No white flash at power-on on the tinySA ULTRA (ZS405)

Report: switching an Ultra (ZS405) on showed a momentary all white screen,
an Ultra+ (ZS406, ZS407) did not.

- The backlight came on before ili9341_init(), so the panel was lit in
  reset and during its init; the ZS405 panel shows white then (the
  ZS406/ZS407 panels, the ones with display inversion, stay black). It
  now comes on at DEFAULT_BRIGHTNESS once the screen is cleared.
- That left a very brief flash: the backlight came on before the panel
  had settled. The init now waits the 120 ms the controller needs after
  SLEEP_OUT before DISPLAY_ON (LCD_WAKE_UP_MS). Only with
  __LCD_BRIGHTNESS__: without backlight control a longer wait would
  only show the uninitialised panel longer, so the BASIC is unchanged.

Tested on a ZS405 (HW V0.4.5.1) with the ST-Link: DAC2 at 0 darkens the
backlight, the backlight is dark with the CPU held in reset, the
backlight switched on 2 s after the init shows no flash, and with this
build power-ons show no flash. Start-up 120 ms longer; ULTRA +12 bytes,
BASIC binary unchanged in size.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ili9341.c

@@ -504,6 +504,12 @@ void lcd_setBrightness(uint16_t b){
 }
 #endif
 
+#ifdef __LCD_BRIGHTNESS__
+#define LCD_WAKE_UP_MS  120   // after sleep out the panel supplies settle, the backlight comes on after the init
+#else
+#define LCD_WAKE_UP_MS  2     // backlight not controlled: a longer wait only shows the uninitialised panel longer
+#endif
+
 void ili9341_init(void)
 {
   LCD_DC_DATA;
@@ -513,8 +519,8 @@ void ili9341_init(void)
   const uint8_t *p;
   for (p = LCD_INIT; *p; ) {
     send_command(p[0], p[1], &p[2]);
+    chThdSleepMilliseconds(p[0] == ILI9341_SLEEP_OUT ? LCD_WAKE_UP_MS : 2);
     p += 2 + p[1];
-    chThdSleepMilliseconds(2);
   }
 #ifdef TINYSA4
   if ((hwid % 100) >= 2) {

main.c

@@ -3550,9 +3550,6 @@ int main(void)
   adc_init();
   PULSE
   DAC->CR|= DAC_CR_EN1 | DAC_CR_EN2; // Use DAC: CH1 and CH2
-  #ifdef  __LCD_BRIGHTNESS__
-    lcd_setBrightness(DEFAULT_BRIGHTNESS);
-  #endif
   DAC->DHR12R1 = 0;                  // Setup DAC: CH1 value
 
   /*
@@ -3563,6 +3560,11 @@ int main(void)
 #endif
   ili9341_init();
   PULSE
+  #ifdef  __LCD_BRIGHTNESS__
+    // Backlight on only now the screen is cleared: a panel in reset or before its init
+    // shows white (the ZS405 panel, ZS406/ZS407 stay black)
+    lcd_setBrightness(DEFAULT_BRIGHTNESS);
+  #endif
 
 #ifdef TINYSA4
   ili9341_set_foreground(LCD_FG_COLOR);