add ultrasonic sending interval constant macro definition

src/main.c

@@ -13,6 +13,7 @@
 #include "ultrasonic_driver.h"
 
 #define ULTRASONIC_CYCLES     0x05U
+#define ULTRASONIC_TRAN_US    50  // (ms)
 
 /*!
     \brief      main function
@@ -47,7 +48,7 @@ int main(void)
 
     while(1)
     {
-        delay_ms(50);
+        delay_ms(ULTRASONIC_TRAN_US);
         ultrasonic_pwm_out_cycles(ULTRASONIC_CYCLES);
         // printf("Send ultra sonic driver signal!\r\n");
     }