diff --git a/homework4/src/homework4.h b/homework4/src/homework4.h
index 5b0803b..ef5919a 100644
--- a/homework4/src/homework4.h
+++ b/homework4/src/homework4.h
@@ -43,4 +43,125 @@ void set_38_2SEG()
     ADDR2 = 1;
     ADDR3 = 1;
     ENLED = 0;
+}
+
+// 7SEG number display function:
+// 0-9
+void show_number(unsigned char number)
+{
+    switch (number) {
+        case 0: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 0;
+            BUS6F  = 0;
+            BUS7G  = 1;
+            BUS8DP = 1;
+            break;
+        }
+        case 1: {
+            BUS1A  = 1;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 1;
+            BUS5E  = 1;
+            BUS6F  = 1;
+            BUS7G  = 1;
+            BUS8DP = 1;
+            break;
+        }
+        case 2: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 1;
+            BUS4D  = 0;
+            BUS5E  = 0;
+            BUS6F  = 1;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 3: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 1;
+            BUS6F  = 1;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 4: {
+            BUS1A  = 1;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 1;
+            BUS5E  = 1;
+            BUS6F  = 0;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 5: {
+            BUS1A  = 0;
+            BUS2B  = 1;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 1;
+            BUS6F  = 0;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 6: {
+            BUS1A  = 0;
+            BUS2B  = 1;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 0;
+            BUS6F  = 0;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 7: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 1;
+            BUS5E  = 1;
+            BUS6F  = 1;
+            BUS7G  = 1;
+            BUS8DP = 1;
+            break;
+        }
+        case 8: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 0;
+            BUS6F  = 0;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        case 9: {
+            BUS1A  = 0;
+            BUS2B  = 0;
+            BUS3C  = 0;
+            BUS4D  = 0;
+            BUS5E  = 1;
+            BUS6F  = 0;
+            BUS7G  = 0;
+            BUS8DP = 1;
+            break;
+        }
+        default: {
+            break;
+        }
+    }
 }
\ No newline at end of file
diff --git a/homework4/src/test3.c b/homework4/src/test3.c
index 942e525..5d02048 100644
--- a/homework4/src/test3.c
+++ b/homework4/src/test3.c
@@ -3,9 +3,35 @@
 
 #include "homework4.h"
 
+unsigned char cnt    = 0; // 定时器溢出计数
+unsigned char second = 0; // 秒计数
+
 void test3()
 {
     set_38_2SEG();
+    TMOD = 0x01;
+    TH0  = 0x3C;
+    TL0  = 0xB0;
+    TR0  = 1;
+
+    // 初始化时立即显示0
+    second = 0;
+    show_number(0);
+
+    while (1) {
+        if (TF0 == 1) {
+            TF0 = 0;
+            TH0 = 0x3C;
+            TL0 = 0xB0;
+            if (++cnt >= 20) {
+                cnt = 0;
+
+                second = (second < 9) ? ++second : 0;
+
+                show_number(second); // 调用显示函数
+            }
+        }
+    }
 }
 
 void main()