Fix 3.5" black screen: drive backlight on GPIO27, not GPIO21.

GPIO21 is GT911 INT on 3248S035C; PWM on the wrong pin left the panel dark.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -139,3 +139,4 @@ git reset --hard v1.1-case-snaps
 | 2026-10-17 | 顶栏改单行，避免 up 文案被表盘面板盖住 |
 | 2026-10-17 | 归档预编译固件到 `firmware/cyd_28r/`，附一键刷写脚本 |
 | 2026-10-25 | 3.5" 表盘读数：数值+单位整体居中，数值变宽后不再盖住单位 |
+| 2026-10-25 | 修复 3.5" 黑屏：背光改回 GPIO27（GPIO21 为 GT911 INT） |

firmware/cyd_35r/VERSION.txt

@@ -1,15 +1,11 @@
 name: CYD temperature (cyd_35r)
-git: de177b2
+git: 0a1b612
 date: 2026-10-25
-board: ESP32-3248S035 (ST7796)
+board: ESP32-3248S035 (ST7796 + GT911)
 flash_size: 4MB
 offsets:
   0x1000  bootloader.bin
   0x8000  partitions.bin
   0xe000  boot_app0.bin
   0x10000 firmware.bin
-sha256:
-  bootloader.bin  3D234A7471F67B013686DABD4DEE7C1FA915C9928463616A94BC9297ACF1ABF8
-  boot_app0.bin   F94C5D786A7A8FAB06AC5D10E33BF37711A6697636DC037559EA19CC410A17F0
-  firmware.bin    A8033223DDE79874B4CA41AFE3DBB247938184123FB66BCAA6EAF7F5C1D32516
-  partitions.bin  0A8B5720E7B77FF11F1462458C3A509DEE79224E5279898F26D6A2E3AE0517B7
+note: TFT_BL=GPIO27

firmware/export_from_build.ps1

@@ -73,8 +73,8 @@ if (Test-Path $metaPath) {
   try {
     $meta = Get-Content $metaPath -Raw | ConvertFrom-Json
     $meta.boardId = $boardIdHint
-    if ($Env -eq "cyd_35r" -and $meta.name -notmatch "3\.5") {
-      $meta.name = ($meta.name -replace "\s*$","") + " (3.5\")"
+    if ($Env -eq "cyd_35r" -and $meta.name -notmatch '3\.5') {
+      $meta.name = ($meta.name -replace '\s*$','') + ' (3.5")'
     }
     $meta | ConvertTo-Json -Depth 8 | Set-Content -Encoding utf8 $metaPath
   } catch {

platformio.ini

@@ -69,6 +69,7 @@ build_flags =
 ; Optional 3.5" CYD ESP32-3248S035C (ST7796 + GT911 capacitive). Default remains cyd_28r.
 ; Build: pio run -e cyd_35r
 ; Keep TOUCH_CS=-1 — GPIO33 is GT911 SDA, not resistive touch CS.
+; TFT_BL=GPIO27 (not 21 — 21 is GT911 INT on C boards).
 [env:cyd_35r]
 lib_deps =
   lvgl/lvgl@9.2.2
@@ -83,7 +84,7 @@ build_flags =
   -DTFT_WIDTH=320
   -DTFT_HEIGHT=480
   -UTFT_BL
-  -DTFT_BL=21
+  -DTFT_BL=27
   -DCYD_BOARD_3248S035=1
   -DSUPPORT_TRANSACTIONS
   -DTFT_RGB_ORDER=TFT_BGR

src/main.cpp

@@ -1,6 +1,7 @@
 /**
  * CYD ESP32-2432S028R — LVGL 温湿度仪表盘
- * DHT11 DATA -> GPIO22 (3.5 TFT_BL on GPIO21)
+ * DHT11 DATA -> GPIO22
+ * Backlight: 2.8" GPIO21, 3.5" GPIO27 (GT911 INT is GPIO21 — not BL)
  */
 
 #include <Arduino.h>
@@ -12,7 +13,7 @@
 
 #define DHT_PIN 22
 #if defined(CYD_BOARD_3248S035)
-#define TFT_BL_PIN 21
+#define TFT_BL_PIN 27
 #else
 #define TFT_BL_PIN 21
 #endif

src/touch.cpp

@@ -76,9 +76,8 @@ static void gt_hw_reset(bool addr_0x14) {
   delay(5);
   digitalWrite(CYD_GT911_INT, LOW);
   delay(50);
-  /* INT shares TFT_BL on 3.5 — keep backlight HIGH after address select. */
-  pinMode(CYD_GT911_INT, OUTPUT);
-  digitalWrite(CYD_GT911_INT, HIGH);
+  /* Backlight is GPIO27 — leave INT as input for polling / optional IRQ. */
+  pinMode(CYD_GT911_INT, INPUT);
   delay(50);
 }