read time led blink from serial

Difficulty 01/serial-read-blink/serial-read-blink.ino

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+/*
+  Blink BUILTIN LED example
+
+  Author: Alexander Popov
+  License: Unlicense
+*/
+
+int user_input;
+int delay_time = 1000;
+bool led_state = HIGH;
+unsigned long timer = 0;
+
+void setup() {
+  pinMode(LED_BUILTIN, OUTPUT);
+  Serial.begin(9600);
+}
+
+void loop() {
+  // read blink delay
+  if (Serial.available()) {
+    user_input = Serial.parseInt();
+    if (user_input >= 100) {
+      delay_time = user_input;
+
+      Serial.print("Delay set to ");
+      Serial.print(user_input);
+      Serial.println(" ms");
+    }
+    else if (user_input > 0 && user_input < 100) {
+      Serial.println("Error: set delay time >=100ms");
+    }
+  }
+
+  // blink LED
+  if (millis() - timer > delay_time) {
+    led_state =! led_state;
+    digitalWrite(LED_BUILTIN, led_state);
+
+    timer = millis();
+  }
+}