broadcast tx -> rx esp8266 tested. IT WORKS.

broadcast/platformio.ini

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+; < NOTE >
+
+; to enable verbose output add option -->
+; $ platformio run --verbose
+
+; to make this permanent for the proj. -->
+; $ platformio settings set force_verbose Yes
+
+; then confirm the change -->
+; $ platformio settings get
+
+
+; // pio v 4.0 'Build options'
+;    - build_type
+;    - build_flags
+;    - src_build_flags
+;    - build_unflags
+;    - src_filter
+;    - targets
+
+
+[platformio]
+default_envs = d1_mini_pro
+
+[env]
+framework = arduino
+upload_port = /dev/ttyUSB0
+lib_deps =
+  721 ; TaskScheduler
+
+[env:nodemcuv2]
+platform = espressif8266
+board = nodemcuv2
+lib_deps =
+  ${env.lib_deps}
+upload_speed = 921600 ; 9600, 19200, 38400, 57600, 115200, 230400, 460800, 921600
+
+[env:d1_mini_pro]
+platform = espressif8266
+board = d1_mini_pro
+lib_deps =
+  ${env.lib_deps}
+upload_speed = 460800 ; 9600, 19200, 38400, 57600, 115200, 230400, 460800, 921600

broadcast/src/main.cpp

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+//
+// wirelessly connected cloud (based on ESP-NOW, a kind of LPWAN?)
+//
+
+//
+// Conversation about the ROOT @ SEMA warehouses, Seoul
+//
+
+//
+// 2021 02 21
+//
+
+// broadcast tx/rx test with esp8266
+
+//============<parameters>============
+//
+#define LED_PERIOD (11111)
+#define LED_ONTIME (1)
+#define LED_GAPTIME (222)
+//
+#define WIFI_CHANNEL 5
+//
+//============</parameters>===========
+
+//============<board-specifics>============
+#if defined(ARDUINO_FEATHER_ESP32) // featheresp32
+#define LED_PIN 13
+#else
+#define LED_PIN 2
+#endif
+//============</board-specifics>===========
+
+//arduino
+#include <Arduino.h>
+
+//espnow
+#include <ESP8266WiFi.h>
+#include <espnow.h>
+
+// on 'sent'
+void onDataSent(uint8_t *mac_addr, uint8_t sendStatus) {
+  if (sendStatus != 0) Serial.println("Delivery failed!");
+}
+
+// on 'receive'
+void onDataReceive(uint8_t * mac, uint8_t *incomingData, uint8_t len) {
+  Serial.write(incomingData, len);
+  Serial.println();
+}
+
+//task
+#include <TaskScheduler.h>
+Scheduler runner;
+
+//task #0 : blink led
+extern Task blink_task;
+void blink() {
+  //
+  static int count = 0;
+  count++;
+  //
+  switch (count % 4) {
+  case 0:
+    digitalWrite(LED_PIN, LOW); // first ON
+    blink_task.delay(LED_ONTIME);
+    break;
+  case 1:
+    digitalWrite(LED_PIN, HIGH); // first OFF
+    blink_task.delay(LED_GAPTIME);
+    break;
+  case 2:
+    digitalWrite(LED_PIN, LOW); // second ON
+    blink_task.delay(LED_ONTIME);
+    break;
+  case 3:
+    digitalWrite(LED_PIN, HIGH); // second OFF
+    blink_task.delay(LED_PERIOD - 2* LED_ONTIME - LED_GAPTIME);
+    break;
+  }
+}
+Task blink_task(0, TASK_FOREVER, &blink, &runner, true); // -> ENABLED, at start-up.
+
+//task #1 : regular tx
+void espnow_transmit() {
+  //
+  String hello = "hello. from ... " + String(WiFi.macAddress());
+  esp_now_send(NULL, (uint8_t *) hello.c_str(), hello.length());
+  //
+}
+Task espnow_transmit_task(1000, TASK_FOREVER, &espnow_transmit, &runner, true); // by default, ENABLED
+
+//
+void setup() {
+
+  //led
+  pinMode(LED_PIN, OUTPUT);
+
+  //serial
+  Serial.begin(115200);
+  delay(100);
+
+  //wifi
+  WiFiMode_t node_type = WIFI_AP_STA;
+  WiFi.mode(node_type);
+
+  //esp-now
+  if (esp_now_init() != 0) {
+    Serial.println("Error initializing ESP-NOW");
+    return;
+  }
+  esp_now_set_self_role(ESP_NOW_ROLE_COMBO);
+  esp_now_register_send_cb(onDataSent);
+  esp_now_register_recv_cb(onDataReceive);
+
+  //
+  uint8_t broadcastmac[6] = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
+  esp_now_add_peer(broadcastmac, ESP_NOW_ROLE_COMBO, 1, NULL, 0);
+}
+
+void loop() {
+  //
+  runner.execute();
+  //
+}