Power-cycle-friendly Nano boot and ESP relay handling

Nano / driver:
- No full on/off flip at boot; the first display() after boot is a forced
  full redraw instead, since the physical dot state is unknown
- Send READY when booted

ESP32:
- relayOn() only switches and waits when the relay was off, and waits for
  the Nano's READY (3 s timeout) instead of a fixed 3 s on every frame
- relayOff() ends Serial and releases the TX pin so the idle-high line no
  longer back-powers the Nano through its RX pin
- No display commands are sent while the relay is off
- Switching the relay on cancels a pending power-off countdown

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/README.md

@@ -26,7 +26,13 @@ BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent over `S
   - `FlipDotDisplayClock`: clock on, or the Nintendo logo when off
   - `FlipDotDisplayPower`: turns the display off through the relay
 - **Relay on GPIO 13** switches the power for both the panels and the Nano. `LOW` turns it
-  on. After switching on, the ESP waits 3 s so the Nano can boot before frames are sent.
+  on.
+  - **On:** the ESP waits for the Nano's `READY` message, at most 3 s
+    (`NANO_BOOT_TIMEOUT`), before it sends frames. The relay stays on while it is needed;
+    the ESP only waits when it actually switches the relay on.
+  - **Off:** the ESP stops sending and releases its TX pin (`Serial.end()` and the pin set
+    to input). An idle UART line is high and would otherwise power the Nano through its RX
+    pin.
 - **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
   off at night or when the display is powered off.
 

src/esp32/libs.h

@@ -44,7 +44,11 @@ HASwitch powerSwitch("FlipDotDisplayPower");
 bool power = true;
 
 const int led1 = BUILTIN_LED; // Pin of the LED
-const int relay = 13; // Pin of the relay
+const int relay = 13; // Pin of the relay, powers the panels and the Nano (LOW = on)
+const int nano_tx = 1; // UART0 TX, wired to the Nano RX
+bool relay_on = false;
+// Max wait for the Nano's READY after power on (also the wait when Nano TX is not wired to our RX)
+const uint32_t NANO_BOOT_TIMEOUT = 3000;
 
 int hour = 0;
 int minute = 0;

src/esp32/main.cpp

@@ -5,6 +5,60 @@ void toggleLED(void *parameter);
 uint32_t power_off_delay;
 int triggered_power_off = false;
 int turn_hassio_power_off = false;
+
+// Wait until the Nano reports READY after booting, or until NANO_BOOT_TIMEOUT
+void waitForNano()
+{
+  char line[16];
+  uint8_t len = 0;
+  uint32_t start = millis();
+  while (millis() - start < NANO_BOOT_TIMEOUT)
+  {
+    while (Serial.available())
+    {
+      char c = Serial.read();
+      if (c == '\n')
+      {
+        line[len] = '\0';
+        if (strstr(line, "READY"))
+          return;
+        len = 0;
+      }
+      else if (len < sizeof(line) - 1)
+        line[len++] = c;
+    }
+    delay(10);
+  }
+}
+
+void relayOn()
+{
+  // cancel a pending power off countdown
+  triggered_power_off = false;
+
+  if (relay_on)
+    return;
+
+  Serial.begin(115200);
+  // drop old data (e.g. ERR replies) so only a new READY counts
+  while (Serial.available())
+    Serial.read();
+
+  digitalWrite(relay, LOW);
+  waitForNano();
+  relay_on = true;
+}
+
+void relayOff()
+{
+  digitalWrite(relay, HIGH);
+  relay_on = false;
+
+  // An idle UART TX line is high and would power the Nano through its RX pin
+  Serial.flush();
+  Serial.end();
+  pinMode(nano_tx, INPUT);
+}
 void turnPowerOff(uint32_t delay = 30000, bool _turn_hassio_power_off = false)
 {
     if(triggered_power_off == false)
@@ -25,7 +79,7 @@ void updatePower()
         triggered_power_off = false;
 
         // OFF
-        digitalWrite(relay, HIGH);
+        relayOff();
 
         if(turn_hassio_power_off)
         {
@@ -46,10 +100,13 @@ void onPowerSwitchStateChanged(bool state, HASwitch *s)
 
   if(power == false)
   {
-      display.fillScreen(0);
-      display.display();
+      if (relay_on)
+      {
+        display.fillScreen(0);
+        display.display();
 
-      delay(1000);
+        delay(1000);
+      }
 
       turnPowerOff(0);
     
@@ -230,9 +287,7 @@ void setup()
   otaPanicMode = false;
 
   pinMode(relay, OUTPUT);
-  digitalWrite(relay, LOW);
-
-  delay(4000);
+  relayOn();
 
   display.begin(&Serial);
   // display.setTiming(200);
@@ -379,11 +434,7 @@ void drawRotatedBitmap(int16_t x, int16_t y, const uint8_t *bitmap, int16_t w, i
 void turnPowerOn(bool overide = false)
 {
   if(power == true || overide == true)
-  {
-    // ON
-    digitalWrite(relay, LOW);
-    delay(3000);
-  }
+    relayOn();
 }
 
 void loop()
@@ -434,8 +485,11 @@ void loop()
     
       if(hour <= 7 || (hour >= 23 && minute > 30))
       {
-        display.fillScreen(0);
-        display.display();
+        if (relay_on)
+        {
+          display.fillScreen(0);
+          display.display();
+        }
 
         turnPowerOff();
       }
@@ -446,28 +500,32 @@ void loop()
       {
         turnPowerOn();
 
-        int xoffset = 3;
+        // power can be off (Home Assistant power switch): then nothing is sent
+        if (relay_on)
+        {
+          int xoffset = 3;
 
-        display.fillScreen(0);
-        // display.setTextSize(2);
-        display.setTextSize(1);
-        // display.setCursor(7, 3);
-        display.setCursor(xoffset + 0, 19 - 2);
-        display.setTextWrap(false);
-        // display.print(t);
+          display.fillScreen(0);
+          // display.setTextSize(2);
+          display.setTextSize(1);
+          // display.setCursor(7, 3);
+          display.setCursor(xoffset + 0, 19 - 2);
+          display.setTextWrap(false);
+          // display.print(t);
 
-        // if(minute % 2 == 0)
-        if(hour >= 17 || hour <= 6)
-          display.drawBitmap(xoffset + 0, 0, yawning_pikachu_bitmap_yawning_pikachu_black_border, 35, 19, 1);
-        else
-          display.drawBitmap(xoffset + 0, 0, pikachu_bitmap_pikachu_black_border, 35, 19, 1);
+          // if(minute % 2 == 0)
+          if(hour >= 17 || hour <= 6)
+            display.drawBitmap(xoffset + 0, 0, yawning_pikachu_bitmap_yawning_pikachu_black_border, 35, 19, 1);
+          else
+            display.drawBitmap(xoffset + 0, 0, pikachu_bitmap_pikachu_black_border, 35, 19, 1);
 
-        display.drawBitmap(xoffset + 35, 0, clock_thought_bubble_bitmap_clock_thought_bubble, 46, 19, 1);
+          display.drawBitmap(xoffset + 35, 0, clock_thought_bubble_bitmap_clock_thought_bubble, 46, 19, 1);
         
-        display.setCursor(xoffset + 43, (19/2) - 5);
-        display.print(timeStringBuff);
+          display.setCursor(xoffset + 43, (19/2) - 5);
+          display.print(timeStringBuff);
       
-        display.display();
+          display.display();
+        }
       }
       else
       {
@@ -476,34 +534,37 @@ void loop()
     }
     else
     {
-      display.fillScreen(0);
-      display.drawBitmap(0, 0, nintendo_bitmap_Nintendo_new, 84, 19, 1);
+      if (relay_on)
+      {
+        display.fillScreen(0);
+        display.drawBitmap(0, 0, nintendo_bitmap_Nintendo_new, 84, 19, 1);
 
-      // display.drawBitmap(0, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
-      // display.drawBitmap(18, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
-      // display.drawBitmap(18*2, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
-      // display.drawBitmap(18*3, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
-      // display.drawBitmap(18*4, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
+        // display.drawBitmap(0, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
+        // display.drawBitmap(18, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
+        // display.drawBitmap(18*2, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
+        // display.drawBitmap(18*3, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
+        // display.drawBitmap(18*4, 0, nintendo_bitmap_panda_inverted, 17, 18, 1);
 
-      // drawRotatedBitmap(0, 0, nintendo_bitmap_Nintendo_new, 17, 18, 1, 90);
+        // drawRotatedBitmap(0, 0, nintendo_bitmap_Nintendo_new, 17, 18, 1, 90);
 
-      // Text inside border.
-      // display.drawBitmap(0, 0, nintendo_bitmap_nintendo_border, 84, 19, 1);
-      // display.setFont();
-      // display.setTextSize(1);
-      // display.setTextWrap(false);
-      // display.setCursor(5, 4);
-      // display.print("Nintendo");
+        // Text inside border.
+        // display.drawBitmap(0, 0, nintendo_bitmap_nintendo_border, 84, 19, 1);
+        // display.setFont();
+        // display.setTextSize(1);
+        // display.setTextWrap(false);
+        // display.setCursor(5, 4);
+        // display.print("Nintendo");
 
-      // display.setFont();
-      // // display.setTextSize(2);
-      // display.setTextSize(1);
-      //  display.setCursor(0, 0);
-      // display.setTextWrap(true);
-      // // display.print(t);
-      // display.print("Carla & Jan   Kragt");
+        // display.setFont();
+        // // display.setTextSize(2);
+        // display.setTextSize(1);
+        //  display.setCursor(0, 0);
+        // display.setTextWrap(true);
+        // // display.print(t);
+        // display.print("Carla & Jan   Kragt");
 
-      display.display();
+        display.display();
+      }
 
       turnPowerOff();
     }

src/nano/README.md

@@ -8,8 +8,11 @@ logic.
 
 ## Behaviour
 
-- **Boot:** the Nano loads the flip time from EEPROM and flips the whole display on and
-  then off.
+- **Boot:** the Nano loads the flip time from EEPROM, sets up the pins and sends `READY`.
+  It doesn't flip any dots at boot. The relay powers the Nano, so it boots every time the
+  display is switched on.
+- **First `display()` after boot** redraws every dot, because the Nano doesn't know which
+  state the dots were left in.
 - **`display()`:** compares the new buffer with the last one that was shown and strobes
   only the dots that changed.
 - **Every 2 minutes** (`REFRESH_TIME`) it does a full redraw to fix dots that got stuck.
@@ -47,7 +50,10 @@ skipped and the next line starts clean. Coordinates are 2-digit **hex**. Color i
 | `R` | `R` | Print the buffer as ASCII art on the serial port |
 | `T` | `Tnnn` | Set the flip pulse length in µs (decimal, 50–2000), stored in EEPROM |
 
-Invalid commands get a reply that starts with `ERR:` on the Nano's TX pin.
+Replies on the Nano's TX pin:
+
+- `READY` after booting. The ESP waits for it before sending the first frame.
+- `ERR: ...` for invalid commands.
 
 Example (what the ESP32 sends for one pixel and a refresh):
 

src/nano/main.cpp

@@ -265,6 +265,9 @@ void setup()
 
 	last_refresh = millis();
 	last_rx = millis();
+
+	// Tells the ESP that frames can be sent (the relay powers the Nano, so this happens at every power on)
+	Serial.println(F("READY"));
 }
 
 void loop()