diff --git a/homework4/.eide/eide.json b/homework4/.eide/eide.json
index 0f7e955..00cb4bf 100644
--- a/homework4/.eide/eide.json
+++ b/homework4/.eide/eide.json
@@ -21,7 +21,8 @@
       "excludeList": [
         "src/test1.c",
         "src/test2.c",
-        "src/test3.c"
+        "src/test3.c",
+        "src/test4.c"
       ],
       "toolchain": "Keil_C51",
       "compileConfig": {
diff --git a/homework4/src/test5.c b/homework4/src/test5.c
new file mode 100644
index 0000000..d5a181f
--- /dev/null
+++ b/homework4/src/test5.c
@@ -0,0 +1,38 @@
+// test5.c
+// 数码管静态显示秒表的倒计时；
+#include "homework4.h"
+
+unsigned char cnt    = 0; // 定时器溢出计数
+unsigned char second = 0; // 秒计数
+
+void test5()
+{
+    set_38_2SEG();
+    TMOD = 0x01;
+    TH0  = 0x3C;
+    TL0  = 0xB0;
+    TR0  = 1;
+
+    // 初始化时立即显示9
+    second = 9;
+    show_number(second);
+
+    while (1) {
+        if (TF0 == 1) {
+            TF0 = 0;
+            TH0 = 0x3C;
+            TL0 = 0xB0;
+            if (++cnt >= 20) {
+                cnt = 0;
+
+                second = (second > 0) ? --second : 9;
+                show_number(second);
+            }
+        }
+    }
+}
+
+void main()
+{
+    test5();
+}
\ No newline at end of file