Also as per the SDK
• system_get_vdd33 can only be called when TOUT pin is suspended
• The 107th byte in esp_init_data_default.bin(0〜~127byte)is named as
“vdd33_const“ , when TOUT pin is suspended vdd33_const must be set as
0xFF, that is 255
In my case I have grounded TOUT in one my apps and it chugs along nicely, but then I only call readvdd33 once per on/suspend cycle. YMMV