2 layers101 × 68 mm
Led panel 1
tmarhguy/alu
8-bit combinational ALU built from scratch with 3,488 CMOS transistors (KiCad + SPICE + Logisim + 1.24M tests
131 parts48MIT
2 layers110 × 117 mm
Add sub
tmarhguy/alu488-bit combinational ALU built from scratch with 3,488 CMOS transistors (KiCad + SPICE + Logisim + 1.24M tests
- Size
- 110 × 117 mm
- Copper layers
- 2
- Thickness
- 1.6 mm
- Parts
- 200
- Nets
- 234
- Pads
- 796
- Tracks, vias
- 0, 0
- Routed
- Not yet
- KiCad
- KiCad 9.0 (file 20241229)
- Created
- 21 Jul 2025
- Updated
- 5 Jun 2026
Overview
Add sub is an open-source printed circuit board by tmarhguy, from the tmarhguy/alu repository on GitHub, published under the MIT licence. It is a 2-layer board of 110 × 117 mm with 200 parts on 234 nets.
It is built around the 74HC157D and 74HC86D. Its parts include 178 transistors, 12 ICs, 6 capacitors and 4 connectors. It belongs with the retro computing and games boards of this library.
Main components
2
74HC157D
SO-16-40℃~+125℃ 10ns@6.0V,50pF 160uA 2V~6V 5.2mA 5.2mA 74HC 8/4 Multiplexer Single Supply SOIC-16 Signal Switches, Multiplexers, Decoders ROHSNexperiaSignal Switches / Encoders & Decoders / Multiplexers
74HC86D
SO-14-40℃~+125℃ 1.5V~4.2V 20ns@6V,50pF 2V~6V 2uA 4 5.2mA 5.2mA 500mV~1.8V 74HC Series XOR Gate SOIC-14 Logic Gates ROHSNexperiaLogic Gates
README
schematics/kicad/boards/flags/README.mdFlags Generation Board
Comparison and status flag generation
Function
Generate 4 flags for ALU status and comparison:
- LESS: A < B (magnitude comparison)
- EQUAL: A == B (equality check)
- POSITIVE: OUT > 0 (positive result)
- COUT: Carry out (from adder)
Schematic
 Flags generation circuit
Flag Implementations
EQUAL Flag (~100T)
Function: Detect A == B
Logic:
EQUAL = ~((A[7] XOR B[7]) | (A[6] XOR B[6]) | ... | (A[0] XOR B[0]))Implementation:
- 8× XOR gates (96T)
- 8-input NOR gate (16T)
LESS Flag (~120T)
Function: Detect A < B
Logic: Cascaded priority comparison (MSB first)
Implementation: ~120T comparator logic
POSITIVE Flag (~20T)
Function: Detect OUT > 0
Logic:
NOT_ZERO = OUT[7] | ... | OUT[0]
POSITIVE = ~OUT[7] & NOT_ZEROCOUT Flag (0T)
Function: Pass carry from adder
Implementation: Direct wire
Signals
Inputs:
- A[7:0], B[7:0] - For comparison
- OUT[7:0] - For POSITIVE detection
- COUT_IN - From adder
Outputs:
- LESS, EQUAL, POSITIVE, COUT
Last Updated: 2026-01-16
Parts
200| Qty | Value | Footprint | References |
|---|---|---|---|
| 89 | NMOS | Package_TO_SOT_SMD:TSOT-23 | Q301, Q304, Q1002, Q1502, Q1702, Q1901, Q1904, Q1906, Q2002, Q2302, Q2402, Q2502, Q2802, Q3001, Q3004, Q3006 +73 |
| 89 | PMOS | Package_TO_SOT_SMD:TSOT-23 | Q302, Q303, Q1001, Q1501, Q1701, Q1902, Q1903, Q1905, Q2001, Q2301, Q2401, Q2501, Q2801, Q3002, Q3003, Q3005 +73 |
| 6 | 100nF | Capacitor_THT:C_Disc_D5.0mm_W2.5mm_P5.00mm | C201, C701, C702, C1101, C2201, C3301 |
| 6 | 74HC157 | Package_SO:SOIC-16_3.9x9.9mm_P1.27mm | U801, U802, U803, U804, U805, U806 |
| 4 | 74HC86 | Package_SO:SOIC-14_3.9x8.7mm_P1.27mm | U201, U1101, U2201, U3301 |
| 3 | Conn_01x12_Pin | Connector_Harwin:Harwin_Gecko-G125-FVX1205L0X_2x06_P1.25mm_Vertical | J101, J103, J104 |
| 2 | 74HC86 | Package_DIP:DIP-14_W7.62mm | U701, U702 |
| 1 | Conn_01x34_Pin | Connector_Harwin:Harwin_Gecko-G125-FVX3405L0X_2x17_P1.25mm_Vertical | J102 |
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Questions
How big is the Add sub PCB?
It measures 110 × 117 mm and has 2 copper layers, 1.6 mm thick.
What parts does the Add sub use?
200 parts, built around the 74HC157D and 74HC86D. The full list of its parts is on this page, with links to each part's symbol and footprint where PCBKit's library has them.
Where can I download the Add sub KiCad files?
From the tmarhguy/alu repository on GitHub, at the commit this page was made from: the board is schematics/kicad/boards/add_sub/add_sub.kicad_pcb. PCBKit opens the board in the browser, with nothing to install.
Can I use the Add sub design in my own project?
Its repository publishes it under the MIT licence: you may reuse it on that licence's terms (keep the author's notice, and for a copyleft licence share your changes alike).