419 DJ8 8-bit CPU :: Quicker, easier and cheaper to make your own chip!

How it works

DJ8 is a 8-bit CPU implemented in VHDL, originally developped for XCS10XL featuring:

8 x 8-bit register file
3-4 cycles per instruction
15-bit address bus
8-bit data bus
Built-in 256-bytes demo ROM with 2 demos

Sample assembly code could be found in test bench and demo ROM.

Other implementations:

Memory Map

From	To	Description
0x0000	0x7fff	External memory
0x8000	0xffff	Internal Test ROM (256 bytes, mirrored)

External memory map if using the recommended setup (see pinout)

From	To	Description
0x2000	0x3fff	External RAM (32 bytes)
0x4000	0x5fff	External Flash ROM (16KB)

Registers

There are 8 general purposes 8-bit registers (A,B,C,D,E,F,G,H), two flag registers (CF, ZF), and 16-bit PC.

For memory addressing, 16-bit combined registers EF and GH are used.

At reset time, PC is set to 0x4000. All other registers are set to 0x80.

Instruction Set

For future compatibility, please set the don't care bits (?) to 0.

ALU reg, imm8: Immediate ALU operation

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	1	A	A	A	D	D	D	I	I	I	I	I	I	I	I

A : ALU operation
- 000: ADD: reg = reg + imm8
- 001: ADC: reg = reg + imm8 + CF
- 010: SUBC: reg = reg - (imm8 + CF)
- 011: MOVR: reg = reg
- 100: XOR: reg = reg ^ imm8
- 101: OR: reg = reg | imm8
- 110: AND: reg = reg & imm8
- 111: MOVI: reg = imm8
D : register
I : imm8

ALU dest, src, A {,shift}: ALU operation with src register & register A

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	0	A	A	A	D	D	D	S	S	S	?	F	F	0	0

A : ALU operation
- 000: ADD: dest = src + A
- 001: ADC: dest = src + A + CF
- 010: SUBC: dest = src - (A + CF)
- 011: MOVR: dest = src
- 100: XOR: dest = src ^ A
- 101: OR: dest = src | A
- 110: AND: dest = src & A
- 111: MOVI: dest = A
D : dest register
S : src register
F : final shift operation
- 00: No shift
- 01: Shift right logical (shr)
- 10: Shift right arithmetic (sar)

ALU dest, [mem], A {,shift}: ALU operation with memory & register A

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	0	A	A	A	D	D	D	?	?	?	M	F	F	1	0

A : ALU operation
- 000: ADD: dest = [mem] + A
- 001: ADC: dest = [mem] + A + CF
- 010: SUBC: dest = [mem] - (A + CF)
- 011: MOVR: dest = [mem]
- 100: XOR: dest = [mem] ^ A
- 101: OR: dest = [mem] | A
- 110: AND: dest = [mem] & A
- 111: MOVI: dest = A
D : dest register
M: memory mode
- 0: [GH]
- 1: [EF]
F : final shift operation
- 00: No shift
- 01: Shift right logical (shr)
- 10: Shift right arithmetic (sar)

MOVR [mem], reg: Store content of register in memory

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
1	0	0	1	1	D	D	D	?	?	?	M	?	?	0	1

D: register
M: memory mode
- 0: [GH]
- 1: [EF]

Jxx imm12: Conditional or unconditional jump to absolute address

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	0	J	J	I	I	I	I	I	I	I	I	I	I	I	I

J: jmpcode
- 01: Jump if zero (JZ)
- 10: Jump if not zero (JNZ)
- 11: Unconditional jump (JMP)
I: imm12
- PC = (PC & 0xe000) | (imm12 << 1)

JMP GH: Unconditional jump to address GH

15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0	1	?	?	?	?	?	?	?	?	?	?	?	?	?	?

Pinout

Due to TT06 IO constraints, pins are shared between Address bus LSB and Data bus OUT. It means that during memory write instructions, the address space is only 128 bytes.

Pins	Standard mode	During memory write execute+writeback cycles
ui[7..0]	Data bus IN	Data bus IN
uio[7..0]	Address bus LSB (7..0)	Data bus OUT
uo[6..0]	Address bus MSB (14..8)	Address bus MSB (14..8)
uo[7]	Write Enable	Write Enable

You can connect a 8KB parallel Flash ROM + 32b SRAM without external logic and use uo[6] for RAM OE# and uo[5] for Flash ROM OE#.

To get a bidirectional data bus (needed for SRAM), uio bus must be connected to ui bus with resistors. To be tested!

How to test

An internal test ROM with two demos is included for easy testing. Just select the corresponding DIP switches at reset time to start the demo (technically, a jmp GH instruction will be seen on the data bus thanks to the DIP switches values, with GH=0x8080 at reset).

Demo 1: Rotating LED indicator

SW1	SW2	SW3	SW4	SW5	SW6	SW7	SW8
0	0	0	0	0	0	1	0

No external hardware needed. This demo shows a rotating indicator on the 7-segment display. Its speed can be changed with DIP switches, the internal delay loop is entirely deactivated when all switches are reset.

Demo 2: Bytebeat Synthetizer

SW1	SW2	SW3	SW4	SW5	SW6	SW7	SW8
0	0	0	0	0	1	1	0

Modem handshakes sound like music to your hears? It's your lucky day! Become a bit-crunching DJ thanks to 256 lo-fi glitchy settings.

Connect a speaker to uo[4] or use Tiny Tapeout Simon Says PMOD. Play with the DIP switches to change the loop settings.

It is highly recommended to add a simple low-pass RC filter on the speaker line to filter out the buzzing 8kHz carrier. Ideal cut-off frequency between 3kHz and 8kHz, TBD.

Set SW1 and/or SW2 at reset time to adjust speed in case the design doesn't run at 14MHz.

External hardware

No external hardware for Demo 1
Speaker for Demo 2
Otherwise: Parallel Flash ROM + optional SRAM

#	Input	Output	Bidirectional
0	data in 0	address out 8	address out 0 / data out 0
1	data in 1	address out 9	address out 1 / data out 1
2	data in 2	address out 10	address out 2 / data out 2
3	data in 3	address out 11	address out 3 / data out 3
4	data in 4	address out 12	address out 4 / data out 4
5	data in 5	address out 13	address out 5 / data out 5
6	data in 6	address out 14	address out 6 / data out 6
7	data in 7	write enable	address out 7 / data out 7

419 DJ8 8-bit CPU

419 : DJ8 8-bit CPU