Not Enough Pins? Matrix Expansion

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Not Enough Pins for Your Keyboard Matrix?

SPI Shift Registers 74HC595 · 74HC165 · 74HC589 Matrix Remapping (5×13 → 8×9) QMK & ZMK

So you decided to wire a custom mechanical keyboard, but your favorite compact microcontroller doesn't expose enough GPIO pins for the full switch matrix. This is the exact challenge I hit when moving the 65-key m65 onto the tiny 11-pin Seeed Studio XIAO RP2040.

Common Pin-Reduction Approaches

Before settling on SPI shift registers, I surveyed the existing approaches used in the community:

  • Duplex Matrix: Folds long rows/columns by wiring two switches per row/column intersection with opposing diodes. Effective, but makes schematic reading and routing more mentally taxing.
  • Direct-Pin Shift Registers: Used in boards like tzarc's Ghoul with chained 74HC589 shift registers (one bit per switch, no matrix). Great scan speed, but requires many ICs for 65+ keys.
  • I²C IO Expanders & Demuxers: Popular on split boards (e.g., Ferris IO Expander guide) or 74HC138 3-to-8 decoders in QMK.
  • SPI Shift-Register Matrix Scanning: Using inexpensive 74HC595 (serial-in, parallel-out) and 74HC165 / 74HC589 (parallel-in, serial-out) shift registers to drive columns and/or read rows over hardware SPI.

Official QMK Support & Acknowledgments

When I started experimenting with these circuits, QMK had very few public examples. Since QMK `0.20.0`, boards like Keychron and `hazel/bad_wings` use `74HC595`, and `handwired/dqz11n1g` uses `74HC165`. Huge thanks to **sigprof** and **tzarc** for their guidance on SPI timing and tri-state buffering.

Disclaimer

While the schematics and firmware below work reliably together to scan a 13×5 / 14×5 matrix at 4,000–6,000+ Hz, remember I am a hardware enthusiast rather than an electrical engineer — use and adapt them at your own risk!

Method 1: Scan Columns via 74HC595 + Direct GPIO Rows

Uses 2× 74HC595 output shift registers chained over SPI (MOSI, SCK, RCLK) to drive up to 16 columns using only 3 MCU pins, while the 5 rows are read directly on 5 MCU GPIO pins (total: 8 pins for a 5×16 matrix).

Matrix Version 1 — 2x 74HC595 column scanning

Method 2a: Scan Columns (74HC595) & Rows (74HC165) — Unbuffered

Uses 2× 74HC595 (columns) and 1× 74HC165 (rows). Both ICs are inexpensive and widely available in through-hole (DIP-16) and SMD (SOIC-16) packages. Reads the entire 5×14 matrix using only 4 MCU pins (MOSI, MISO, SCK, shared Latch).

Limitation: Unbuffered MISO

Because the `74HC165` serial output (`Q7`) does not have a true high-impedance (3-state) output when deselected, **you cannot easily share the `MISO` pin with other SPI peripherals** without adding external tri-state buffer circuitry.

Matrix Version 2a — 2x 74HC595 + 1x 74HC165

Method 2b: Scan Columns (74HC595) & Rows (74HC589) — Buffered 3-State SPI

Replaces the 74HC165 with a 1× 74HC589 (mostly SMD today). The 74HC589 includes an internal 3-state output buffer, allowing the MISO line to be cleanly shared with other SPI devices (such as SPI e-ink or Sharp Memory displays).

Variant 1 (Single Shared Latch — Deprecated)

Do Not Use Variant 1 for New Designs

While this single-latch circuit (`m65 Rev 10` / `m66 Rev 2`) works with carefully tuned firmware timing, it relies on edge-case latch/output-enable behavior. Use **Variant 2** below instead.

Matrix Version 2b Variant 1

Variant 2 (Separate Chip Selects — Recommended)

Uses separate latch / chip-select lines for the 2× 74HC595 column driver and the 1× 74HC589 row reader (costing 1 extra pin compared to Variant 1, for 5 SPI pins total), making SPI bus sharing and matrix scanning clean and deterministic.

Matrix Version 2b Variant 2

Hybrid Scanning

All three shift-register methods above can be combined with direct GPIO pins in custom `matrix.c` routines if you want to mix shift-register columns with extra direct keys.

Method 3: Mathematical Matrix Restructuring (Zero Extra Hardware)

If you are only short by 1 or 2 GPIO pins, you don't need any shift registers at all — simply decouple the electrical matrix dimensions from the physical key layout so the row and column counts are closer to a square (Rows ≈ Cols maximizes Rows × Cols for a fixed pin budget Rows + Cols).

  • Example 1: A physical 6×17 board (68 keys) normally requires 6 + 17 = 23 pins. Electrically wiring it as a 10×7 or 9×8 matrix reduces the pin count to 17 pins — saving 6 pins!
  • Example 2 (m65 5×13 Ortho):
    • Standard 5×13 Matrix: Uses 5 + 13 = 18 pins (65 positions).
    • Restructured 8×9 Matrix: Uses 8 + 9 = 17 pins (72 positions, saving 1 pin with 7 spare switch slots!).

Both QMK (via data-driven info.json "matrix": [row, col] coordinates) and ZMK (via zmk,matrix-transform) make this remapping trivial:

Standard 5×13 Matrix (18 Pins)

1. info.json Pin Definition (5 + 13 = 18 pins):

"matrix_pins": {
    "cols": ["GP1", "GP6", "GP7", "GP8", "GP9", "GP15", "GP14", "GP13", "GP12", "GP11", "GP10", "GP17", "GP21"],
    "rows": ["GP22", "GP16", "GP18", "GP19", "GP20"]
}

2. Physical-to-Electrical Mapping (1:1 Grid):

┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│0A││0B││0C││0D││0E││0F││0G││0H││0I││0J││0K││0L││0M│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│1A││1B││1C││1D││1E││1F││1G││1H││1I││1J││1K││1L││1M│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│2A││2B││2C││2D││2E││2F││2G││2H││2I││2J││2K││2L││2M│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│3A││3B││3C││3D││3E││3F││3G││3H││3I││3J││3K││3L││3M│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│4A││4B││4C││4D││4E││4F││4G││4H││4I││4J││4K││4L││4M│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘

3. Full LAYOUT_ortho_5x13 Definition:

Expand 5×13 info.json Layout Definition 

```json
"LAYOUT_ortho_5x13": {
    "layout": [
        { "label": "⎋" , "matrix": [0, 0] , "w": 1, "x": 0 , "y": 0 },
        { "label": "1" , "matrix": [0, 1] , "w": 1, "x": 1 , "y": 0 },
        { "label": "2" , "matrix": [0, 2] , "w": 1, "x": 2 , "y": 0 },
        { "label": "3" , "matrix": [0, 3] , "w": 1, "x": 3 , "y": 0 },
        { "label": "4" , "matrix": [0, 4] , "w": 1, "x": 4 , "y": 0 },
        { "label": "5" , "matrix": [0, 5] , "w": 1, "x": 5 , "y": 0 },
        { "label": "6" , "matrix": [0, 6] , "w": 1, "x": 6 , "y": 0 },
        { "label": "7" , "matrix": [0, 7] , "w": 1, "x": 7 , "y": 0 },
        { "label": "8" , "matrix": [0, 8] , "w": 1, "x": 8 , "y": 0 },
        { "label": "9" , "matrix": [0, 9] , "w": 1, "x": 9 , "y": 0 },
        { "label": "0" , "matrix": [0, 10], "w": 1, "x": 10, "y": 0 },
        { "label": "-" , "matrix": [0, 11], "w": 1, "x": 11, "y": 0 },
        { "label": "⌫" , "matrix": [0, 12], "w": 1, "x": 12, "y": 0 },
        { "label": "↹", "matrix": [1, 0] , "w": 1, "x": 0 , "y": 1 },
        { "label": "q" , "matrix": [1, 1] , "w": 1, "x": 1 , "y": 1 },
        { "label": "w" , "matrix": [1, 2] , "w": 1, "x": 2 , "y": 1 },
        { "label": "e" , "matrix": [1, 3] , "w": 1, "x": 3 , "y": 1 },
        { "label": "r" , "matrix": [1, 4] , "w": 1, "x": 4 , "y": 1 },
        { "label": "t" , "matrix": [1, 5] , "w": 1, "x": 5 , "y": 1 },
        { "label": "y" , "matrix": [1, 6] , "w": 1, "x": 6 , "y": 1 },
        { "label": "u" , "matrix": [1, 7] , "w": 1, "x": 7 , "y": 1 },
        { "label": "i" , "matrix": [1, 8] , "w": 1, "x": 8 , "y": 1 },
        { "label": "o" , "matrix": [1, 9] , "w": 1, "x": 9 , "y": 1 },
        { "label": "p" , "matrix": [1, 10], "w": 1, "x": 10, "y": 1 },
        { "label": "[" , "matrix": [1, 11], "w": 1, "x": 11, "y": 1 },
        { "label": "]" , "matrix": [1, 12], "w": 1, "x": 12, "y": 1 },
        { "label": "#" , "matrix": [2, 0] , "w": 1, "x": 0 , "y": 2 },
        { "label": "a" , "matrix": [2, 1] , "w": 1, "x": 1 , "y": 2 },
        { "label": "s" , "matrix": [2, 2] , "w": 1, "x": 2 , "y": 2 },
        { "label": "d" , "matrix": [2, 3] , "w": 1, "x": 3 , "y": 2 },
        { "label": "f" , "matrix": [2, 4] , "w": 1, "x": 4 , "y": 2 },
        { "label": "g" , "matrix": [2, 5] , "w": 1, "x": 5 , "y": 2 },
        { "label": "h" , "matrix": [2, 6] , "w": 1, "x": 6 , "y": 2 },
        { "label": "j" , "matrix": [2, 7] , "w": 1, "x": 7 , "y": 2 },
        { "label": "k" , "matrix": [2, 8] , "w": 1, "x": 8 , "y": 2 },
        { "label": "l" , "matrix": [2, 9] , "w": 1, "x": 9 , "y": 2 },
        { "label": ";" , "matrix": [2, 10], "w": 1, "x": 10, "y": 2 },
        { "label": "'" , "matrix": [2, 11], "w": 1, "x": 11, "y": 2 },
        { "label": "⏎" , "matrix": [2, 12], "w": 1, "x": 12, "y": 2 },
        { "label": "⇧" , "matrix": [3, 0] , "w": 1, "x": 0 , "y": 3 },
        { "label": "\\", "matrix": [3, 1] , "w": 1, "x": 1 , "y": 3 },
        { "label": "z" , "matrix": [3, 2] , "w": 1, "x": 2 , "y": 3 },
        { "label": "x" , "matrix": [3, 3] , "w": 1, "x": 3 , "y": 3 },
        { "label": "c" , "matrix": [3, 4] , "w": 1, "x": 4 , "y": 3 },
        { "label": "v" , "matrix": [3, 5] , "w": 1, "x": 5 , "y": 3 },
        { "label": "b" , "matrix": [3, 6] , "w": 1, "x": 6 , "y": 3 },
        { "label": "n" , "matrix": [3, 7] , "w": 1, "x": 7 , "y": 3 },
        { "label": "m" , "matrix": [3, 8] , "w": 1, "x": 8 , "y": 3 },
        { "label": "," , "matrix": [3, 9] , "w": 1, "x": 9 , "y": 3 },
        { "label": "." , "matrix": [3, 10], "w": 1, "x": 10, "y": 3 },
        { "label": "↑" , "matrix": [3, 11], "w": 1, "x": 11, "y": 3 },
        { "label": "/" , "matrix": [3, 12], "w": 1, "x": 12, "y": 3 },
        { "label": "⎈" , "matrix": [4, 0] , "w": 1, "x": 0 , "y": 4 },
        { "label": "" , "matrix": [4, 1] , "w": 1, "x": 1 , "y": 4 },
        { "label": "⇓" , "matrix": [4, 2] , "w": 1, "x": 2 , "y": 4 },
        { "label": "⎇" , "matrix": [4, 3] , "w": 1, "x": 3 , "y": 4 },
        { "label": "⇑" , "matrix": [4, 4] , "w": 1, "x": 4 , "y": 4 },
        { "label": " " , "matrix": [4, 5] , "w": 1, "x": 5 , "y": 4 },
        { "label": " " , "matrix": [4, 6] , "w": 1, "x": 6 , "y": 4 },
        { "label": " " , "matrix": [4, 7] , "w": 1, "x": 7 , "y": 4 },
        { "label": "⎇" , "matrix": [4, 8] , "w": 1, "x": 8 , "y": 4 },
        { "label": "⇧" , "matrix": [4, 9] , "w": 1, "x": 9 , "y": 4 },
        { "label": "←", "matrix": [4, 10], "w": 1, "x": 10, "y": 4 },
        { "label": "↓" , "matrix": [4, 11], "w": 1, "x": 11, "y": 4 },
        { "label": "→" , "matrix": [4, 12], "w": 1, "x": 12, "y": 4 }
    ]
}
```
Restructured 8×9 Matrix (17 Pins — Saves 1 Pin)

1. info.json Pin Definition (8 + 9 = 17 pins):

"matrix_pins": {
    "cols": ["GP1", "GP6", "GP7", "GP8", "GP9", "GP15", "GP14", "GP13", "GP12"],
    "rows": ["GP11", "GP10", "GP17", "GP21", "GP16", "GP18", "GP19", "GP20"]
}

2. Physical-to-Electrical Mapping (8 Rows 0–7 × 9 Cols A–I):

Wire all keys sharing the same digit (0–7) to the corresponding row pin, and all keys sharing the same letter (A–I) to the corresponding column pin:

┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│0A││0B││0C││0D││0E││0F││0G││0H││0I││1A││1B││1C││1D│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│1E││1F││1G││1H││1I││2A││2B││2C││2D││2E││2F││2G││2H│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│2I││3A││3B││3C││3D││3E││3F││3G││3H││3I││4A││4B││4C│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│4D││4E││4F││4G││4H││4I││5A││5B││5C││5D││5E││5F││5G│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘
┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐┌──┐
│5H││5I││6A││6B││6C││6D││6E││6F││6G││6H││6I││7A││7B│
└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘└──┘

3. Full Remapped LAYOUT_ortho_5x13 Definition:

Expand Remapped 8×9 info.json Layout Definition 

```json
"LAYOUT_ortho_5x13": {
    "layout": [
        { "label": "⎋" , "matrix": [0, 0] , "w": 1, "x": 0 , "y": 0 },
        { "label": "1" , "matrix": [0, 1] , "w": 1, "x": 1 , "y": 0 },
        { "label": "2" , "matrix": [0, 2] , "w": 1, "x": 2 , "y": 0 },
        { "label": "3" , "matrix": [0, 3] , "w": 1, "x": 3 , "y": 0 },
        { "label": "4" , "matrix": [0, 4] , "w": 1, "x": 4 , "y": 0 },
        { "label": "5" , "matrix": [0, 5] , "w": 1, "x": 5 , "y": 0 },
        { "label": "6" , "matrix": [0, 6] , "w": 1, "x": 6 , "y": 0 },
        { "label": "7" , "matrix": [0, 7] , "w": 1, "x": 7 , "y": 0 },
        { "label": "8" , "matrix": [0, 8] , "w": 1, "x": 8 , "y": 0 },
        { "label": "9" , "matrix": [1, 0] , "w": 1, "x": 9 , "y": 0 },
        { "label": "0" , "matrix": [1, 1], "w": 1, "x": 10, "y": 0 },
        { "label": "-" , "matrix": [1, 2], "w": 1, "x": 11, "y": 0 },
        { "label": "⌫" , "matrix": [1, 3], "w": 1, "x": 12, "y": 0 },
        { "label": "↹", "matrix": [1, 4] , "w": 1, "x": 0 , "y": 1 },
        { "label": "q" , "matrix": [1, 5] , "w": 1, "x": 1 , "y": 1 },
        { "label": "w" , "matrix": [1, 6] , "w": 1, "x": 2 , "y": 1 },
        { "label": "e" , "matrix": [1, 7] , "w": 1, "x": 3 , "y": 1 },
        { "label": "r" , "matrix": [1, 8] , "w": 1, "x": 4 , "y": 1 },
        { "label": "t" , "matrix": [2, 0] , "w": 1, "x": 5 , "y": 1 },
        { "label": "y" , "matrix": [2, 1] , "w": 1, "x": 6 , "y": 1 },
        { "label": "u" , "matrix": [2, 2] , "w": 1, "x": 7 , "y": 1 },
        { "label": "i" , "matrix": [2, 3] , "w": 1, "x": 8 , "y": 1 },
        { "label": "o" , "matrix": [2, 4] , "w": 1, "x": 9 , "y": 1 },
        { "label": "p" , "matrix": [2, 5], "w": 1, "x": 10, "y": 1 },
        { "label": "[" , "matrix": [2, 6], "w": 1, "x": 11, "y": 1 },
        { "label": "]" , "matrix": [2, 7], "w": 1, "x": 12, "y": 1 },
        { "label": "#" , "matrix": [2, 8] , "w": 1, "x": 0 , "y": 2 },
        { "label": "a" , "matrix": [3, 0] , "w": 1, "x": 1 , "y": 2 },
        { "label": "s" , "matrix": [3, 1] , "w": 1, "x": 2 , "y": 2 },
        { "label": "d" , "matrix": [3, 2] , "w": 1, "x": 3 , "y": 2 },
        { "label": "f" , "matrix": [3, 3] , "w": 1, "x": 4 , "y": 2 },
        { "label": "g" , "matrix": [3, 4] , "w": 1, "x": 5 , "y": 2 },
        { "label": "h" , "matrix": [3, 5] , "w": 1, "x": 6 , "y": 2 },
        { "label": "j" , "matrix": [3, 6] , "w": 1, "x": 7 , "y": 2 },
        { "label": "k" , "matrix": [3, 7] , "w": 1, "x": 8 , "y": 2 },
        { "label": "l" , "matrix": [3, 8] , "w": 1, "x": 9 , "y": 2 },
        { "label": ";" , "matrix": [4, 0], "w": 1, "x": 10, "y": 2 },
        { "label": "'" , "matrix": [4, 1], "w": 1, "x": 11, "y": 2 },
        { "label": "⏎" , "matrix": [4, 2], "w": 1, "x": 12, "y": 2 },
        { "label": "⇧" , "matrix": [4, 3] , "w": 1, "x": 0 , "y": 3 },
        { "label": "\\", "matrix": [4, 4] , "w": 1, "x": 1 , "y": 3 },
        { "label": "z" , "matrix": [4, 5] , "w": 1, "x": 2 , "y": 3 },
        { "label": "x" , "matrix": [4, 6] , "w": 1, "x": 3 , "y": 3 },
        { "label": "c" , "matrix": [4, 7] , "w": 1, "x": 4 , "y": 3 },
        { "label": "v" , "matrix": [4, 8] , "w": 1, "x": 5 , "y": 3 },
        { "label": "b" , "matrix": [5, 0] , "w": 1, "x": 6 , "y": 3 },
        { "label": "n" , "matrix": [5, 1] , "w": 1, "x": 7 , "y": 3 },
        { "label": "m" , "matrix": [5, 2] , "w": 1, "x": 8 , "y": 3 },
        { "label": "," , "matrix": [5, 3] , "w": 1, "x": 9 , "y": 3 },
        { "label": "." , "matrix": [5, 4], "w": 1, "x": 10, "y": 3 },
        { "label": "↑" , "matrix": [5, 5], "w": 1, "x": 11, "y": 3 },
        { "label": "/" , "matrix": [5, 6], "w": 1, "x": 12, "y": 3 },
        { "label": "⎈" , "matrix": [5, 7] , "w": 1, "x": 0 , "y": 4 },
        { "label": "" , "matrix": [5, 8] , "w": 1, "x": 1 , "y": 4 },
        { "label": "⇓" , "matrix": [6, 0] , "w": 1, "x": 2 , "y": 4 },
        { "label": "⎇" , "matrix": [6, 1] , "w": 1, "x": 3 , "y": 4 },
        { "label": "⇑" , "matrix": [6, 2] , "w": 1, "x": 4 , "y": 4 },
        { "label": " " , "matrix": [6, 3] , "w": 1, "x": 5 , "y": 4 },
        { "label": " " , "matrix": [6, 4] , "w": 1, "x": 6 , "y": 4 },
        { "label": " " , "matrix": [6, 5] , "w": 1, "x": 7 , "y": 4 },
        { "label": "⎇" , "matrix": [6, 6] , "w": 1, "x": 8 , "y": 4 },
        { "label": "⇧" , "matrix": [6, 7] , "w": 1, "x": 9 , "y": 4 },
        { "label": "←", "matrix": [6, 8], "w": 1, "x": 10, "y": 4 },
        { "label": "↓" , "matrix": [7, 0], "w": 1, "x": 11, "y": 4 },
        { "label": "→" , "matrix": [7, 1], "w": 1, "x": 12, "y": 4 }
    ]
}
```
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Exhaustive engineering tutorial on driving large matrices from tiny MCUs via SPI shift registers and matrix folding.

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