diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..62514e8
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+*
+.*
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..5fef702
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,11 @@
+all:
+	mkdir -p build
+	sdcc src/main.c -o build/
+
+clean:
+	rm -rf build
+
+PORT ?= /dev/ttyUSB0
+
+flash: all
+	stcgal build/main.ihx -p $(PORT)
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..1cc9f84
--- /dev/null
+++ b/README.md
@@ -0,0 +1,32 @@
+Digital Timer Relay
+===================
+
+Digital Timer Relay based on STC89C51RC chip.
+
+## Dependencies
+
+```
+sdcc
+stcgal
+```
+
+## Preparing
+
+### Obtain the source
+
+```
+git clone --recursive https://github.com/redchenjs/digital_timer_relay_mcs51.git
+```
+
+### Update an existing repository
+
+```
+git pull
+git submodule update --init --recursive
+```
+
+## Building
+
+```
+make flash
+```
diff --git a/src/main.c b/src/main.c
new file mode 100644
index 0000000..02f3aaf
--- /dev/null
+++ b/src/main.c
@@ -0,0 +1,194 @@
+/*
+ * main.c
+ *
+ *  Created on: 2020-12-31 06:26
+ *      Author: Jack Chen <redchenjs@live.com>
+ */
+
+#include <8051.h>
+#include <stdint.h>
+
+#define RELAY   P1_7
+#define SEG_DAT P0
+#define KEY_DAT P1
+#define SEG_SEL P2
+
+// STC89C51RC 12T@12.000MHz
+#define FOSC  (12000000UL)
+#define T1MS  (65536 - FOSC / 12 / 1000 * 1)
+#define T50MS (65536 - FOSC / 12 / 1000 * 50 + 15)
+
+// Common-Anode Display
+const uint8_t table[] = {0xc0, 0xf9, 0xa4, 0xb0, 0x99, 0x92, 0x82, 0xf8, 0x80, 0x90};
+
+static uint8_t hour = 0, minute = 0, second = 0;
+static uint8_t mode = 0, count_10ms = 0, count_1s = 0;
+
+void tf0_isr(void) __interrupt TF0_VECTOR
+{
+    TL0 = T50MS & 0xff;
+    TH0 = T50MS >> 8;
+
+    if (++count_1s == 20) {
+        count_1s = 0;
+
+        if (hour || minute || second) {
+            RELAY = 0;
+
+            if (second-- == 0) {
+                second = 59;
+
+                if (minute-- == 0) {
+                    minute = 59;
+
+                    if (hour-- == 0) {
+                        hour = 0;
+                    }
+                }
+            }
+        } else {
+            RELAY = 1;
+        }
+    }
+}
+
+void tf1_isr(void) __interrupt TF1_VECTOR
+{
+    static uint8_t digit = 0, key_p = 0, key_n = 0;
+
+    TL1 = T1MS & 0xff;
+    TH1 = T1MS >> 8;
+
+    if (++count_10ms == 10) {
+        count_10ms = 0;
+
+        key_n = KEY_DAT & 0x07;
+
+        if ((key_p & 0x01) && !(key_n & 0x01)) {
+            if (mode == 0) {
+                TR0 = 0;
+
+                mode = 6;
+                count_1s = 0;
+            } else if (--mode == 0) {
+                TR0 = 1;
+            }
+        }
+
+        if ((key_p & 0x02) && !(key_n & 0x02)) {
+            switch (mode) {
+                case 1:
+                    second += 60 + 1;
+                    break;
+                case 2:
+                    second += 60 + 10;
+                    break;
+                case 3:
+                    minute += 60 + 1;
+                    break;
+                case 4:
+                    minute += 60 + 10;
+                    break;
+                case 5:
+                    hour += 100 + 1;
+                    break;
+                case 6:
+                    hour += 100 + 10;
+                    break;
+                default:
+                    break;
+            }
+
+            hour %= 100;
+            minute %= 60;
+            second %= 60;
+        }
+
+        if ((key_p & 0x04) && !(key_n & 0x04)) {
+            switch (mode) {
+                case 1:
+                    second += 60 - 1;
+                    break;
+                case 2:
+                    second += 60 - 10;
+                    break;
+                case 3:
+                    minute += 60 - 1;
+                    break;
+                case 4:
+                    minute += 60 - 10;
+                    break;
+                case 5:
+                    hour += 100 - 1;
+                    break;
+                case 6:
+                    hour += 100 - 10;
+                    break;
+                default:
+                    break;
+            }
+
+            hour %= 100;
+            minute %= 60;
+            second %= 60;
+        }
+
+        key_p = key_n;
+    }
+
+    SEG_SEL = 0xff;
+
+    switch (digit) {
+        case 0:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[second % 10];
+            SEG_SEL = 0xfe;
+            break;
+        case 1:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[second / 10];
+            SEG_SEL = 0xfd;
+            break;
+        case 2:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[minute % 10];
+            SEG_SEL = 0xfb;
+            break;
+        case 3:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[minute / 10];
+            SEG_SEL = 0xf7;
+            break;
+        case 4:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[hour % 10];
+            SEG_SEL = 0xef;
+            break;
+        case 5:
+            SEG_DAT = mode && mode != (digit + 1) ? 0xbf : table[hour / 10];
+            SEG_SEL = 0xdf;
+            break;
+        default:
+            break;
+    }
+
+    if (++digit == 6) {
+        digit = 0;
+    }
+}
+
+int main(void)
+{
+    TMOD = 0x11;
+
+    TL0 = T50MS & 0xff;
+    TH0 = T50MS >> 8;
+    TR0 = 1;
+    PT0 = 1;
+    ET0 = 1;
+
+    TL1 = T1MS & 0xff;
+    TH1 = T1MS >> 8;
+    TR1 = 1;
+    PT1 = 0;
+    ET1 = 1;
+
+    EA = 1;
+
+    while (1);
+}