LEGO Powered Up Protocol — Megahub ESP32

Hardware Setup

• UART chip: SC16IS752 (I²C-to-dual-UART bridge)
• Ports: 0–3 (physical LEGO connectors)
• Baud rate: negotiated per device during handshake (typically starts at low speed, switches to 115200)
• Library: lib/lpfuart/ — legodevice.cpp, lumpparser.cpp, SC16IS752serialadapter.cpp

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Device Initialization Sequence

Devices send identification messages on connect. Never skip any step.

1. LUMP_CMD_TYPE — device reports its type ID and name
2. LUMP_CMD_MODES — device reports number of supported modes
3. LUMP_CMD_SPEED — device requests baud rate to switch to
4. LUMP_CMD_VERSION — device reports firmware + hardware versions
5. LUMP_INFO_NAME/RAW/PCT/SI/UNITS/MAPPING/FORMAT — per-mode metadata (repeats for each mode)
6. LUMP_INFO_MODE_COMBOS — supported mode combinations
7. finishHandshake() — switch to negotiated baud rate (e.g., 115200)
8. selectMode() — set default operating mode

Example (BOOST Interactive Motor, device ID 0x26):

• 4 modes: POWER (0), SPEED (1), POS (2), TEST (3)
• Default mode: 2 (POS — position feedback)
• Handshake takes ~3 seconds from connect to ready

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Protocol Rules

• Command rate limit: ~20ms minimum between commands for most devices
• Device timeout: some devices sleep if no command within ~1 second — send keepalives
• Byte order: LEGO uses little-endian in most messages
• Port cleanup: on disconnect, cancel all pending commands for that port
• Mode selection: always use device type to validate supported modes before CMD_SELECT

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UART Corruption from Motor Noise

This is a known hardware issue. When motors run, EMI corrupts the UART RX line.

Symptoms

• [WARN] LUMP sync lost after N good frames (checksum expected=0xXX actual=0xYY)
• Repeated 0xD2 bytes in discarded data (spurious detections of LEGO header byte)
• uartOvr=1 in parser stats — SC16IS752 FIFO overflow

Root Cause

Motor control via M1/M2 generates high-frequency switching noise → current spikes → EMI coupling into UART RX trace → SC16IS752 FIFO fills faster than ESP32 reads via I²C.

Software Mitigations (already implemented)

• UART read batch size increased to 64 bytes per call (reduces FIFO overrun probability)
• delayMicroseconds(100) after motor speed changes (allows transients to settle)
• FIFO overrun counter tracked and logged as uartOvr

Hardware Fixes (if software mitigations are insufficient)

Priority	Fix	Expected improvement
1 (best)	68–100pF NP0/C0G ceramic cap on UART RX pin to GND	50–70% error reduction
2	Clip-on ferrite bead on RX wire near PCB	40–60% error reduction
3	100Ω + 100pF RC filter on UART TX	10–20% additional
4	100nF decoupling cap on SC16IS752 VCC pin	standard practice

Target: < 5 sync recoveries per minute with motor at 100% speed.

Parser Recovery

The parser (lumpparser.cpp) handles corruption gracefully:

• Detects checksum mismatch
• Scans forward to find next valid frame header
• Logs discarded bytes
• Recovers without device reset

Good state: Parser: ok=XXX csErr=0 discarded=0 recoveries=0 uartOvr=0

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Debugging LEGO

Watch parser stats in serial log (logged every ~5 seconds):

[INFO] logParserStats() - Parser: ok=138 csErr=0 discarded=0 recoveries=0 overflow=0 unknownSys=0 invalidSize=0 uartOvr=1

Field	Meaning
ok	Good frames parsed
csErr	Checksum errors
discarded	Bytes thrown away during resync
recoveries	Number of sync-loss recovery events
uartOvr	SC16IS752 FIFO overflow count