Initialisation depot
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ESP32/DCC-Bench/doc/LM18200_DUAL_MODE.md
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ESP32/DCC-Bench/doc/LM18200_DUAL_MODE.md
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# LM18200 Dual-Mode Operation
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## System Architecture
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```
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┌─────────────────────────────────────────────────────────────┐
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│ ESP32-2432S028R Module │
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│ │
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│ ┌──────────────┐ ┌────────────┐ ┌──────────────────┐ │
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│ │ Touchscreen │ │ DCC │ │ Motor │ │
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│ │ UI Control │→→│ Generator │ │ Controller │ │
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│ └──────────────┘ └────────────┘ └──────────────────┘ │
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│ │ │ │
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│ ▼ ▼ │
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│ GPIO 18 (PWM/DCC_A) │
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│ GPIO 19 (DIR/DCC_B) │
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│ GPIO 23 (BRAKE) │
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│ GPIO 35 (ADC - ACK Detect) │
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└─────────────────────────────────────────────────────────────┘
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│
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▼
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┌─────────────────┐
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│ LM18200 │
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│ H-Bridge │
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│ │
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│ Universal │
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│ DC/DCC Driver │
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└─────────────────┘
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│
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▼
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┌─────────────────┐
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│ Current Sense │
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│ 0.1Ω 1W │
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└─────────────────┘
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│
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┌────────────┴────────────┐
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│ │
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▼ ▼
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Track Rail 1 Track Rail 2
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│ │
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└────── LOCOMOTIVE ───────┘
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```
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## Mode Comparison
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### DC Analog Mode
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```
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GPIO 18 ──→ PWM Signal (20kHz, 0-100% duty) ──→ LM18200 ──→ Variable Voltage
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GPIO 19 ──→ Direction (HIGH/LOW) ──→ LM18200 ──→ Polarity
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GPIO 23 ──→ Brake (active when needed) ──→ LM18200 ──→ Both outputs LOW
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Result: Traditional DC motor control with variable speed
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```
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### DCC Digital Mode
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```
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GPIO 18 ──→ DCC Signal A (58μs/100μs pulses) ──→ LM18200 ──→ Track +
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GPIO 19 ──→ DCC Signal B (inverted A) ──→ LM18200 ──→ Track -
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GPIO 23 ──→ Brake (emergency stop) ──→ LM18200 ──→ Both outputs LOW
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Result: NMRA DCC digital control with 128 speed steps + functions
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```
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### Programming Track Mode (DCC Service Mode)
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```
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GPIO 18 ──→ Service Mode Packets (22-bit preamble) ──→ LM18200 ──→ Track +
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GPIO 19 ──→ Inverted service packets ──→ LM18200 ──→ Track -
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│
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▼
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Current Sense (0.1Ω)
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│
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▼
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Voltage Divider
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│
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▼
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GPIO 35 ◄──────────────── ADC reads ACK pulse (60mA = 6mV across 0.1Ω)
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Result: Decoder programming with ACK verification
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```
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## Signal Characteristics
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### DC Mode Signals
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```
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GPIO 18 (PWM):
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▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁
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▔▔▔▔▔▔▔▔▔▔ (20kHz square wave, variable duty cycle)
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GPIO 19 (Direction):
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▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔ (Forward: HIGH)
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▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁ (Reverse: LOW)
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```
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### DCC Mode Signals
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```
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GPIO 18 (DCC Signal A):
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▔▁▔▁▔▁▔▁▔▁▔▁▔▁▔▁ ← '1' bits (58μs per half)
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▔▔▁▁▔▔▁▁▔▔▁▁▔▔▁▁ ← '0' bits (100μs per half)
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GPIO 19 (DCC Signal B):
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▁▔▁▔▁▔▁▔▁▔▁▔▁▔▁▔ ← Inverted from Signal A
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▁▁▔▔▁▁▔▔▁▁▔▔▁▁▔▔
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```
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### Programming Track ACK
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```
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Current Draw During Programming:
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Normal: ───────────────────────────── (baseline ~10-20mA)
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ACK: ────────┏━━━━━━┓───────────── (60mA spike for 6ms)
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↑ ↑
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Valid End
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Command ACK
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ADC Reading (GPIO 35):
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─────────┏━━━━━┓────────────── (voltage spike detected)
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```
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## LM18200 Pin Configuration
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```
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┌────────────────────────────────┐
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│ LM18200 H-Bridge │
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├────────────────────────────────┤
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│ │
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│ PWM (Pin 3) ← GPIO 18 │ } Dual purpose:
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│ DIR (Pin 5) ← GPIO 19 │ } DC: PWM+Direction
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│ BRAKE(Pin 4) ← GPIO 23 │ } DCC: Signal A+B
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│ │
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│ VCC (Pin 1) ← 5V Logic │
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│ GND (Pin 2) ← GND │
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│ │
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│ VS (Pin 10) ← 12-18V Track │
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│ │
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│ OUT1 (Pin 8) → Rail 1 ────┐ │
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│ OUT2 (Pin 9) → Rail 2 ────┤ │
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└────────────────────────────┼───┘
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│
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┌──────┴──────┐
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│ 0.1Ω Sense │
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└──────┬──────┘
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│
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To Track
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```
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## Current Flow and ACK Detection
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### Programming Track Current Sensing
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```
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LM18200 OUT1
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│
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▼
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┌─────────┐
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│ 0.1Ω 1W │ ← Sense Resistor
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└─────────┘
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│
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┌────┴────┐
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│ │
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1kΩ To Track Rail 1
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│
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GPIO 35 (ADC)
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│
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10kΩ
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│
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GND ──── Track Rail 2 ──── LM18200 OUT2
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Voltage Calculation:
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- Decoder ACK = 60mA
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- Voltage across 0.1Ω = I × R = 0.06A × 0.1Ω = 6mV
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- Voltage divider (1kΩ/10kΩ): V_adc = 6mV × (10/(1+10)) ≈ 5.45mV
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- ESP32 ADC: 12-bit (0-4095) for 0-3.3V
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- Expected ADC value: (5.45mV / 3300mV) × 4095 ≈ 6-7 counts
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Note: In practice, use higher resistance for better ADC reading,
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or amplify signal with op-amp for more reliable detection.
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```
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## Why This Works for Programming
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### Traditional DCC System
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- **Main Track**: 3-5A continuous, many locomotives
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- **Programming Track**: 250mA max, one decoder at a time
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- **Separation Required**: Different boosters to prevent overcurrent
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### DCC-Bench Approach
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- **Single Track**: Only one locomotive under test
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- **Low Current**: Programming current well within LM18200 limits
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- **No Isolation Needed**: Same track for operation and programming
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- **Mode Selection**: Software-controlled (touchscreen UI)
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### LM18200 Specifications
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- **Continuous Current**: 3A (plenty for single loco)
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- **Peak Current**: 6A (handles inrush)
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- **Current Limit**: Built-in thermal protection
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- **Perfect for**: Small test bench with one locomotive
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## Safety Features
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### Hardware Protection
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1. **LM18200 thermal shutdown**: 165°C junction temperature
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2. **Current limiting**: Automatic under-voltage lockout
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3. **Brake function**: Forces outputs LOW (GPIO 23)
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4. **Optional fuse**: 250mA on track output for extra safety
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### Software Safety
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1. **Power-off on mode change**: Prevents accidental high current
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2. **CV range validation**: Only CV 1-1024 allowed
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3. **Address validation**: 1-10239 range check
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4. **Write verification**: Confirms successful programming
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5. **Timeout handling**: Aborts if no ACK after retries
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## Limitations and Considerations
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### Current Implementation ✅
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- Sends NMRA-compliant programming packets
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- Proper timing and packet structure
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- Retry logic for reliability
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- Basic ACK detection framework
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### With Hardware Addition 📋
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- Full ACK detection with current sensing
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- Verified programming success
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- Reliable decoder communication
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- Professional-grade test bench
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### Not Supported ⚠️
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- **Operations Mode Programming**: Requires main track operation
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- **RailCom**: Needs additional hardware and timing
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- **Multiple Locomotives**: Bench designed for single loco testing
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- **High Current Ops**: Not a layout controller (test bench only)
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## Advantages Summary
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✅ **Simplicity**: One driver for everything
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✅ **Cost**: No separate programming booster
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✅ **Reliability**: LM18200 proven design
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✅ **Flexibility**: Easy mode switching
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✅ **Safety**: Built-in protection
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✅ **Completeness**: Full NMRA compliance
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✅ **Practicality**: Perfect for test bench use
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This design leverages the fact that a test bench only ever has ONE locomotive,
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eliminating the need for separate main track and programming track boosters!
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