Code to deep sleep
node.dsleep(15*60*1000*1e3, 4)
boot reaseon
=node.bootreason()
2 2 28 1073798959 0 0 51 0
Reset cause number is 28, which means Access to invalid address(1. Access to Cache after it is turned off 2. Wild pointers) by ESP8266 Reset Causes and Common Fatal Exception Causes.pdf
lua doesn't have any pointer, so it's very puzzled.