diff --git a/heart_rate_sensor_protocol.h b/heart_rate_sensor_protocol.h
index 5b00334..ce0da4e 100644
--- a/heart_rate_sensor_protocol.h
+++ b/heart_rate_sensor_protocol.h
@@ -24,8 +24,47 @@ typedef enum {
   kifyhrs_ecode_electrode_is_not_inserted = 9,
   kifyhrs_ecode_device_exception          = 10,
   kifyhrs_ecode_invalid_state             = 11,
+
+  kifyhrs_ecode_overtime         = 200,
+  kifyhrs_ecode_channel_is_close = 201,
+
 } ify_hrs_error_code_t;
 
+static inline const char *ify_hrs_error_code_to_string(ify_hrs_error_code_t code) {
+  switch (code) {
+    case kifyhrs_ecode_success:
+      return "success";
+    case kifyhrs_ecode_unkown_error:
+      return "unkown error";
+    case kifyhrs_ecode_cmd_not_support:
+      return "cmd not support";
+    case kifyhrs_ecode_illegal_parameter:
+      return "illegal parameter";
+    case kifyhrs_ecode_device_busy:
+      return "device busy";
+    case kifyhrs_ecode_hardware_error:
+      return "hardware error";
+    case kifyhrs_ecode_sensor_drop:
+      return "sensor drop";
+    case kifyhrs_ecode_no_record_find:
+      return "no record find";
+    case kifyhrs_ecode_parameter_error:
+      return "parameter error";
+    case kifyhrs_ecode_electrode_is_not_inserted:
+      return "electrode is not inserted";
+    case kifyhrs_ecode_device_exception:
+      return "device exception";
+    case kifyhrs_ecode_invalid_state:
+      return "invalid state";
+    case kifyhrs_ecode_overtime:
+      return "overtime";
+    case kifyhrs_ecode_channel_is_close:
+      return "channel is close";
+    default:
+      return "unkown error";
+  }
+}
+
 typedef enum {
   kifyhrs_pt_cmd           = 1,
   kifyhrs_pt_cmd_receipt   = 2,