8-Bit CPU
About the 8-Bit CPU PCB
8-Bit CPU is an open source PCB design by otterbox901, published on GitHub. It is a 2-layer board measuring 160 × 140 mm, with 65 components from 36 distinct parts.
Other key parts include the AT28C256 (ctrl ROM 1), AT28C256 (ctrl ROM 2), AT28C64B (program ROM), HM6116 (data RAM) and 74HC161 (PC lo). By type, the board carries 27 capacitors, 24 ICs, 7 resistors, 3 connectors, 1 crystal and 1 fuse.
From the project
Microcoded 8-bit CPU built from 24 74HC-series ICs: a custom 10-instruction ISA, 4 general-purpose registers, and an EEPROM-driven control unit.
A microcoded 8-bit CPU built from 24 74HC-series logic and memory ICs: custom instruction set, 4 general-purpose registers, add/subtract ALU with Zero and Carry flags, and a control unit driven entirely by two EEPROMs.
- [x] Instruction set (10 instructions, 1- and 2-byte encodings) - [x] Microcode ROM generator (ROM/microcoderomgen.py) - [x] Emulator + assembler, full test program passing - [x] Schematic: 24 ICs, 0 ERC errors - [x] Schematic verified by simulation: the KiCad netlist runs the test program at the chip-pin level with the real ROM images (kicadfiles/simnetlist.py) - [x] PCB…
Main components on the 8-Bit CPU
8-Bit CPU bill of materials (BOM)
65 components, 36 distinct parts — part numbers from the project's BOM.
| Qty | Part | Footprint | Refs |
|---|---|---|---|
| 1 | AT28C256 (ctrl ROM 1) | DIP-28_W15.24mm_Socket | U1 |
| 1 | AT28C256 (ctrl ROM 2) | DIP-28_W15.24mm_Socket | U2 |
| 1 | AT28C64B (program ROM) | DIP-28_W15.24mm_Socket | U3 |
| 1 | HM6116 (data RAM) | DIP-24_W15.24mm_Socket | U4 |
| 1 | 74HC161 (PC lo) | SOIC-16_3.9x9.9mm_P1.27mm | U5 |
| 1 | 74HC161 (PC hi) | SOIC-16_3.9x9.9mm_P1.27mm | U6 |
| 1 | 74HC161 | SOIC-16_3.9x9.9mm_P1.27mm | U7 |
| 1 | 74HC377 (MAR) | SOIC-20W_7.5x12.8mm_P1.27mm | U8 |
| 1 | 74HC377 (IR) | SOIC-20W_7.5x12.8mm_P1.27mm | U9 |
| 1 | 74HC377 | SOIC-20W_7.5x12.8mm_P1.27mm | U10 |
| 1 | 74HC377 (A) | SOIC-20W_7.5x12.8mm_P1.27mm | U11 |
| 1 | 74HC377 (TEMP) | SOIC-20W_7.5x12.8mm_P1.27mm | U12 |
| 1 | 74HC670 (R0-R3 bits 0-3) | SOIC-16_3.9x9.9mm_P1.27mm | U13 |
| 1 | 74HC670 (R0-R3 bits 4-7) | SOIC-16_3.9x9.9mm_P1.27mm | U14 |
| 1 | 74HC157 | SOIC-16_3.9x9.9mm_P1.27mm | U15 |
| 2 | 74HC86 | SOIC-14_3.9x8.7mm_P1.27mm | U16, U17 |
| 2 | 74HC283 | SOIC-16_3.9x9.9mm_P1.27mm | U18, U19 |
| 1 | 74HC245 (ALU out) | SOIC-20W_7.5x12.8mm_P1.27mm | U20 |
| 1 | 74HC688 (zero detect) | SOIC-20W_7.5x12.8mm_P1.27mm | U21 |
| 1 | 74HC173 (flags) | SOIC-16_3.9x9.9mm_P1.27mm | U22 |
Show 16 more
| Qty | Part | Footprint | Refs |
|---|---|---|---|
| 1 | 74HC00 | SOIC-14_3.9x8.7mm_P1.27mm | U23 |
| 1 | 74HC14 | SOIC-14_3.9x8.7mm_P1.27mm | U24 |
| 1 | 1MHz osc, tri-state OE | Oscillator_SMD_SeikoEpson_SG8002CA-4Pin_7.0x5.0mm | Y1 |
| 1 | USB-C 5V | USB_C_Receptacle_GCT_USB4125-xx-x_6P_TopMnt_Horizontal | J1 |
| 1 | CLK SEL | PinHeader_1x03_P2.54mm_Vertical | J2 |
| 1 | DEBUG | PinHeader_1x10_P2.54mm_Vertical | J3 |
| 1 | 750mA hold | Fuse_1206_3216Metric | F1 |
| 1 | RESET | SW_SPST_TL3342 | SW1 |
| 1 | PWR | LED_0603_1608Metric | D1 |
| 1 | 1UF 1uF | C_0603_1608Metric | C1 |
| 1 | 10UF 10uF | C_0805_2012Metric | C2 |
| 25 | 100NF 100nF | C_0603_1608Metric | C10, C11, C12, C13, C14, C15, C16, C17 +17 |
| 2 | 10K 10k | R_0603_1608Metric | R1, R2 |
| 2 | 5.1K 5.1k | R_0603_1608Metric | R3, R4 |
| 1 | 1k | R_0603_1608Metric | R5 |
| 2 | 10k x4 | R_Array_Convex_4x0603 | RN1, RN2 |
8-Bit CPU design files
The KiCad project lives in the otterbox901/8-Bit_CPU repository on GitHub; these links point at the commit this page was built from.
8-Bit CPU: common questions
How big is the 8-Bit CPU PCB?
The 8-Bit CPU measures 160 × 140 mm, has 2 copper layers and is 1.6 mm thick.
How many components are on the 8-Bit CPU?
65 components, from 36 distinct parts, with part numbers taken from the project's own BOM. The full bill of materials is listed on this page.
Where can I download the 8-Bit CPU design files?
From the otterbox901/8-Bit_CPU repository on GitHub, which has the KiCad layout, the schematic and the BOM; the links under Design files point to each one.
Can I use the 8-Bit CPU design in my own project?
The repository has no license, so all rights stay with otterbox901. Use it as a reference, and ask the author before reusing the design.