are a few small shops but they have exorbitant prices.  I was going to get a roll of 22AWG hookup wire from them.  They were charging $25 for a 50 foot roll.   I've got several different 100 foot rolls on order for $8.50 USD each.  I couldn't bring myself to pay that much for something that I've already ordered.  I've spent more that I wanted to on this already.   I'm just going to have to be patient and wait a bit until I can actually start building things.

Discussions

Steve Toner wrote 12/18/2018 at 02:38 point

If you've only got 2 '181s (8 bit ALU), you don't really need a '182.  It will only gain you about 10ns best case, probably less.

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2-Zons wrote 12/18/2018 at 03:43 point

good tip.  I will adjust my plans.

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