Introduction: MP-4 Simplest 4 Bit TTL CPU

About: Orthopaedic Surgeon, Paragliding, Skiing, Digital electronics, Microcomputer lover.

only 12 TTL chips and 3 eeproms and magic :)

Step 1: CPU Architecture

MP-4 is simplest CPU that you can build and run your programs, including conditional branches and a few arithmetic logical operations. As you can see in block diagram, MP-4 is an harvard architecture microcontroller.

In this very first version there is only one register (accumulator). General purpose RAM not implemented yet.

This CPU design is very helpful to understand how CPU's work.

You can easily modify it to add RAM, add more registers or add more instructions.

Step 2: Byte Structure

instruction and operand combined in MP-4 bus. This makes possible less hardware and single instruction in single cycle. Most significant 3 bits are for instruction and lower 5 bits for operand.

Total 8 instructions and 32 bytes of memory awailable with this configuration.

MP-4 is a MIPS class CPU

Step 3: ALU and Accumulator

74ls181 ALU used for arithmetic and logical operations. 74hc574 used as accumulator.

With this configuration seen in diagram, every results can be stored in accumulator again.

Do not confuse in schematic, it seems there is no direct acces to Acc but its not true,

If you arrange 74181's B input to pass data directly out; data stores in acc directly by passing through 74ls181

74ls181 may be seen very useful in first look but unfortunately this is not true (its a big mess)

To understand work of this CPU; I recommend setup an 74ls181 circuit first and play with it a few days. (I did)

74ls181 have two 4 bit inputs, one 4 bit output and 6 control inputs.
this control inputs determines output function.

Step 4: Clock, Program Counter and Phases

The main philosophy of MP-4 is simplicity so, I used ne555 as clock generator and its working about 1Hz to understand timing. 555 produces clock signal and 74hct4017 decade counter produces phases.

(DO NOT USE CD4017 !)

In first phase, CPU fetches instruction to microcode and puts data to bus.

in second phase instruction executing and accumulator refreshing with new value.

MP-4 can run reliably up to 8 Mhz

Step 5: Microcode and Control

MP-4 have low encoded horizontal microcode. This mean, we need tons of control pins.

So, I used two 8 bit eeproms. Microcode sequencer directly reads instruction from ROM data output and produces necessary control signals.

Bit 0 and 5-6-7 of microcode1 not used. also LDA,M and LDM,A instructions not implemented in this published version.

PcLoad/ signal is producing by zero flag and JP, JPZ signals.

Step 6: Conditional Jump

Conditional jump is one of important part of this system.

Acc output goes to 4 input OR gates and generates Zero flag.

Zero signal, JP signal and JPZ signal determines conditional or unconditional jumps.

Step 7: Complete Circuit

Im putting here pdf version to view and realise your MP-4 also DSN files for Proteus ISIS design suit

in ISIS you can simulate your own CPU design. simply right click on ROM and put your own hex program to run

But please note that Proteus-isis cannot simulate 74ls181 properly

Step 8: Instructions and Usage

00 : NOP You have only 8 instructions and you waste one of them with "no operation" command !

NOP instruction is very important for upgrade to multicycle CPU. When upper microcode fetching instruction, operand or adress bytes, lower microcode must run NOP command.

We will see this in next version of MP-4E ( E: enchanced, multicycle CPU)

20: LDA,n (2 for command, lower nibble is operand: 20,2f, 2a etc all loads acc with low nibble)

40: ADD,n : adds operand to acc

60: DEC : decrements accumulator operand ignored

80 IN A : gets data from input port to acc

A0: OUT A :puts acc content to out port

for in or out commands you may use bit as adress decoder to add one more port

then BX command puts data to second out port

C0: JP : unconditional jump to adress n (4 bit adress)

Dn: JP unconditional jump to 5 bit adress

En: JPZ : conditional jump to adress n (4 bit adress): jump if zero
Fn: JPZ : conditional jump to 5 bit adress

HACK1: If you want to add RAM, install LDA,M (load acc with adress content) and LDM,A (load memory with acc) instructions.

Also you must add MAR register, then remove instructions NOP and DEC (ADD ,0e is same with DEC)

dont forget to modify in microcode CS/, WE/ and RD/ signals. In this setup, you may use unused PCload/ signal in microcode ;)

HACK2: you may add 8 more instructions by sacrifying 16 bytes of memory then, upper four bits goes for code and lower four bits for operand ;)

Step 9: Microcode and Test Programs for ROM

I used 28c16's for microcodes and 28c64 for main ROM unfortunately proteus design suit cant recognized so that I published circuit with 2764 eproms here is microcodes and test programs