993 Nibble - 4-bit CPU :: Quicker, easier and cheaper to make your own chip!

How it works

Nibble is a minimalistic 4-bit accumulator CPU with a Harvard architecture, designed to fit in a single tile on the TinyTapeout Sky130 process. It executes a simple 16-instruction ISA over a 2-cycle fetch-execute pipeline.

Architecture Overview

The CPU consists of:

4-bit Accumulator (A): The primary register for all arithmetic and logic operations
4-bit Program Counter (PC): Addresses up to 16 external instructions
Condition Flags: Carry (C) and Zero (Z) flags updated by ALU operations
Status Registers: Halted flag and phase indicator (0=FETCH, 1=EXECUTE)
External Program Memory: Up to 16 instructions provided via the input pins (Harvard architecture)

Instruction Set Architecture

The CPU decodes 8-bit instructions with a 4-bit opcode [7:4] and 4-bit immediate/address [3:0]:

Opcode	Instruction	Operation	C	Z
0x0	NOP	No operation	-	-
0x1	LDI imm	A ← imm	-	✓
0x2	ADD imm	A ← A + imm	✓	✓
0x3	SUB imm	A ← A − imm	✓	✓
0x4	AND imm	A ← A & imm	-	✓
0x5	OR imm	A ← A \| imm	-	✓
0x6	XOR imm	A ← A ^ imm	-	✓
0x7	NOT	A ← ~A	-	✓
0x8	SHL	{C,A} ← {A[3],A<<1}	✓	✓
0x9	SHR	{A,C} ← {A>>1,A[0]}	✓	✓
0xA	JMP addr	PC ← addr	-	-
0xB	JZ addr	if Z: PC ← addr	-	-
0xC	JC addr	if C: PC ← addr	-	-
0xD	JNZ addr	if ¬Z: PC ← addr	-	-
0xE	IN	A ← port_in	-	✓
0xF	HLT	Halt execution	-	-

Legend: ✓ = flag updated, - = flag unchanged

Pipeline Execution

The CPU operates in a 2-cycle fetch-execute pipeline:

FETCH Phase: The current PC value is output on uio[3:0]. External memory (RP2040 or EEPROM) responds with the instruction on ui_in[7:0], which is latched into the Instruction Register (IR).
EXECUTE Phase: The latched instruction is decoded and executed. The ALU computes results, flags are updated, and the PC is incremented (or modified by branch operations).

Each instruction takes exactly 2 clock cycles. The phase output (uo[7]) toggles between 0 (FETCH) and 1 (EXECUTE) so that the external memory controller can respond synchronously.

Pin Configuration

Inputs (ui[7:0]):

ui[3:0] - Instruction bits 0–3
ui[7:4] - Instruction bits 4–7 (opcode)

Outputs (uo[7:0]):

uo[3:0] - Accumulator value (connect to LEDs!)
uo[4] - Carry flag
uo[5] - Zero flag
uo[6] - Halted flag
uo[7] - Phase (0=FETCH, 1=EXECUTE)

Bidirectional (uio[7:0]):

uio[3:0] - Program Counter output (address bus for external ROM)
uio[7:4] - General-purpose input port (for IN instruction)

How to test

Basic Setup

Connect the design to a clock source. The design is rated for 1 MHz operation.
Provide an RP2040 microcontroller or external ROM as the program memory:
- Monitor uio[3:0] (PC output) to determine which instruction address is being requested.
- When uo[7] (phase) is 0 (FETCH), drive the corresponding instruction onto ui[7:0] within one clock cycle.
Connect LEDs to uo[3:0] to visualize the accumulator value. Optionally monitor uo[4:7] for flags and status.

Example Program: Count to 15

Load the following 16 instructions into external memory:

0x0 (PC=0):  0x10  LDI 0      // A ← 0
0x1 (PC=1):  0x21  ADD 1      // A ← A + 1
0x2 (PC=2):  0x5F  OR 15      // A ← A | 0xF (ensure A ≤ 15)
0x3 (PC=3):  0xB0  JZ 0       // if Z: jump to 0 (never from OR)
...
0xF (PC=15): 0xF0  HLT        // Halt

After reset, the accumulator will increment from 0 to 15 on each execution, with LEDs reflecting the count.

Testing with TinyTapeout Demo Board

The TinyTapeout Sky130 demo board integrates an RP2040 that can act as the program memory controller:

The RP2040 samples uio[3:0] using GPIO pins to determine the current PC.
It drives the instruction corresponding to that PC onto ui[7:0].
Each cycle, uo[7] (phase) toggles, allowing the RP2040 to synchronize instruction delivery.

Load your program into the RP2040 firmware, and the CPU will execute it automatically with LEDs showing the accumulator state.

External Hardware

Program Memory: 16×8-bit instruction ROM (can be implemented as:
- RP2040 microcontroller firmware (recommended for TinyTapeout)
- External EEPROM or SRAM with address decoder
- Combinational ROM in additional Verilog/FPGA logic)
Clock Source: Any clock generator capable of 1 MHz (can vary up to a few MHz with timing analysis)
LEDs (optional): 4 LEDs connected to uo[3:0] to visualize the accumulator. Additional LEDs can monitor uo[4:6] for carry, zero, and halted flags.
Input Port (optional): 4-bit input or button circuit connected to uio[7:4] for the IN instruction.

#	Input	Output	Bidirectional
0	instruction bit 0	accumulator bit 0	program counter bit 0 (output)
1	instruction bit 1	accumulator bit 1	program counter bit 1 (output)
2	instruction bit 2	accumulator bit 2	program counter bit 2 (output)
3	instruction bit 3	accumulator bit 3	program counter bit 3 (output)
4	instruction bit 4 (opcode LSB)	carry flag	input port bit 0 (input)
5	instruction bit 5	zero flag	input port bit 1 (input)
6	instruction bit 6	halted	input port bit 2 (input)
7	instruction bit 7 (opcode MSB)	phase (0 = fetch, 1 = execute)	input port bit 3 (input)

993 Nibble - 4-bit CPU