diff --git a/homework5/.eide/eide.json b/homework5/.eide/eide.json
index a837da6..ff0053b 100644
--- a/homework5/.eide/eide.json
+++ b/homework5/.eide/eide.json
@@ -20,7 +20,8 @@
     "Debug": {
       "excludeList": [
         "src/test.c",
-        "src/test1.c"
+        "src/test1.c",
+        "src/test2.c"
       ],
       "toolchain": "Keil_C51",
       "compileConfig": {
diff --git a/homework5/src/test3.c b/homework5/src/test3.c
new file mode 100644
index 0000000..a02d0e1
--- /dev/null
+++ b/homework5/src/test3.c
@@ -0,0 +1,83 @@
+// test3.c
+// 让数码管动态显示秒表，但只显示有效位，高位的0不显示
+// display the stopwatch on the Seg. but only display the valid digits.
+
+/*
+ * how it work?
+ * 1. based on test2.c.
+ * 2. add check to the buffer.
+ * 3. only this unit is not 0 OR the previous unit is not 0, log the number in the buffer.
+ * 4. so this is my niubility code.
+ * 5. I'm so proud of myself.
+ * 6. msksbr NB!!!
+ * 7. msksbr NB!!!
+ * 8. msksbr NB!!!
+ */
+*/
+
+#include "homework5.h"
+    unsigned char LedBuff[6] = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}; // buffer to store the seg chars.
+
+void test3()
+{
+    unsigned char second       = 0; // to count the seconds.
+    unsigned char minute       = 0; // to count the minutes.
+    unsigned char hour         = 0; // to count the hours.
+    unsigned char i            = 0; // to count the times.
+    unsigned long time_counter = 0;
+
+    while (1) {
+        for (i = 0; i < 6; i++) {
+            // display number
+            set38Transistors(i);
+            P2 = LedBuff[i];
+            // display dot
+            if (i == 2 || i == 4) {
+                // 2nd dot between the minutes and seconds.
+                // 4th dot between the hours and minutes.
+                BUS8DP = 0; // display the dot.
+            } else {
+                BUS8DP = 1; // hide the dot on other places.
+            }
+
+            timer(1); // delay 1ms.
+            time_counter++;
+
+            // update time when 1000ms passed.
+            if (time_counter >= 1000) {
+                time_counter = 0; // reset the counter.
+                second++;         // count the seconds.
+                // solve unit overflow.
+                // second overflow
+                if (second >= 60) {
+                    second = 0; // reset the second.
+                    minute++;   // count the minutes.
+                }
+                // minute overflow.
+                if (minute >= 60) {
+                    minute = 0; // reset the minute.
+                    hour++;     // count the hours.
+                }
+                // everything overflow.
+                if (hour >= 99 && second >= 59 && minute >= 59) {
+                    hour   = 0; // reset the hour.
+                    second = 0; // reset the second.
+                    minute = 0; // reset the minute.
+                }
+            }
+            // log the number in the buffer.
+            // only log the valid digits.
+            LedBuff[5] = hour >= 10 ? LedChar[hour / 10] : 0XFF;                     // log the hour tens.
+            LedBuff[4] = hour > 0 ? LedChar[hour % 10] : 0XFF;                       // log the hour units.
+            LedBuff[3] = (minute >= 10 || hour > 0) ? LedChar[minute / 10] : 0XFF;   // log the minute tens.
+            LedBuff[2] = minute > 0 ? LedChar[minute % 10] : 0XFF;                   // log the minute units.
+            LedBuff[1] = (second >= 10 || minute > 0) ? LedChar[second / 10] : 0XFF; // log the second tens.
+            LedBuff[0] = LedChar[second % 10];                                       // log the second units.
+        }
+    }
+}
+
+void main()
+{
+    test3();
+}
\ No newline at end of file