291 Minibyte CPU :: Quicker, easier and cheaper to make your own chip!

How it works

The Minibyte CPU is a simple "toy" 8-bit CPU that uses a custom RISC instruction set

The CPU also has some built in DFT (Design For Test) features and a Demo ROM that can be enabled for easy testing (some external hardware required)

This was created mostly as a learning/reference project to get more familiar with Verilog

Specs

Max CLK Frequency: 50Mhz (untested)

Data Buss Width:    8 bits
Address Buss Width: 8 bits (only 7 bits usable due to limited IO)

Registers:
    A   - 8 bits wide - Accumulator
    M   - 8 bits wide - Memory Address Pointer
    PC  - 8 bits wide - Program Counter
    IR  - 8 bits wide - Instruction Register
    CCR - 2 bits wide - Condition Code Register

Memory Mapped Registers:
    R0  - 8 bits wide - Gen Purpose Reg 0
    R1  - 8 bits wide - Gen Purpose Reg 1
    R2  - 8 bits wide - Gen Purpose Reg 2
    R3  - 8 bits wide - Gen Purpose Reg 3
    R4  - 8 bits wide - Gen Purpose Reg 4
    R5  - 8 bits wide - Gen Purpose Reg 5
    R6  - 8 bits wide - Gen Purpose Reg 6
    R7  - 8 bits wide - Gen Purpose Reg 7

Number of Instructions: 37

ALU:
    Data Inputs: 2x 8 bit inputs
    Data Output: 8 bits (result) + 2 bits (flags)

    Operations Supported:
        PASSA - Passthrough input A
        PASSB - Passthrough input B
        ADD   - Add A and B
        SUB   - Subtract B from A
        AND   - Logical and of A, B
        OR    - Logical or of A, B
        XOR   - Logical xor of A, B
        LSL   - Logical shift A left by B
        LSR   - Logical shift A right by B
        ASL   - Arithmetic shift A left by B
        ASR   - Arithmetic shift A right by B
        RSL   - Rotary shift A left by B
        RSR   - Rotary shift A right by B

    Flags:
        Z - Set if result is 0, otherwise clear
        N - Set if result is a negative signed int, otherwise clear

Pinout

uio[7:0]    - DATA IN/OUT BUSS
ui_in[7:0]  - DFT Test and Configuration Select
uo_out[7]   - WE (Write Enable Signal)
uo_out[6:0] - ADDR OUT BUSS

Architecture

The Minibyte CPU uses a very traditional architecture where most data is manipulated via a single accumulator (A Register)

The ALU operates on data from the A Register and either direct data (from memory indexed by the M register), or immediate data (from the current instruction's operand indexed by the PC register)

Minibyte Block Diagram

*Note that DFT and testing features are not represented in the above block diagram

Power Up State

Upon reset, the device will be initialized with all registers cleared to 0x00. This includes the program counter (PC register). It is expected that the program memory will start at address 0x00 to begin execution.

Instruction Set

The Minibyte CPU has 4 instruction format types. The program memory is chunked into bytes. Some instructions only occupy a single byte, while others occupy 2 bytes for an opcode and a following operand

Type	Length	Desc
Inherent	8 - bits	IR with no operand
Immediate	16 - bits	IR with an operand containing DATA
Direct	16 - bits	IR with an operand containing an ADDRESS
Indirect	16 - bits	IR with an operand containing an ADDRESS that points to another ADDRESS

As a visual reference, here is how we would expect a basic program to be structured in memory. Please note that all programs start execution from address 0x00 as shown.

Example Program Memory

The above program adds the numbers 0x05 and 0x03 together, and then loops back to the starting IP of 0x00

Inherent IR:

Type	OP[7:0]
Inherent	IR OPCODE

Immediate/Direct IR:

Type	OP[15:8]	OP[7:0]
Immediate	IR OPCODE	OPERAND DATA
Direct	IR OPCODE	OPERAND ADDRESS
Indirect	IR OPCODE	OPERAND ADDRESS

Opcode Table:

OPCODE	HEX	Operand	CCR
NOP	0x00	N/A	N/A
LDA_IMM	0x01	Immediate	N/A
LDA_DIR	0x02	Direct	N/A
STA_DIR	0x03	Direct	N/A
STA_IND	0x04	Indirect	N/A
ADD_IMM	0x05	Immediate	N/A
ADD_DIR	0x06	Direct	N/A
SUB_IMM	0x07	Immediate	N/A
SUB_DIR	0x08	Direct	N/A
AND_IMM	0x09	Immediate	N/A
AND_DIR	0x0A	Direct	N/A
OR_IMM	0x0B	Immediate	N/A
OR_DIR	0x0C	Direct	N/A
XOR_IMM	0x0D	Immediate	N/A
XOR_DIR	0x0E	Direct	N/A
LSL_IMM	0x0F	Immediate	N/A
LSL_DIR	0x10	Direct	N/A
LSR_IMM	0x11	Immediate	N/A
LSR_DIR	0x12	Direct	N/A
ASL_IMM	0x13	Immediate	N/A
ASL_DIR	0x14	Direct	N/A
ASR_IMM	0x15	Immediate	N/A
ASR_DIR	0x16	Direct	N/A
RSL_IMM	0x17	Immediate	N/A
RSL_DIR	0x18	Direct	N/A
RSR_IMM	0x19	Immediate	N/A
RSR_DIR	0x1A	Direct	N/A
JMP_DIR	0x1B	Direct	N/A
JMP_IND	0x1C	Indirect	N/A
BNE_DIR	0x1D	Direct	Z==CLEAR
BNE_IND	0x1E	Indirect	Z==CLEAR
BEQ_DIR	0x1F	Direct	Z==SET
BEQ_IND	0x20	Indirect	Z==SET
BPL_DIR	0x21	Direct	N==CLEAR
BPL_IND	0x22	Indirect	N==CLEAR
BMI_DIR	0x23	Direct	N==SET
BMI_IND	0x24	Indirect	N==SET

OPCODE	Desc
NOP	No Operation
LDA_IMM	Load A with immediate operand data
LDA_DIR	Load A with the data stored at the operand addr
STA_DIR	Store A at the operand addr
STA_IND	Store A at the addr contained at the operand addr
ADD_IMM	Add the immediate operand data to A
ADD_DIR	Add the data stored at the operand addr to A
SUB_IMM	Subtract the immediate operand data from A
SUB_DIR	Subtract the data stored at the operand addr from A
AND_IMM	And the immediate operand data with A
AND_DIR	And the data stored at the operand addr with A
OR_IMM	Or the immediate operand data with A
OR_DIR	Or the data stored at the operand addr with A
XOR_IMM	Xor the immediate operand data with A
XOR_DIR	Xor the data stored at the operand addr with A
LSL_IMM	Logical shift A left by the immediate operand data
LSL_DIR	Logical shift A left by the data at the operand addr
LSR_IMM	Logical shift A right by the immediate operand data
LSR_DIR	Logical shift A right by the data at the operand addr
ASL_IMM	Arithmetic shift A left by the immediate operand data
ASL_DIR	Arithmetic shift A left by the data at the operand addr
ASR_IMM	Arithmetic shift A right by the immediate operand data
ASR_DIR	Arithmetic shift A right by the data at the operand addr
RSL_IMM	Rotate A left by the immediate operand
RSL_DIR	Rotate A left by the data stored at the operand addr
RSR_IMM	Rotate A right by the immediate operand data
RSR_DIR	Rotate A right by the data stored at the operand addr
JMP_DIR	Jump PC to the operand addr
JMP_IND	Jump PC to the addr stored at the operand addr
BNE_DIR	Jump PC (if Z is clear) to the operand addr
BNE_IND	Jump PC (if Z is clear) to the addr stored at the operand addr
BEQ_DIR	Jump PC (if Z is set) to the operand addr
BEQ_IND	Jump PC (if Z is set) to the addr stored at the operand addr
BPL_DIR	Jump PC (if N is clear) to the operand addr
BPL_IND	Jump PC (if N is clear) to the addr stored at the operand addr
BMI_DIR	Jump PC (if N is set) to the operand addr
BMI_IND	Jump PC (if N is set) to the addr stored at the operand addr

DFT and Extra Features

The Minibyte CPU has a few DFT features that might be useful on live silicon for debug/testing. All DFT functions are enabled by an active high signal on one of the ui_in[7:0] pins (ui_in[7:0] should be tied to zero during normal operation)

ui_in Bit	Feature
ui_in [7]	Enable Memory Mapped Gen Purpose Registers
ui_in [6:5]	Unused
ui_in [4]	Enable Demo ROM
ui_in [3]	Halt Control Unit on Next Fetch
ui_in [2:0]	Debug Output Signal Select

Gen Purpose Registers:

The Gen Purpose Registers are a set of 8 memory mapped general purpose registers that can be accessed at the following addresses as long as ui_in[7] is held high

Reg Name	Mem Address
Register R0	0x78
Register R1	0x79
Register R2	0x7A
Register R3	0x7B
Register R4	0x7C
Register R5	0x7D
Register R6	0x7E
Register R7	0x7F

Debug Out Select:

The CPU has an extra mux between the normal addr out mux and the uo_out pins. To leverage this ui_in[2:0] can be used to select a debug signal to output on the uo_out[6:0] pins in place of the normal address buss

Debug Out Select	Function
ui_in[2:0] = 0b000	Normal Operation
ui_in[2:0] = 0b001	Output A[6:0] to uo_out[6:0]
ui_in[2:0] = 0b010	Output A[7] to uo_out[0]
ui_in[2:0] = 0b011	Output M[6:0] to uo_out[6:0]
ui_in[2:0] = 0b011	Output PC[6:0] to uo_out[6:0]
ui_in[2:0] = 0b011	Output IR[6:0] to uo_out[6:0]
ui_in[2:0] = 0b011	Output CCR[1:0] to uo_out[1:0]
ui_in[2:0] = 0b011	Output CU_STATE[6:0] to uo_out[6:0]

How to test

Simulation

The Minibyte CPU has fairly exhaustive cocotb test suite that is able to test and verify most of the device's intended functionality.

To run the test suite, cd into the ./test directory of the project and run "make"

Simulation Results

On Live Silicon

The easiest way to test the Minibyte CPU on live silicon is to use the built-in Demo ROM

To enable the Demo ROM, make sure that ui_in[4] and ui_in[7] are held high on reset, and remain high while the program runs

Holding ui_in[4] high will enable the Demo Rom

Holding ui_in[7] high will enable the General Purpose Registers, which are required/utilized by the Demo ROM program

The Demo ROM will run the following program

PSEUDOCODE:
    WHILE FOREVER{
        //Part 1: Binary Count
        SET A to 0

        WHILE A <= 255 {
            INCREMENT A

            WRITE A to ADDRESS 0x40
        }

        //Part 2: Walking 1
        SET A to 1

        WHILE A > 0 {
            LEFT SHIFT A by 1

            WRITE A to ADDRESS 0x40
        }

        //Part 3: Deadbeef to RAM/Gen Purpose Registers and back out
        LOAD 0xDEADBEEF into R0->R3

        WRITE R0 to ADDRESS 0x40
        WRITE R1 to ADDRESS 0x40
        WRITE R2 to ADDRESS 0x40
        WRITE R3 to ADDRESS 0x40
    }

To capture the output of the program with LEDs, it is recommended to add a D-Flip Flop (such as a 74x273 series chip) on the output of the data buss (uio[7:0]). See External Hardware section below for more details

External hardware

Demo Setup

Demo Schematic

Something similar to the above schematic is recommended when running the Demo ROM. Note that an inverter (such as a 74x04 series chip) should be used as shown on the CLK input of the DFF. We want data to be latched when WE falls to 0 (after the data has had time to set up and make its way out of the chip). Please also note that you will probably need to run the CPU at a fairly low CLK frequency in order to see any LED activity with the naked eye.

Other Setups

The sky is the limit as far as as what devices you attach to the CPU. If you are writing your own programs, you probably are going to want to attach some sort of external ROM to the main address and data buss. Here is a recommended setup adding an external EEPROM.

External EEPROM Schematic

Beyond this, you will hopefully find that the Minibyte CPU can be paired with a wide variety 3.3V compatible parallel ROM/EPROM/EEPROM, SRAM, and IO expander modules.

#	Input	Output	Bidirectional
0	DEBUG_OUT_SELECT_0	ADDR_OUT_0	DATA_0
1	DEBUG_OUT_SELECT_1	ADDR_OUT_1	DATA_1
2	DEBUG_OUT_SELECT_2	ADDR_OUT_2	DATA_2
3	DEMO_ROM_ENABLE	ADDR_OUT_3	DATA_3
4		ADDR_OUT_4	DATA_4
5		ADDR_OUT_5	DATA_5
6		ADDR_OUT_6	DATA_6
7		WE_OUT	DATA_7

291 Minibyte CPU

291 : Minibyte CPU