<dd>1. It is recommended that users set the timer to the periodic mode for periodic checks. </dd>
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(1). In freeRTOS timer or os_timer, do not delay by while(1) or in the manner that will block the thread. </dd>
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(2). The timer callback should not occupy CPU more than 15ms. </dd>
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(3). os_timer_t should not define a local variable, it has to be global varialbe or memory got by malloc.</dd>
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2. Since esp_iot_rtos_sdk_v1.0.4, functions are stored in CACHE by default, need not be added ICACHE_FL