diff --git a/homework4/homework4.pdsprj b/homework4/homework4.pdsprj
new file mode 100644
index 0000000..1c94be2
Binary files /dev/null and b/homework4/homework4.pdsprj differ
diff --git a/homework4/src/homework4.h b/homework4/src/homework4.h
new file mode 100644
index 0000000..8e701c5
--- /dev/null
+++ b/homework4/src/homework4.h
@@ -0,0 +1,22 @@
+// homework4.h
+
+/*
+ * this file defines the pin numbers for the LED and 38 transistors.
+ */
+
+#include <REG52.H>
+
+sbit ADDR0 = P1 ^ 0;
+sbit ADDR1 = P1 ^ 1;
+sbit ADDR2 = P1 ^ 2;
+sbit ADDR3 = P1 ^ 3;
+sbit ENLED = P1 ^ 4;
+
+sbit D1 = P2 ^ 7;
+sbit D2 = P2 ^ 6;
+sbit D3 = P2 ^ 5;
+sbit D4 = P2 ^ 4;
+sbit D5 = P2 ^ 3;
+sbit D6 = P2 ^ 2;
+sbit D7 = P2 ^ 1;
+sbit D8 = P2 ^ 0;
\ No newline at end of file
diff --git a/homework4/src/test1.c b/homework4/src/test1.c
new file mode 100644
index 0000000..8eb9065
--- /dev/null
+++ b/homework4/src/test1.c
@@ -0,0 +1,33 @@
+// test1.c
+// 通过定时器实现以0.5Hz的频率闪烁一个小灯
+
+#include "homework4.h"
+
+unsigned char cnt = 0; // log timer T0 overflow counts
+
+void main()
+{
+    ENLED = 0;
+    ADDR3 = 1;
+    ADDR2 = 1;
+    ADDR1 = 1;
+    ADDR0 = 0;
+
+    TMOD = 0x01; // timer T0 mode 1, 16-bit timer
+    TH0  = 0XB8;
+    TL0  = 0X00;
+    TR0  = 1; // timer T0 run
+
+    while (1) {
+        if (TF0 == 1) { // timer T0 overflow
+            TF0 = 0;    // clear timer T0 overflow flag
+            TH0 = 0xB8; // reload timer T0 value
+            TL0 = 0x00;
+            cnt++; // log timer T0 overflow counts
+            if (cnt >= 50) {
+                cnt = 0;    // clear log timer T0 overflow counts
+                D1 = ~D1; // toggle LED
+            }
+        }
+    }
+}
\ No newline at end of file