g wifi_fpm_open. This API can not be called after calling wifi_fpm_close. </dd>
<dd>
2. If this API returned 0 means that the configuration is set successfully, but the ESP8266 will not enter sleep mode immediately, it is going to sleep in the system idle task. Please do not call other WiFi related function right after calling this API.</dd></dl>
<dl class="params"><dt>Parameters</dt><dd>
  <table class="params">
    <tr><td class="paramname">uint32</td><td>sleep_time_in_us : sleep time, ESP8266 will wake u