可以使用串口1打印数据

app/MDK-ARM/app.uvoptx

@@ -133,7 +133,56 @@
           <Name>-U-O142 -O2254 -S0 -C0 -TO18 -TC10000000 -TP21 -TDS8007 -TDT0 -TDC1F -TIEFFFFFFFF -TIP8 -FO7 -FD20000000 -FC1000 -FN1 -FF0STM32L4xx_128.FLM -FS08000000 -FL020000 -FP0($$Device:STM32L432KBUx$CMSIS\Flash\STM32L4xx_128.FLM)</Name>
         </SetRegEntry>
       </TargetDriverDllRegistry>
-      <Breakpoint/>
+      <Breakpoint>
+        <Bp>
+          <Number>0</Number>
+          <Type>0</Type>
+          <LineNumber>80</LineNumber>
+          <EnabledFlag>1</EnabledFlag>
+          <Address>0</Address>
+          <ByteObject>0</ByteObject>
+          <HtxType>0</HtxType>
+          <ManyObjects>0</ManyObjects>
+          <SizeOfObject>0</SizeOfObject>
+          <BreakByAccess>0</BreakByAccess>
+          <BreakIfRCount>0</BreakIfRCount>
+          <Filename>../Core/Src/main.c</Filename>
+          <ExecCommand></ExecCommand>
+          <Expression></Expression>
+        </Bp>
+        <Bp>
+          <Number>1</Number>
+          <Type>0</Type>
+          <LineNumber>87</LineNumber>
+          <EnabledFlag>1</EnabledFlag>
+          <Address>0</Address>
+          <ByteObject>0</ByteObject>
+          <HtxType>0</HtxType>
+          <ManyObjects>0</ManyObjects>
+          <SizeOfObject>0</SizeOfObject>
+          <BreakByAccess>0</BreakByAccess>
+          <BreakIfRCount>0</BreakIfRCount>
+          <Filename>../Core/Src/main.c</Filename>
+          <ExecCommand></ExecCommand>
+          <Expression></Expression>
+        </Bp>
+        <Bp>
+          <Number>2</Number>
+          <Type>0</Type>
+          <LineNumber>107</LineNumber>
+          <EnabledFlag>1</EnabledFlag>
+          <Address>0</Address>
+          <ByteObject>0</ByteObject>
+          <HtxType>0</HtxType>
+          <ManyObjects>0</ManyObjects>
+          <SizeOfObject>0</SizeOfObject>
+          <BreakByAccess>0</BreakByAccess>
+          <BreakIfRCount>0</BreakIfRCount>
+          <Filename>../Core/Src/main.c</Filename>
+          <ExecCommand></ExecCommand>
+          <Expression></Expression>
+        </Bp>
+      </Breakpoint>
       <Tracepoint>
         <THDelay>0</THDelay>
       </Tracepoint>

usersrc/usermain.c

@@ -5,18 +5,13 @@
 #include "main.h"
 #include "usart.h"
 
-int fputc(int ch, FILE *stream)
-{
-  uint8_t c = ch;
-  HAL_UART_Transmit(&huart1, &c, 1, 100);
-  return ch;
-}
-
 void user_main()
 {
+  uint8_t c = 'o';
 
   while (1)
   {
-    printf("ok\r\n");
+    HAL_UART_Transmit(&huart1, &c, 1, 100);
+    HAL_Delay(100);
   }
 }