Memory Management & FreeRTOS — Megahub ESP32

Memory Budget (real device measurements)

Point in startup	Free heap
After IMU init	~170 KB
After Lua state creation	~140 KB
After LEGO motor init	~140 KB
After configuration load	~139 KB
After BT stack start	~76 KB
After BLE characteristics setup	~76 KB
During BLE connection (connected)	~63–73 KB

PSRAM: ~4.1 MB available (confirmed present on this board — ESP.getFreePsram()).

BT stack alone consumes ~65 KB of heap. Every BLE connection consumes an additional ~30 KB.

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Memory Rules

• Stack: Never allocate large arrays as local variables. Use static, heap, or PSRAM.
• Heap: Minimize new/malloc. Every allocation needs a matching delete/free.
• Strings: Avoid String += loops. Prefer std::string::reserve() or fixed char[] buffers.
• PSRAM: Available, but don't assume — always check. Use for large buffers if needed.
• Constants: Use const and PROGMEM for large constant data to keep it in flash.
• Lua VM: Runs in heap. Large scripts reduce available heap for everything else.

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FreeRTOS Task Reference

Known tasks and their stack high-water marks (from real device logs):

Task	Priority	Stack HWM (words)	Notes
loopTask	1	272	Arduino main loop
BTMsgProcessor	2	2968–7528	Processes BLE messages
HIDEventTask	5	1920	Classic BT HID events
PortStatus	1	1420	LEGO port status reporting
StatusMonitor	1	2452	Heap/task monitoring
Device monitor	1	1776	Lua thread for device monitoring
BTU_TASK	20	3624–4988	BT stack internal
BTC_TASK	19	4328–4364	BT stack internal
hciT	22	1516–1544	BT HCI layer
btController	23	2088	BT controller
esp_timer	22	4048	ESP timer service
ipc0/1	24	356/476	IPC (highest priority)

⚠ loopTask HWM of 272 words is critically low — avoid adding stack usage to the main loop.

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FreeRTOS Rules

• Mutexes: Always use xSemaphoreTake with a timeout — never block indefinitely
• Task stacks: Monitor with uxTaskGetStackHighWaterMark(). Stack overflow = silent crash.
• Inter-task communication: Use queues or semaphores — not global variables without synchronization
• BLE callbacks: Run in BLE task context — post work to a queue, handle in application task
• Never block in BLE callbacks, HID callbacks, or ISRs
• Task priorities: BT stack tasks run at 19–24. Application tasks at 1–5. Don't starve the BT stack.

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Monitoring Pattern

// Log heap + PSRAM
INFO("Free heap: %d bytes, Free PSRAM: %d bytes",
     ESP.getFreeHeap(), ESP.getFreePsram());

// Log task stack (for current task)
INFO("Stack HWM: %d words", uxTaskGetStackHighWaterMark(NULL));

// Log all task stacks (use vTaskList for full snapshot)