73 ECE 298A 8-Bit CPU Control Block :: Quicker, easier and cheaper to make your own chip!

How it works

This project implements the control block of an 8-bit CPU design building off the SAP-1.

The control block is implemented using a 6 stage sequential counter for sequencing micro-instructions, and a LUT for corresponding op-code to operation(s).

Supported Instructions

Mnemonic	Opcode	Function
HLT	0x0	Stop processing
NOP	0x1	No operation
ADD {address}	0x2	Add B register to A register, leaving result in A
SUB {address}	0x3	Subtract B register from A register, leaving result in A
LDA {address}	0x4	Put RAM data at {address} into A register
OUT	0x5	Put A register data into Output register and display
STA {address}	0x6	Store A register data in RAM at {address}
JMP {address}	0x7	Change PC to {address}

Instruction Notes

All instructions consist of an opcode (most significant 4 bits), and an address (least significant 4 bits, where applicable)

Control Signal Descriptions

Control Signal	Array	Component	Function
CP	14	PC	Increments the PC by 1
EP	13	PC	Enable signal for PC to drive the bus
LP	12	PC	Tells PC to load value from the bus
nLma	11	MAR	Tells MAR when to load address from the bus
nLmd	10	MAR	Tells MAR when to load memory from the bus
nCE	9	RAM	Enable signal for RAM to drive the bus
nLr	8	RAM	Tells RAM when to load memory from the MAR
nLi	7	IR	Tells IR when to load instruction from the bus
nEi	6	IR	Enable signal for IR to drive the bus
nLa	5	A Reg	Tells A register to load data from the bus
Ea	4	A Reg	Enable signal for A register to drive the bus
Su	3	ALU	Activate subtractor instead of adder
Eu	2	ALU	Enable signal for Adder/Subtractor to drive the bus
nLb	1	B Reg	Tells B register to load data from the bus
nLo	0	Output Reg	Tells Output register to load data from the bus

Sequencing Details

The control sequencer is negative edge triggered, so that control signals can be steady for the next positive clock edge, where the actions are executed.
In each clock cycle, there can only be one source of data for the bus, however any number components can read from the bus.
Before each run, a CLR signal is sent to the PC and the IR.

Instruction Micro-Operations

Stage	HLT	NOP	STA	JMP
T0	Ep, nLma	Ep, nLma	Ep, nLma	Ep, nLma
T1	Cp	Cp	Cp	Cp
T2	nCE, nLi	nCE, nLi	nCE, nLi	nCE, nLi
T3	**	-	nEi, nLma	nEi, Lp
T4		-	Ea, nLmd	-
T5		-	nLr	-

Stage	LDA	ADD	SUB	OUT
T0	Ep, nLma	Ep, nLma	Ep, nLma	Ep, nLma
T1	Cp	Cp	Cp	Cp
T2	nCE, nLi	nCE, nLi	nCE, nLi	nCE, nLi
T3	nEi, nLma	nEi, nLma	nEi, nLma	Ea, nLo
T4	nCE, nLa	nCE, nLb	nCE, nLb	-
T5	-	Eu, nLa	Su, Eu, nLa	-

Instruction Micro-Operations Notes

First three micro-operations are common to all instructions.
NOP instruction executes only the first three micro-operations.
HLT instruction transitions to a holding stage after T3, preventing the system for continuing

IO Table

Name	Description	I/O	Width	Trigger
clk	Clock signal	I	1	Edge Transition
rst_n	Set stage to 0	I	1	Active Low
ui_in[3:0]	Opcode	I	4	NA
uo_out[7]	If 1, the system is halted	O	1	Active High
uo_out[6:0]	control_signals[14:8]	O	7	NA
uio_out[7:0]	control_signals[7:0]	O	8	NA
ui_oe[7:0]	All Bidirectional pins are outputs	O	8	NA
uio_in[7:0]	Unused	I	8	NA
ena	Unused	I	1	Active High

IO Table Notes

See Control Signal Descriptions for the list of output control signals, and their correspondance in the control_signal vector

How to test

The control block can be tested by:

Providing an opcode through the ui_in[3:0] input pins.
Monitoring the uo_out[7:0] and uio_out[7:0] output pins for the control signals and halt status
For a given opcode, follow its Instruction Micro-Operation table to validate the control signal sequences
Consider using a logic analyzer to generate a waveform and analyze the stages, or slow down the clock to manually observe the control signals at various times

#	Input	Output	Bidirectional
0	opcode[0]	SIG_RAM_LOAD_N	SIG_OUT_LOAD_N
1	opcode[1]	SIG_RAM_EN_N	SIG_REGB_LOAD_N
2	opcode[2]	SIG_MAR_MEM_LOAD_N	SIG_REGB_EN
3	opcode[3]	SIG_MAR_ADDR_LOAD_N	SIG_ADDER_SUB
4		SIG_PC_LOAD	SIG_REGA_EN
5		SIG_PC_EN	SIG_REGA_LOAD_N
6		SIG_PC_INC	SIG_IR_EN_N
7		halted	SIG_IR_LOAD_N

73 ECE 298A 8-Bit CPU Control Block

73 : ECE 298A 8-Bit CPU Control Block