Merge branch 'docs/pwm_and_sniffer_coexist' into 'master'

How to deal with PWM and sniffer co-exist

See merge request sdk/ESP8266_RTOS_SDK!717

Author: donghengqaz

docs/en/api-guides/pwm-and-sniffer-coexists.md

@@ -0,0 +1,30 @@
+PWM & Sniffer Co-exists
+=======================
+
+1. Overview
+-----------
+
+Without hardware PWM, ESP8266 has to use the hardware timer to simulate the PWM. We are using the Wi-Fi internal timer to drive the PWM, so there may be resource competition issue when using PWM and sniffer/SmartConfig at the same time. 
+
+
+
+2. Root Cause
+-------------
+
+To ensure the high precision of the PWM, the hardware Timer1 will trigger the interrupt AHEAD_TICKS1(6us by default) earlier. And in the interrupt, it will poll to wait for AHEAD_TICKS1(6us by default). After handling the GPIO invert in one channel, the system will check the remaining time (T1) to the next channel invert. If the T1 < AHEAD_TICKS2(8us by default), the system will not exit the interrupt, but poll to wait till timeout, and then invert the GPIO in the next channel; then the system will repeat these steps until all channels inverted.
+So theoretically, the max time that PWM may occupy the CPU is 6 + 8 * n, n means the channel count. For example, if there are 3 channels, then PWM may take 30us at most. In this case, PWM will affect the Wi-Fi sniffer/SmartConfig function, especially for the capture of the LDPC packets, or HT40 packets which require the CPU to handle them in time, otherwise those packets will loss. 
+
+
+3. Issue that may happen
+------------------------
+
+If your application used both PWM and sniffer/SmartConfig, the sniffer/SmartConfig may take a long time to connect to an AP.   
+You can stop the PWM and try it again. If the sniffer/SmartConfig becomes much faster, then it is the PWM that affect the sniffer/SmartConfig. In this case, you should adjust the frequency, duty cycle and phase of the PWM.
+
+4. Suggestion
+-------------
+
+When using the PWM and SmartConfig at the same time, please note:
+
+1. The PWM's frequency cannot be too high, 2KHz at most.
+2. Revise the PWM's duty cycle and phase, make the time intervals (Tn) between each channel inverting be equal to 0 or be larger than 50us (Tn = 0, or Tn > 50).