Lesson 40. EMC/ESD and industrial-input protection: do not hide poor circuit design with software debounce

long cable / external connector
→ energy limiting
→ overvoltage protection
→ filtering
→ galvanic isolation or transceiver
→ clean digital-level conditioning
→ ESP32 or STM32 GPIO/ADC
→ software diagnostics

ESD from a person or connector
EFT from switching an inductive load
surge on an outdoor cable
prolonged connection to 24 V
ground-potential difference

connector
→ primary protection
→ current/energy limiting
→ filter
→ isolation or protected receiver
→ secondary protection/conditioning
→ MCU

FIELD_IN
  ├─ TVS to field return
  ├─ series resistors
  ├─ RC filter
  └─ optocoupler / comparator
         └─ Schmitt output → MCU GPIO

PE / chassis
field ground
isolated bus ground
digital ground
analog ground

VRWM — working voltage
VBR — breakdown voltage
VCL — clamping voltage at the specified current
IPP / pulse power — pulse energy
capacitance — interface impact

τ = R × C
fc = 1 / (2πRC)

R = 10 kOhm
C = 10 nF
τ = 100 us
fc ≈ 1.59 kHz

optocoupler
→ pull-up
→ RC
→ Schmitt buffer
→ GPIO

24V_FIELD
→ fuse / fusible resistor
→ reverse polarity protection
→ TVS
→ resistor chain
→ RC
→ optocoupler LED
→ isolated transistor
→ pull-up 3.3 V
→ Schmitt/GPIO

R ≈ (VIN − VF_LED − Vbridge) / ILED

VPEAK ≈ 230 × √2 ≈ 325 V

connector
→ TVS array
→ optional pulse-proof resistors / PTC
→ optional common-mode choke
→ termination/bias
→ fault-protected transceiver
→ optional isolation
→ MCU

typedef struct {
    uint32_t raw_edge_count;
    uint32_t accepted_edge_count;
    uint32_t rejected_glitch_count;
    uint32_t conflict_count;
    uint32_t impossible_transition_count;
    uint32_t timeout_count;
    uint32_t min_pulse_us;
    uint32_t max_pulse_us;
    uint32_t min_interval_us;
    uint32_t max_interval_us;
    int64_t last_raw_edge_us;
    int64_t last_valid_change_us;
} emc_input_diag_t;

input_state = INPUT_DEGRADED_NOISY;
exti_temporarily_mask();
schedule_polled_recovery();
fault_report(FAULT_INPUT_GLITCH_STORM);

1. Placing TVS near the MCU.
2. Choosing TVS only by VRWM.
3. Connecting all TVS to sensitive digital GND.
4. Treating ESD, EFT, and surge as the same.
5. Protecting prolonged 24 V connection only with TVS.
6. Using internal GPIO clamps as primary protection.
7. An input capacitor that is too large.
8. Filtering damaging voltage in software.
9. Selecting high GPIO speed everywhere.
10. No idle state when the cable is disconnected.
11. Ignoring RS-485/CAN ground differences.
12. Testing interference only with a software generator.

# EMC / ESD policy
1. Every external connector has a documented threat model.
2. Protection components are placed close to the connector.
3. Transient current return paths do not cross sensitive logic.
4. GPIO internal clamps are not system-level protection.
5. Every input has a defined idle state.
6. Hardware filtering is calculated from useful pulse timing.
7. Software debounce does not replace overvoltage protection.
8. Field and logic grounds are explicitly documented.
9. Every communication interface has an overrun/recovery policy.
10. EMC tests include both hardware survival and functional behavior.

| Interface | Normal signal | Possible fault | Hardware reaction | Firmware reaction |
|---|---|---|---|---|
| RED input | 220 V AC via opto | EFT/glitch | RC + opto + Schmitt | reject short pulse |
| 24 V input | 0/24 V | reverse/36 V | series R + TVS | mark channel fault |
| RS-485 | differential | ESD/surge/DC fault | TVS/fault transceiver | framing/error counters |
| CAN | differential | common-mode pulse | TVS/CMC/isolation | bus-off recovery |
| EC25 UART | logic | ringing | series R, short traces | UART error counters |

emc diag:
RED:
raw=12540
valid=11980
rejected=560
glitch_storms=1
min_pulse=42us
max_pulse=10220us
RS485:
frame_errors=2
crc_errors=4
collisions=0
recovery=1

stable level;
1 ms pulses;
1-100 us glitches;
a burst of fast pulses;
bouncing near an edge;
a missing expected pulse.

external cable
→ connector protection
→ controlled transient return
→ current limiting
→ filtering
→ isolation/protected transceiver
→ Schmitt/digital receiver
→ MCU input
→ timestamp/filter/diagnostics

1. Identify the disturbance type before selecting protection.
2. Place TVS near the connector.
3. The transient current path matters as much as the TVS rating.
4. ESD, EFT, surge, and DC fault require different measures.
5. Internal GPIO clamps are not main external protection.
6. Choose RC from useful-signal and disturbance timing.
7. Schmitt trigger prevents repeated switching on a slow edge.
8. A direct mains input requires insulation and safety design.
9. RS-485/CAN protection includes common-mode and ground strategy.
10. Low GPIO speed and series resistors reduce emissions.
11. Firmware should count glitches and errors, not merely hide them.
12. Real EMC resilience is shown by circuit testing, not debounce alone.

31. Time architecture: monotonic, UTC, NTP, PPS, drift.
32. UART/RS-485 and Modbus RTU: DE, t1.5/t3.5, CRC, DMA.
33. Low-level crash debug: STM32 HardFault and ESP32 Guru Meditation.
34. MPU and memory protection: XN RAM, read-only config, stack guards.
35. TrustZone/Secure Boot: key isolation and firmware verification.
36. TLS/PKI: CA, hostname, mTLS, certificates, EC25 vs ESP32 TLS.
37. Secure commands: HMAC, signature, anti-replay, idempotency.
38. Deterministic memory: static allocation, object pools, heap budget.
39. Brownout and power: EC25 peak current, power-good, safe shutdown.
40. EMC/ESD: TVS, RC, Schmitt, industrial inputs, CAN/RS-485 protection.