# Rotary Encoder — Setpoint Control Wiring

Adds an EC11-class incremental rotary encoder to the kttl controller so the
temperature **setpoint** can be dialled in live, and so the encoder's
**push-switch** acts as the on-switch (identical to the board's B1 button).

Target board: **STM32F4-Discovery (STM32F411VE)**. Firmware: `src/main.rs`
(`SetpointKnob`, TIM1 in encoder mode).

---

## 1. Pin assignment

| Encoder leg     | Discovery pin    | Function                  | Extra parts                 |
|-----------------|------------------|---------------------------|-----------------------------|
| **A** (CLK)     | **PE9**  (TIM1_CH1) | quadrature channel A   | 10 kΩ → 3V, 100 nF → GND    |
| **B** (DT)      | **PE11** (TIM1_CH2) | quadrature channel B   | 10 kΩ → 3V, 100 nF → GND    |
| **C** (common)  | **GND**          | rotary common             | —                           |
| **SW1**         | **3V**           | push-switch leg           | —                           |
| **SW2**         | **PA0**          | push-switch leg (‖ B1)    | —                           |
| mount ×2        | **GND**          | shield / mechanical       | optional                    |

> **Use 3V, not 5V.** The STM32F411 is a 3.3 V part; the pull-ups set the logic
> high level seen by the pins, and the switch drives PA0 directly. 3V matches
> the MCU and the existing B1 button exactly — no level shifting.

---

## 2. Schematic

```
   STM32F4-Discovery                         Rotary encoder (EC11, 7-leg)
   ┌──────────────┐
   │           3V ●───────────┬───────────┬──────────────┐
   │              │           │            │              │
   │              │          ┌┴┐          ┌┴┐             │
   │              │     10k  │ │     10k  │ │             │   (pull-ups
   │              │          └┬┘          └┬┘             │    to 3V)
   │              │           │            │              │
   │ PE9  (CH1) ●─┼───────────┼────────────●  A (CLK)     │
   │              │           │            │              │
   │              │      100n═╪═      100n═╪═             │
   │              │           │            │              │
   │ PE11 (CH2) ●─┼───────────●────────────┼──● B (DT)    │
   │              │                        │              │
   │              │                        ●  C (common)──┼──┐
   │              │                                       │  │
   │              │                  SW1 ●────────────────┘  │
   │          PA0 ●───────────────────┐  │   (push-switch)   │
   │              │                    │  ●  SW2 ─────────────┼──┐
   │          GND ●──────┬─────────────┼─────────────────────┘  │
   │              │      │             │   mount legs ●●─────────┘
   └──────────────┘      │             │              (to GND, shield)
                         │             │
                  ┌──────┴──────┐      │  B1 (on-board) also sits
                  │ both 100n   │      │  3V ↔ PA0 — encoder SW is
                  │ caps → GND  │      └─ in PARALLEL with it.
                  └─────────────┘
```

---

## 3. Breadboard view

```
        3V rail  ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━●  ← jumper from Discovery 3V
        GND rail ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━●  ← jumper from Discovery GND
        ┌──────────────────────────────────────────────────┐
        │  [10k]   [10k]          encoder straddles trench   │
        │   │ │     │ │         ┌──────────────────────┐    │
   3V ──┴─┘ └──┬──┘ └──┬──────┤ A    C    B │ SW1  SW2 ├─┐  │
        │      │       │       └──┼────┼────┼───┼────┼──┘ │  │
        │   [100n]  [100n]        │   GND   │  3V   PA0   │  │
        │     │       │           │         │             │  │
   GND ─┴─────┴───────┴───────────┘         │             │  │
        │                                   │             │  │
        │   A → PE9      B → PE11      SW2 → PA0          │  │
        └──────────────────────────────────────────────────┘
            (jumper)     (jumper)      (jumper, ‖ B1)
```

Power both breadboard rails from the Discovery's **3V** and **GND** pins; tap
everything else off those rails.

---

## 4. Why the resistors and caps

- **10 kΩ pull-ups (A→3V, B→3V):** *required.* This HAL's QEI driver puts the
  pins in alternate-function mode without enabling the internal pull-ups, so the
  channels would float. With the encoder common tied to GND, the lines idle high
  and each detent pulls them low.
- **100 nF caps (A→GND, B→GND):** *recommended.* Each forms an RC low-pass with
  its 10 kΩ pull-up, τ = R·C = 10 kΩ · 100 nF = **1 ms** — long enough to absorb
  the ~1–5 ms of mechanical contact bounce, short enough that no hand-turned knob
  outruns it. Combined with the STM32 inputs' Schmitt-trigger hysteresis, this
  turns each bounced click into one clean edge. The cap's closure surge also
  provides wetting current to keep the low-current (~0.3 mA) contacts reliable.

---

## 5. Behaviour (firmware)

- **TIM1 encoder mode** counts quadrature edges (×4). EC11s emit one full cycle
  per detent → **4 counter steps per detent** (`COUNTS_PER_DETENT`).
- Each detent moves the setpoint by **`SETPOINT_STEP_C` (1 °C)**, clamped to
  **`[SETPOINT_MIN_C, SETPOINT_MAX_C]` = [25, 100] °C**. Power-on default is
  **`SETPOINT_DEFAULT_C` = 25 °C**.
- The encoder is sampled every `POLL_MS` (20 ms) in the main poll loops (normal
  and fault), so the setpoint tracks smoothly and the per-second status log
  prints the live value.
- **Push-switch:** wired in parallel with B1 (3V ↔ PA0). Pressing the knob is
  electrically identical to the board button and is handled by the existing PA0
  polling — no extra code.

---

## 6. Alternatives

- **Drop the caps:** enable TIM1's built-in digital input filter (sample-count
  debounce) in firmware instead. The RC filter is simpler and also gives wetting
  current + on-wire noise rejection, so it's preferred for a breadboard build.
- **KY-040 module:** has onboard pull-ups — power it from **3.3 V** (not 5 V) so
  CLK/DT/SW idle at a level the MCU pins accept; the external 10 kΩ/100 nF here
  can then be omitted.