add modbus (not tested yet)

F1:F103/FX3U/usart.h

@@ -18,6 +18,8 @@
 
 #pragma once
 
+#include <stdint.h>
+
 // input and output buffers size
 #define UARTBUFSZI  (64)
 #define UARTBUFSZO  (128)
@@ -25,8 +27,6 @@
 #define usartrx()  (usart_linerdy)
 #define usartovr() (usart_bufovr)
 
-extern volatile int usart_txrdy;
-
 int usart_transmit();
 void usart_setup(uint32_t speed);
 int usart_getline(char **line);