rotary
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# Rotary Encoder — Setpoint Control Wiring
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Adds an EC11-class incremental rotary encoder to the kttl controller so the
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temperature **setpoint** can be dialled in live, and so the encoder's
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**push-switch** acts as the on-switch (identical to the board's B1 button).
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Target board: **STM32F4-Discovery (STM32F411VE)**. Firmware: `src/main.rs`
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(`SetpointKnob`, TIM1 in encoder mode).
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---
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## 1. Pin assignment
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| Encoder leg | Discovery pin | Function | Extra parts |
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|-----------------|------------------|---------------------------|-----------------------------|
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| **A** (CLK) | **PE9** (TIM1_CH1) | quadrature channel A | 10 kΩ → 3V, 100 nF → GND |
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| **B** (DT) | **PE11** (TIM1_CH2) | quadrature channel B | 10 kΩ → 3V, 100 nF → GND |
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| **C** (common) | **GND** | rotary common | — |
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| **SW1** | **3V** | push-switch leg | — |
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| **SW2** | **PA0** | push-switch leg (‖ B1) | — |
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| mount ×2 | **GND** | shield / mechanical | optional |
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> **Use 3V, not 5V.** The STM32F411 is a 3.3 V part; the pull-ups set the logic
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> high level seen by the pins, and the switch drives PA0 directly. 3V matches
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> the MCU and the existing B1 button exactly — no level shifting.
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---
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## 2. Schematic
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```
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STM32F4-Discovery Rotary encoder (EC11, 7-leg)
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┌──────────────┐
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│ 3V ●───────────┬───────────┬──────────────┐
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│ │ │ │ │
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│ │ ┌┴┐ ┌┴┐ │
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│ │ 10k │ │ 10k │ │ │ (pull-ups
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│ │ └┬┘ └┬┘ │ to 3V)
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│ │ │ │ │
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│ PE9 (CH1) ●─┼───────────┼────────────● A (CLK) │
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│ │ │ │ │
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│ │ 100n═╪═ 100n═╪═ │
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│ │ │ │ │
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│ PE11 (CH2) ●─┼───────────●────────────┼──● B (DT) │
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│ │ │ │
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│ │ ● C (common)──┼──┐
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│ │ │ │
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│ │ SW1 ●────────────────┘ │
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│ PA0 ●───────────────────┐ │ (push-switch) │
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│ │ │ ● SW2 ─────────────┼──┐
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│ GND ●──────┬─────────────┼─────────────────────┘ │
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│ │ │ │ mount legs ●●─────────┘
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└──────────────┘ │ │ (to GND, shield)
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│ │
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┌──────┴──────┐ │ B1 (on-board) also sits
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│ both 100n │ │ 3V ↔ PA0 — encoder SW is
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│ caps → GND │ └─ in PARALLEL with it.
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└─────────────┘
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```
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---
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## 3. Breadboard view
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```
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3V rail ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━● ← jumper from Discovery 3V
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GND rail ●━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━● ← jumper from Discovery GND
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┌──────────────────────────────────────────────────┐
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│ [10k] [10k] encoder straddles trench │
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│ │ │ │ │ ┌──────────────────────┐ │
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3V ──┴─┘ └──┬──┘ └──┬──────┤ A C B │ SW1 SW2 ├─┐ │
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│ │ │ └──┼────┼────┼───┼────┼──┘ │ │
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│ [100n] [100n] │ GND │ 3V PA0 │ │
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│ │ │ │ │ │ │
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GND ─┴─────┴───────┴───────────┘ │ │ │
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│ │ │ │
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│ A → PE9 B → PE11 SW2 → PA0 │ │
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└──────────────────────────────────────────────────┘
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(jumper) (jumper) (jumper, ‖ B1)
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```
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Power both breadboard rails from the Discovery's **3V** and **GND** pins; tap
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everything else off those rails.
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---
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## 4. Why the resistors and caps
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- **10 kΩ pull-ups (A→3V, B→3V):** *required.* This HAL's QEI driver puts the
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pins in alternate-function mode without enabling the internal pull-ups, so the
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channels would float. With the encoder common tied to GND, the lines idle high
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and each detent pulls them low.
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- **100 nF caps (A→GND, B→GND):** *recommended.* Each forms an RC low-pass with
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its 10 kΩ pull-up, τ = R·C = 10 kΩ · 100 nF = **1 ms** — long enough to absorb
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the ~1–5 ms of mechanical contact bounce, short enough that no hand-turned knob
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outruns it. Combined with the STM32 inputs' Schmitt-trigger hysteresis, this
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turns each bounced click into one clean edge. The cap's closure surge also
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provides wetting current to keep the low-current (~0.3 mA) contacts reliable.
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---
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## 5. Behaviour (firmware)
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- **TIM1 encoder mode** counts quadrature edges (×4). EC11s emit one full cycle
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per detent → **4 counter steps per detent** (`COUNTS_PER_DETENT`).
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- Each detent moves the setpoint by **`SETPOINT_STEP_C` (1 °C)**, clamped to
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**`[SETPOINT_MIN_C, SETPOINT_MAX_C]` = [25, 100] °C**. Power-on default is
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**`SETPOINT_DEFAULT_C` = 25 °C**.
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- The encoder is sampled every `POLL_MS` (20 ms) in the main poll loops (normal
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and fault), so the setpoint tracks smoothly and the per-second status log
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prints the live value.
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- **Push-switch:** wired in parallel with B1 (3V ↔ PA0). Pressing the knob is
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electrically identical to the board button and is handled by the existing PA0
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polling — no extra code.
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---
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## 6. Alternatives
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- **Drop the caps:** enable TIM1's built-in digital input filter (sample-count
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debounce) in firmware instead. The RC filter is simpler and also gives wetting
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current + on-wire noise rejection, so it's preferred for a breadboard build.
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- **KY-040 module:** has onboard pull-ups — power it from **3.3 V** (not 5 V) so
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CLK/DT/SW idle at a level the MCU pins accept; the external 10 kΩ/100 nF here
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can then be omitted.
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