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MCU & RTOS Firmware

Cortex-M4F Lazy FPU Stacking: Budget the First Floating-Point Instruction in an ISR
ByLIU, ZHUOHUI 15/10/2026

An interrupt can enter promptly yet encounter deferred floating-point context-saving work later, at its first FPU instruction. Measuring only the first GPIO transition inside the handler can miss that cost. Qualify the complete deadline-critical path under both floating-point-active and inactive interrupted contexts. This guide concerns Cortex-M4 processors with the optional FPU, commonly called Cortex-M4F, using…

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MCU & RTOS Firmware

FreeRTOS Queue Sets: Keep Selection and Consumption in One Ownership Contract
ByLIU, ZHUOHUI 15/10/2026

A queue set works reliably when selecting a member and consuming one item from that member are treated as a single application-level obligation. Do not let a diagnostic task, a secondary worker or a recovery shortcut read a member independently. Otherwise, the set can retain a readiness event after the corresponding payload has disappeared. This…

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Industrial Protocols & Gateways

Modbus File Record Transfers: Validate Subrequests, Lengths and Device Support
ByLIU, ZHUOHUI 15/10/2026

Functions 20 and 21 operate on file-record references, with nested request groups and their own length accounting. They are not ordinary holding-register reads and writes with a different function byte. A gateway should validate the entire structure, the expected response budget and the device’s supported file map before sending a request. This guide is scoped…

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Industrial Protocols & Gateways

Modbus Serial Diagnostics: Use Function 08 Counters Without Treating Them as Universal Telemetry
ByLIU, ZHUOHUI 15/10/2026

Function 08 counters can help distinguish receive-path problems from protocol-processing exceptions, provided the device supports the relevant subfunctions and their meaning is known. Use timestamped differences over a controlled observation window. Do not interpret one absolute count as a current fault, and do not treat every diagnostic subfunction as a harmless read. The reference boundary…

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Industrial Protocols & Gateways

Linux RS-485 Direction Control: Verify RTS Polarity and Turnaround on the Wire
ByLIU, ZHUOHUI 15/10/2026

A successful RS-485 configuration ioctl does not prove that the transceiver drives and releases the bus at the correct times. Qualify direction control with two records: the settings actually accepted by the driver and a capture that relates those settings to UART data, driver enable and the differential bus. Treat unsupported controls as a capability…

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Bluetooth & BLE Development

EATT in BLE Products: When Parallel GATT Transactions Help and When They Do Not
ByLIU, ZHUOHUI 15/10/2026

EATT is useful when an independent GATT transaction is delayed behind another transaction on the same ATT bearer, and the host stacks can actually schedule the work on separate bearers. It is not an automatic throughput multiplier. Shared application locks, a serialized client wrapper or limited radio service can leave the original bottleneck unchanged. This…

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Bluetooth & BLE Development

BLE Periodic Advertising Synchronization: Acquisition Timeouts, Lost Sync and Recovery
ByLIU, ZHUOHUI 15/10/2026

Use separate controls for finding a periodic advertiser, detecting loss of an established synchronization and deciding that application data is stale. Cancel an expired acquisition through the stack’s documented lifecycle, wait for cancellation to finish, and only then start a replacement attempt. A parameter named synchronization timeout is not automatically an acquisition deadline. This guide…

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Bluetooth & BLE Development

BLE Extended Advertising Reports: Reassemble Data Before Trusting Device Discovery
ByLIU, ZHUOHUI 15/10/2026

Admit an extended-advertising record to device discovery only after the receiving layer has established that its data is complete and the application parser has validated the result. A recognizable name or service identifier in an early fragment is not evidence that the rest of the record arrived. A truncated report must not be repaired by…

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Device Networking & Provisioning

Wi-Fi Wake-on-WLAN for Headless Devices: Define Which Packets Must Wake the Product
ByLIU, ZHUOHUI 15/10/2026

Wake-on-WLAN qualification should prove a chain: a defined network event reaches the radio, the configured trigger is recognized, the platform resumes from the intended sleep state, and the application becomes usable. A successful cloud command after several retries proves only the last part. It does not identify which event woke the host. This guide uses…

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Device Networking & Provisioning

OWE for Headless Wi-Fi Products: Decide What Encrypted Open Access Does Not Authenticate
ByLIU, ZHUOHUI 15/10/2026

Opportunistic Wireless Encryption can protect a password-free wireless link against passive observation. It cannot establish that the access point belongs to the intended operator. For a headless product, the engineering decision therefore has two separate parts: whether the selected connection must use OWE, and how the product authenticates the service that receives its data. An…

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