# HMC472A Attenuator Controller Wiring

## Connection Diagram

![Wiring Diagram](wiring-diagram.svg)

## Pin Mapping (Optimized for Bitwise Ops)

The wiring is arranged so GPIO number = step bit position + 1, enabling single-instruction bitwise operations in firmware.

| ESP32-S2 Mini | HMC472A Module | Step Bit | Attenuation |
|---------------|----------------|----------|-------------|
| GPIO1         | V6             | Bit 0 (LSB) | 0.5 dB   |
| GPIO2         | V5             | Bit 1       | 1 dB     |
| GPIO3         | V4             | Bit 2       | 2 dB     |
| GPIO4         | V3             | Bit 3       | 4 dB     |
| GPIO5         | V2             | Bit 4       | 8 dB     |
| GPIO6         | V1             | Bit 5 (MSB) | 16 dB    |
| 5V (VBUS)     | +5V            | —          | Power    |
| GND           | GND            | —          | Ground   |

### Why This Mapping?

```c
// Old approach (loop required):
for (int i = 0; i < 6; i++) {
    pin = ATTEN_PINS[5-i];  // Reverse mapping
    if (step & (1 << i)) set_low(pin);
}

// New approach (pure bitwise):
GPIO.out_w1tc = (step & 0x3F) << 1;           // Set LOW where step=1
GPIO.out_w1ts = (~step & 0x3F) << 1 & 0x7E;   // Set HIGH where step=0
```

The step value bits map directly to GPIO register positions. No loop, no lookup table.

## HMC472A Module Header Pinout

The 8-pin header on the HMC472A module (top to bottom when viewing from component side):

| Pin | Signal   | ESP32 GPIO | Description              |
|-----|----------|------------|--------------------------|
| 1   | +5V      | 5V VBUS    | Power supply (+5V DC)    |
| 2   | V6       | GPIO1      | 0.5 dB (step bit 0, LSB) |
| 3   | V5       | GPIO2      | 1 dB (step bit 1)        |
| 4   | V4       | GPIO3      | 2 dB (step bit 2)        |
| 5   | V3       | GPIO4      | 4 dB (step bit 3)        |
| 6   | V2       | GPIO5      | 8 dB (step bit 4)        |
| 7   | V1       | GPIO6      | 16 dB (step bit 5, MSB)  |
| 8   | GND      | GND        | Ground                   |

## Logic Levels

**Active-LOW control:**
- **LOW (0V)** = Attenuate (apply the dB value)
- **HIGH (3.3V or 5V)** = Pass (0 dB contribution)

The HMC472A accepts 0–5V TTL/CMOS logic. The ESP32-S2's 3.3V GPIO output is fully compatible.

## Attenuation Examples

| Step | Binary     | GPIO State (1-6) | Total Attenuation |
|------|------------|------------------|-------------------|
| 0    | `0b000000` | `HHHHHH`         | 0 dB              |
| 1    | `0b000001` | `LHHHHH`         | 0.5 dB            |
| 5    | `0b000101` | `LHLHHH`         | 2.5 dB            |
| 31   | `0b011111` | `LLLLLH`         | 15.5 dB           |
| 63   | `0b111111` | `LLLLLL`         | 31.5 dB           |

Formula: `attenuation = step × 0.5 dB` where step = 0–63

## Wiring Diagram (Text)

```
ESP32-S2 Mini                    HMC472A Module
   (Left Header)                   (8-pin Header)

        3V3 ─┤ 1                   ┌─────┐
   ──────────────────────────────┐ │ +5V │← 1
     GPIO1 ─┤ 2 ───────────────────│ V6  │← 2  (0.5 dB)
     GPIO2 ─┤ 3 ───────────────────│ V5  │← 3  (1 dB)
     GPIO3 ─┤ 4 ───────────────────│ V4  │← 4  (2 dB)
     GPIO4 ─┤ 5 ───────────────────│ V3  │← 5  (4 dB)
     GPIO5 ─┤ 6 ───────────────────│ V2  │← 6  (8 dB)
     GPIO6 ─┤ 7 ───────────────────│ V1  │← 7  (16 dB)
        GND ─┤ 8 ───────────────────│ GND │← 8
                                   └─────┘
   (Right Header)
         5V ─┤ 16 ─────────────────┘ (to +5V)
```

**Note:** The diagram shows straight-through wiring (no crossovers) because the optimized mapping aligns GPIO numbers with the module header in reverse order.

## ESP32-S2 Mini Physical Pinout

```
          ┌─────────────────┐
          │     USB-C       │
          └─────────────────┘
     3V3 ─┤ 1            16 ├─ 5V (VBUS) ──► +5V
  GPIO1 ─┤ 2            15 ├─ GPIO15 (LED)    │
  GPIO2 ─┤ 3            14 ├─ GPIO14          │
  GPIO3 ─┤ 4            13 ├─ GPIO13          │
  GPIO4 ─┤ 5            12 ├─ GPIO12          │
  GPIO5 ─┤ 6            11 ├─ GPIO11          │
  GPIO6 ─┤ 7            10 ├─ GPIO10          │
     GND ─┤ 8             9 ├─ GPIO9          │
          └─────────────────┘                 │
            ↓   ↓   ↓   ↓   ↓   ↓             │
           V6  V5  V4  V3  V2  V1 ◄───────────┘
           (HMC472A control pins + power)
```

GPIOs 1–6 are on the left header, pins 2–7. Direct ribbon cable from S2 Mini left header to HMC472A control header (reversed).

## KiCad Schematic

A full KiCad schematic is available at `hmc472-controller.kicad_sch` for detailed EDA work.

## Notes

1. **No level shifting required** — ESP32-S2 3.3V GPIO drives HMC472A directly
2. **Boot state** — GPIOs default HIGH at boot = 0 dB attenuation (safe)
3. **Glitch-free switching** — Firmware uses register-level writes (`GPIO.out_w1ts`/`GPIO.out_w1tc`)
4. **Power** — The S2 Mini's 5V pin sources USB VBUS directly, sufficient for HMC472A's 2.5 mA draw
5. **Bitwise optimization** — GPIO = bit + 1 mapping eliminates loop in firmware