UART-to-UART-communication.md

@@ -40,7 +40,7 @@ For best results, do not yet connect the UART pins.
 In the main loop, add some code (and fill in the missing details like includes):
 ```c
   /* Infinite loop */
-  /* USER CODE BEGIN WHILE */(&huart2, (void*)rx_data, sizeof(rx_data));
+  /* USER CODE BEGIN WHILE */
   while (1)
   {
     char tx_data[MAX_LEN];
@@ -52,7 +52,7 @@ In the main loop, add some code (and fill in the missing details like includes):
     }
     tx_data[tx_len] = '\0';
 
-    printf("%s -> ", tx_data);
+    printf("%10s -> ", tx_data);
     fflush(stdout); // print immediately, don't buffer output until end of line ('\n')
 
     HAL_UART_Transmit(&huart1, (void*)tx_data, tx_len, 100);
@@ -66,3 +66,75 @@ In the main loop, add some code (and fill in the missing details like includes):
 `tx_len` will be initialized by a random int value between 1 and 10.
 The array `tx_data` will be filled with ASCII chars for the decimal digits 0 to 9.
 The `printf` and `fflush` function will do the logging. To receive the logging output on the ST-LINK host, [redirect stdout](redirect stdout) to UART2 output.
+The logging output should look like (keep in mind it is randomized, ymmv)
+```
+012345 ->     678901 ->         23 ->     456789 ->   01234567 ->     890123 ->   
+```
+A logic analyzer attached to UART1_TX and UART2_TX (logging output) should show a pattern like this:
+![LA_sending_only](uploads/4f8f80d11c0e6f7d860395c7085131b8/LA_sending_only.png)
+The leading spaces ' ' of the logging output are not fully visible.
+
+Receiving Data
+===
+For receiving in interrupt mode, the USART2 interrupt must be enabled in STM32CubeMX.
+![UART2_NVIC_Config](uploads/7c700b896e7d685c1b46a51762b9a83f/UART2_NVIC_Config.png)
+
+Define the receiving callback and the error callback in main.c:
+```c
+/* Private user code ---------------------------------------------------------*/
+/* USER CODE BEGIN 0 */
+
+#define MAX_LEN 20
+
+char rx_data[MAX_LEN];
+
+void HAL_UARTEx_RxEventCallback(UART_HandleTypeDef *huart, uint16_t len)
+{
+  HAL_GPIO_TogglePin(LD3_GPIO_Port, LD3_Pin);
+  rx_data[len] = '\0';
+  printf("%s\n", rx_data);
+  HAL_UARTEx_ReceiveToIdle_IT(&huart2, (void*)rx_data, sizeof(rx_data));
+}
+
+void HAL_UART_ErrorCallback(UART_HandleTypeDef *huart)
+{
+  volatile unsigned error = HAL_UART_GetError(huart);
+  printf("HAL_UART_ErrorCallback: %d\n", error);
+  Error_Handler();
+}
+
+/* USER CODE END 0 */
+```
+
+Trigger the very first receive right above the main while loop:
+```c
+  /* Infinite loop */
+  /* USER CODE BEGIN WHILE */
+  HAL_UARTEx_ReceiveToIdle_IT(&huart2, (void*)rx_data, sizeof(rx_data));
+  while (1)
+  {
+```
+
+Connect the UART2_RX pin to the UART1_TX pin and restart the code. 
+The LD3 LED should start blinking. 
+It toggles with each reception. 
+The logging output should look like:
+```
+        12 -> 12
+         3 -> 3
+       456 -> 456
+    789012 -> 789012
+     34567 -> 34567
+      8901 -> 8901
+   2345678 -> 2345678
+       901 -> 901
+     23456 -> 23456
+        78 -> 78
+```
+where the right half is printed at the receiver side.
+And the logic analyzer:
+![LA_sending_and_receiving](uploads/e0deca333743df5bdacf714d95759704/LA_sending_and_receiving.png)
+From left to right: 
+* printf at the transmitter side
+* transmission log "on the wire"
+* printf at the receiver side
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