ADR-002: Wave v2 MCU Selection (MCXN947)

Status: Accepted Date: November 2025

Executive Summary

Recommended MCU for EQ Wave v2: NXP MCXN947

Key reasons:

1. Best fit for power quality application (dual Cortex-M33, DSP, AI/ML acceleration)
2. Superior memory (2 MB Flash, 512 KB RAM) vs LPC54628 (512 KB Flash, 200 KB RAM)
3. Modern security features (TrustZone, EdgeLock) for industrial deployment
4. Excellent power efficiency (57 µA/MHz) for 24/7 operation
5. microSD card support: Yes, can handle 1–2 TB SDXC cards with exFAT

Detailed Comparison

MCU Candidates

Performance Analysis

Raw processing power:

• STM32H747: Highest (3220 CoreMark total)
• MCXN947: Good (1236 CoreMark total, dual 150 MHz)
• RA6M4: Moderate (800 CoreMark, single 200 MHz)
• LPC54628: Moderate (778 CoreMark, single 220 MHz)

Power efficiency (CoreMark/mW):

• MCXN947: Excellent (618 / (0.057 mA/MHz × 150 MHz) = 72 CoreMark/mA)
• RA6M4: Excellent (800 / (0.099 mA/MHz × 200 MHz) = 40 CoreMark/mA)
• LPC54628: Good (778 / (0.1 mA/MHz × 220 MHz) = 35 CoreMark/mA)
• STM32H747: Moderate (high performance but higher power)

For power quality application (PLL residual encoding, DSP, networking):

• MCXN947: Best (dual core + DSP + AI/ML, can offload PLL to NPU or second core)
• STM32H747: Overkill (very high performance but power hungry, expensive)
• RA6M4: Good (single core but fast, good DSP)
• LPC54628: Adequate (single core, older architecture)

Memory Comparison

Why MCXN947’s memory advantage matters:

Critical: LPC54628’s 200 KB RAM is insufficient for v2 requirements:

• Network buffers: ~80 KB
• DAQ buffers: ~80 KB
• PLL residual compression: ~224 KB (per-channel buffers)
• Total needed: ~384 KB
• LPC54628 has: 200 KB ❌

Verdict: LPC54628 eliminated due to insufficient RAM.

Special Features

MCXN947 advantages:

1. Dual core: Can dedicate core 1 to DAQ/PLL, core 2 to networking/compression
2. AI/ML NPU: 4.8 GOPs for edge analytics (future: anomaly detection on-device)
3. PowerQuad DSP: Hardware acceleration for FFT, filtering, PLL computations
4. Security: EdgeLock Secure Enclave for field deployment integrity
5. SmartDMA: Offload memory-intensive operations (SD card writes)

STM32H747 advantages:

1. Highest performance: 480 MHz Cortex-M7 for demanding DSP
2. Asymmetric cores: M7 for processing, M4 for networking/housekeeping
3. Mature ecosystem: Extensive libraries, tools, community support
4. L1 cache: 16 KB I + 16 KB D per core (faster code execution)

Why STM32H747 not chosen:

• Overkill for power quality (don’t need 480 MHz)
• Higher power consumption (~280 µA/MHz vs 57 µA/MHz)
• More expensive ($10–15 vs $7–10)
• Complexity of asymmetric dual-core programming

Why MCXN947 chosen:

• Balanced performance for power quality application
• Best power efficiency for 24/7 operation
• Modern security features for industrial IoT
• AI/ML acceleration for future edge analytics
• Dual symmetric cores easier to program than asymmetric

microSD Card Support: 1–2 TB Capability ✅

Can ARM Cortex-M MCUs support 1–2 TB microSD cards?

Yes, all modern ARM Cortex-M MCUs can support SDXC cards up to 2 TB.

Technical requirements:

1. SDIO/SDMMC interface: All candidates have UHS-I capable SDIO
2. exFAT file system: Required for > 32 GB cards
3. FatFS middleware: Must enable exFAT support (available for all platforms)

MCXN947 specific:

• SDIO interface: Yes, supports UHS-I (up to 50 MB/s)
• DMA support: Yes, for efficient SD card transfers without CPU overhead
• FatFS port: Available in MCUXpresso SDK
• exFAT support: Requires enabling FF_FS_EXFAT in ffconf.h

Storage capacity with PLL residual encoding:

Value Proposition:

• 1–2 TB microSD: ~$30–60 (consumer grade), ~$100–200 (industrial grade)
• EQ Wave COGS: ~$600
• Capability: 6–24 months of continuous 64 kSPS waveform storage
• Use case: Walk-up fault recorder without dedicated gateway
  • Field tech plugs laptop into fiber port
  • Retrieves months/years of historical data
  • No gateway, no cloud, no complexity
  • Killer feature for industrial deployments

Real-World Deployment Scenarios

Scenario 1: Remote Substation (No Gateway)

• Install EQ Wave v2 with 1 TB microSD
• 12 months of continuous waveform storage
• Monthly site visit: Retrieve data via laptop + fiber connection
• Value: Eliminates $2–5k gateway hardware, no network infrastructure

Scenario 2: Semiconductor Fab (Critical Equipment)

• 2 TB microSD for 2-year event history
• Tool goes down → immediate retrieval of pre-failure waveforms
• Value: Million-dollar fab line saved by instant root cause analysis

Scenario 3: Data Center (Compliance)

• Regulatory requirement: 6-month power quality records
• 512 GB microSD stores 6 months continuously
• Retrieve on-demand for audits
• Value: Compliance without ongoing gateway/cloud costs

Implementation Considerations

exFAT licensing:

• Historical issue: Microsoft patents on exFAT
• Current status: Microsoft released exFAT specification to Open Invention Network (OIN) in 2019
• For commercial products: Verify FatFS exFAT implementation is properly licensed
• Alternative: Use FAT32 with multiple partitions (32 GB each), manage as array
  • 1 TB = 32 × 32 GB partitions
  • More complex but avoids any exFAT licensing concerns

Performance:

• SDIO @ 50 MB/s (UHS-I) >> 0.25 MB/s compressed write rate
• DMA-based writes: minimal CPU overhead
• No bottleneck for continuous 64 kSPS recording

Reliability:

• Industrial-grade microSD: Rated for -40°C to +85°C, 10-year retention
• Wear leveling: Modern controllers handle 24/7 writes
• Expected lifetime: 3–5 years continuous writes (varies by card quality)
• Recommendation: Use SLC or pSLC (pseudo-SLC) industrial cards for 24/7 operation

Cost Analysis

Component costs (approximate, 1k qty):

COGS breakdown (v2 with 1 TB storage):

• Electronics (MCU, ADCs, passives): ~$200
• Enclosure, mounting: ~$100
• Fiber optics, connectors: ~$80
• Assembly, test: ~$120
• Sensors (CT/PT): ~$100
• Total: ~$600 COGS

Market positioning:

• EQ Wave v2 with 1-year onboard storage: $2000–2500 retail
• Competitive fault recorders: $5000–15000 (without waveform storage capability)
• Value proposition: 2–3× lower price with superior continuous waveform archival

Recommendation

MCXN947 is the best choice for EQ Wave v2

Summary:

1. ✅ Memory adequate: 2 MB Flash, 512 KB RAM (vs LPC54628’s insufficient 200 KB RAM)
2. ✅ Performance balanced: Dual 150 MHz Cortex-M33 sufficient for PLL residual encoding (17% CPU)
3. ✅ Power efficient: 57 µA/MHz enables 24/7 operation with minimal heat
4. ✅ Modern features: AI/ML NPU, PowerQuad DSP, EdgeLock security
5. ✅ microSD support: Can handle 1–2 TB SDXC cards for 1–2 year continuous storage
6. ✅ Future-proof: Dual core allows offloading to core 2 or NPU as features expand
7. ✅ Price competitive: $7–10 vs STM32H747’s $10–15

LPC54628 eliminated: Insufficient RAM (200 KB vs 384 KB needed)

STM32H747 not needed: Overkill performance, higher power/cost, complexity not justified

Implementation Plan

v2.0 (2026 Q2):

• MCXN947 with 32 GB microSD (baseline)
• Simple per-channel LZ4 compression
• Storage: 10–12 days (network outage resilience)

v2.1 (2026 Q4):

• Add PLL residual encoding
• Option: 128 GB, 512 GB, 1 TB, or 2 TB microSD
• Storage: 40 days to 2 years
• Market as: “Standalone fault recorder with gateway-free operation”

Future (v2.2+):

• Utilize NPU for edge anomaly detection
• Use core 2 for advanced DSP (supraharmonics preprocessing, harmonic extraction)
• OTA firmware updates via microSD

Early Design Verification Items

MCXN947 capabilities to confirm during initial design phase:

1. SHA hardware accelerator: ✅ VERIFY

   • Confirm MCXN947 has hardware SHA-256 for SSL/TLS handshake acceleration
   • LPC54628 had SHA accelerator - ensure MCXN947 has equivalent or better
   • Critical for secure network communications

2. EtherCAT interface support: ✅ VERIFY

   • Determine how to interface external EtherCAT controller (e.g., Beckhoff ET1100) to MCXN947
   • Options: SPI, parallel bus, or other high-speed interface
   • LPC54628 had 180 pins enabling MII for direct EtherCAT - confirm MCXN947 alternative approach
   • Required for optional EtherCAT module in v2

Note: These items were compelling features of LPC54628 that influenced early consideration. Confirm MCXN947 can support equivalent functionality before finalizing schematic.

Conclusion

MCXN947 enables the “walk-up fault recorder” use case:

• Install EQ Wave with 1–2 TB microSD
• Months/years of continuous waveform storage
• Field tech retrieves data directly via laptop + fiber
• No gateway, no cloud dependency, no complexity
• Killer capability for $600 COGS device

Competitive differentiation:

• Traditional fault recorders: $5–15k, limited storage (days/weeks)
• EQ Wave v2: $2–2.5k retail, 6–24 months continuous storage
• 10× cost advantage with superior capability

This positions EQ Wave v2 as both:

1. Gateway-connected smart sensor (with EQ Syntropy platform)
2. Standalone fault recorder (walk-up data retrieval)

Dual-mode capability expands addressable market significantly.