Engineering Standards & Testing Practices

Developing mission-critical embedded software requires strict adherence to memory discipline, static analysis, and deterministic execution bounds.

1. Embedded C & FreeRTOS Coding Rules

A. Zero Dynamic Allocation in Polling Loops

Never call malloc(), calloc(), or pvPortMalloc() during steady-state Modbus polling or telemetry generation.
All telemetry schema builders and Modbus frame buffers must use static or task-stack allocated buffers with strict bounds checking.

B. FreeRTOS Task Stack Budgets

Every FreeRTOS task must declare an explicit stack depth with a verified high-water mark margin of at least 25%:

Task Name	Priority	Stack Allocation	Verified High-Water Mark
`vModbusTask`	`tskIDLE_PRIORITY + 3`	4,096 bytes	2,840 bytes
`vStorageTask`	`tskIDLE_PRIORITY + 2`	6,144 bytes	4,120 bytes
`vCloudTask`	`tskIDLE_PRIORITY + 2`	8,192 bytes	5,680 bytes
`vHttpTask`	`tskIDLE_PRIORITY + 1`	6,144 bytes	3,920 bytes
`vDiagTask`	`tskIDLE_PRIORITY + 4`	2,048 bytes	1,180 bytes

C. Stack Overflow Hook

The firmware configures configCHECK_FOR_STACK_OVERFLOW = 2:

void vApplicationStackOverflowHook(TaskHandle_t xTask, char *pcTaskName) {
    // Record fault in flash diagnostic log
    aslandi_diag_log_fault(FAULT_STACK_OVERFLOW, pcTaskName);
    // Force immediate hardware reboot via watchdog
    watchdog_reboot(0, 0, 0);
}

2. Automated Regression Test Harness

The tests/ directory contains Python test harnesses simulating Modbus RTU slaves, dropping Wi-Fi packets, and verifying telemetry schema conformance:

# Run automated Modbus protocol and timeout simulator
python tests/test_modbus_protocol.py --port /dev/ttyUSB0 --meters 50

# Execute cloud ingestion end-to-end regression
pytest tests/cloud_e2e_test.py -v