1. How often the callback function which was configured by zn_uart_register_rx_callback() API will ba called?

This is called for every time the MCU UART RX interrupt is invoked. For this reason, it is extremely important that is callback is lightweight. All heavy processing should be deferred to a thread context.

2.  The number configuring by zn_event_enable_irq_events() is not clear.

This configures the number of events that can be queued from the IRQ context.

A configuration of 8 allows for events to be queued from the UART RX callback. i.e. zn_event_issue(my_handler, NULL, ZOS_EVENT_FLAG_FROM_IRQ) can be invoked up to 8 times (without my_handler executing in-between) before the API fails with an overflow error.

3. The meaning of flag used by zn_event_issue() is unclear.

This tells the API that the event_handler is being issued from an IRQ context.

Effectively, this flag tells the API to add the handler to the IRQ handler queue configured with zn_event_enable_irq_events().

4. The relation between the data size specified by zn_uart_receive_bytes() and timeout. If the timeout occurred, will the data be received until the timeout occurred? Or, will the whole data discarded? If the former is the case, can we know the actually received size by zn_uart_get_bytes_received()?

If zn_uart_receive_bytes() returns ZOS_TIMEOUT, then the amount of data read up until the timeout is returned.

zn_uart_get_bytes_received() contains the amount of read data returned in the buffer.