Strange behaviour of WiFi and Deep Sleep mode on ESP32S2

[TimLovell]

ESP32S2 using arduino 2.0.16. Static IP IOT node that is going to spend most of its time asleep running from batteries. powered over USB for testing. If ESP32S2 periodically reset by timer connected to ENH/RST pin then boots and reliably connects to WiFi in 85-95ms and reliably pulls information over http from local subnet.

When node allowed to go to deep sleep it unreliably wakes up, sometimes after it has woken a watchdog reboots it before it establishes WiFi connection, other times it just takes an unpredictable, but generally several seconds to connect to WiFi.
The sketch below demonstrates the problem

Use code tags to format code for the forum

#include <WiFi.h>
#include <HTTPClient.h>

RTC_DATA_ATTR int nv;

const char*     ssid        = "****";   // Replace with your network credentials
const char*     password    = "****";

void setup() {
  uint32_t   start;
  int        httpCode;
  HTTPClient http;
  esp_reset_reason_t reason = esp_reset_reason();
  IPAddress       local_IP(192, 168, 18,149);                 // Set Static IP address
  IPAddress       gateway(192, 168, 18, 1);                   // Set Gateway IP address
  IPAddress       subnet(255, 255, 255, 0);
  IPAddress       primaryDNS(8, 8, 8, 8);                     // Needed for NTP
  IPAddress       secondaryDNS(1,1,1,1);                      // Backup for NTP

  Serial1.begin(9600,SERIAL_8N1,-1,39);   // Tx only 
  delay(10);
  Serial1.println("");
  switch (reason) {
    case ESP_RST_POWERON:Serial1.println("Power ON");break;
    case ESP_RST_SW:Serial1.println("Reset ESPrestart()");break;
    case ESP_RST_PANIC:Serial1.println("Reset exception/panic");break;
    case ESP_RST_UNKNOWN:Serial1.println("Reset UNKNOW");break;
    case ESP_RST_EXT:Serial1.println("Reset by external pin (not applicable for ESP32)");break;
    case ESP_RST_INT_WDT:Serial1.println("Reset (software or hardware) by interrupt WATCHDOG");break;
    case ESP_RST_TASK_WDT:Serial1.println("Reset WATCHDOG");break;
    case ESP_RST_WDT:Serial1.println("Reset others WATCHDOG´s");break;                                
    case ESP_RST_DEEPSLEEP:Serial1.println("Reset DEEP SLEEP MODE");break;
    case ESP_RST_BROWNOUT:Serial1.println("Brownout reset (software or hardware)");break;
    case ESP_RST_SDIO:Serial1.println("SDIO Reset");break;
    default:
    break;
  }
  Serial1.printf("nv = %d\r\n",nv);
  nv++;
  start=millis();
  if (!WiFi.config(local_IP, gateway, subnet, primaryDNS, secondaryDNS)) Serial1.println("Unable to set Static IP address");
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    if (millis()-start > 20000) {
      Serial1.println("WiFi Failed - Rebooting");
      Serial1.flush();
      ESP.restart();      // Fatal unable to establish WiFi force reboot
    } else delay(10);
  }
  Serial1.printf(" CONNECTED  %ld ms ",millis()-start);

  start=millis();
  http.begin("http://192.168.18.240/unixtime");

  httpCode = http.GET();
  if (httpCode > 0) {

    if (httpCode == HTTP_CODE_OK) {
      String payload = http.getString();
      Serial1.printf("  HTTP %ld\r\n",millis()-start);
    }
  } else {
    Serial1.printf("[HTTP] GET... failed, error: %s\n", http.errorToString(httpCode).c_str());
  }
  http.end();
  Serial1.flush();

  esp_sleep_enable_timer_wakeup(90000000); // 90 sec
  esp_deep_sleep_start(); 
}

void loop() { // never executed

}

Typical output below

Power ON
nv = 0
CONNECTED 94 ms HTTP 2823

Reset DEEP SLEEP MODE
nv = 1

Reset others WATCHDOG´s
nv = 0
CONNECTED 96 ms HTTP 125

Reset DEEP SLEEP MODE
nv = 1

Reset others WATCHDOG´s
nv = 0

CONNECTED 105 ms HTTP 121

Reset DEEP SLEEP MODE
nv = 1
CONNECTED 3075 ms
Reset others WATCHDOG´s
nv = 0
CONNECTED 94 ms HTTP 46

Reset DEEP SLEEP MODE
nv = 1
CONNECTED 7145 ms HTTP 91

Reset DEEP SLEEP MODE
nv = 2

Reset others WATCHDOG´s
nv = 0
CONNECTED 95 ms HTTP 111

Reset DEEP SLEEP MODE
nv = 1

Reset others WATCHDOG´s
nv = 0
CONNECTED 91 ms HTTP 67

Reset DEEP SLEEP MODE
nv = 1
CONNECTED 5255 ms
Reset others WATCHDOG´s
nv = 0
CONNECTED 95 ms HTTP 72

Reset DEEP SLEEP MODE
nv = 1
CONNECTED 6164 ms
Reset others WATCHDOG´s
nv = 0
CONNECTED 95 ms HTTP 84

Reset DEEP SLEEP MODE
nv = 1

Reset others WATCHDOG´s
nv = 0
CONNECTED 94 ms HTTP 77

If I don't use WiFi the node runs for days waking every 90 seconds and increases nv by 1 each time and never produces a watchdog timeout.
How should I be loading/unloading WiFi whereby it quickly connects to to static network and can communicate to subnet.
A workaround is to use a hardware timer to keep rebooting the ESP32/S2 but that doesn't maintain the RTC_RAM.