feat(uart): simplifies UART example based on MODBUS standard (#11309)

* feat(uart): simplifies UART example based on MODBUS standard

* fix(uart): fixes a uart example typo

* feat(uart): replaces UART0 by Serial0 in the code

* ci(pre-commit): Apply automatic fixes

* fix(uart): typo error message in commentary

---------

Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>

Author: SuGlider

libraries/ESP32/examples/Serial/onReceiveExample/onReceiveExample.ino

@@ -5,20 +5,20 @@
    void HardwareSerial::onReceive(OnReceiveCb function, bool onlyOnTimeout = false)
 
    It is possible to register an UART callback function that will be called
-   every time that UART receives data and an associated interrupt is generated.
+   every time that UART receives data and an associated UART interrupt is generated.
 
-   In summary, HardwareSerial::onReceive() works like an RX Interrupt callback, that can be adjusted
-   using HardwareSerial::setRxFIFOFull() and HardwareSerial::setRxTimeout().
+   In summary, HardwareSerial::onReceive() works like an RX Interrupt callback, that
+   can be adjusted using HardwareSerial::setRxFIFOFull() and HardwareSerial::setRxTimeout().
 
-   OnReceive will be called, while receiving a stream of data, when every 120 bytes are received (default FIFO Full),
-   which may not help in case that the application needs to get all data at once before processing it.
-   Therefore, a way to make it work is by detecting the end of a stream transmission. This can be based on a protocol
-   or based on timeout with the UART line in idle (no data received - this is the case of this example).
+   In case that <onlyOnTimeout> is not changed or it is set to <false>, the callback function is
+   executed whenever any event happens first (FIFO Full or RX Timeout).
+   OnReceive will be called when every 120 bytes are received(default FIFO Full),
+   or when RX Timeout occurs after 1 UART symbol by default.
 
-   In some cases, it is necessary to wait for receiving all the data before processing it and parsing the
-   UART input. This example demonstrates a way to create a String with all data received from UART0 and
-   signaling it using a Mutex for another task to process it. This example uses a timeout of 500ms as a way to
-   know when the reception of data has finished.
+   This example demonstrates a way to create a String with all data received from UART0 only
+   after RX Timeout. This example uses an RX timeout of about 3.5 Symbols as a way to know
+   when the reception of data has finished.
+   In order to achieve it, the sketch sets <onlyOnTimeout> to <true>.
 
    The onReceive() callback is called whenever the RX ISR is triggered.
    It can occur because of two possible events:
@@ -34,90 +34,73 @@
 
    2- UART RX Timeout: it happens, based on a timeout equivalent to a number of symbols at
       the current baud rate. If the UART line is idle for this timeout, it will raise an interrupt.
-      This time can be changed by HardwareSerial::setRxTimeout(uint8_t rxTimeout)
+      This time can be changed by HardwareSerial::setRxTimeout(uint8_t rxTimeout).
+      <rxTimeout> is bound to the clock source.
+      In order to use it properly, ESP32 and ESP32-S2 shall set the UART Clock Source to APB.
 
    When any of those two interrupts occur, IDF UART driver will copy FIFO data to its internal
    RingBuffer and then Arduino can read such data. At the same time, Arduino Layer will execute
    the callback function defined with HardwareSerial::onReceive().
 
-   <bool onlyOnTimeout> parameter (default false) can be used by the application to tell Arduino to
-   only execute the callback when the second event above happens (Rx Timeout). At this time all
-   received data will be available to be read by the Arduino application. But if the number of
-   received bytes is higher than the FIFO space, it will generate an error of FIFO overflow.
-   In order to avoid such problem, the application shall set an appropriate RX buffer size using
+   <bool onlyOnTimeout> parameter can be used by the application to tell Arduino to only execute
+   the callback when Rx Timeout happens, by setting it to <true>.
+   At this time all received data will be available to be read by the Arduino application.
+   The application shall set an appropriate RX buffer size using
    HardwareSerial::setRxBufferSize(size_t new_size) before executing begin() for the Serial port.
-*/
 
-// this will make UART0 work in any case (using or not USB)
-#if ARDUINO_USB_CDC_ON_BOOT
-#define UART0 Serial0
-#else
-#define UART0 Serial
-#endif
+   MODBUS timeout of 3.5 symbol is based on these documents:
+   https://www.automation.com/en-us/articles/2012-1/introduction-to-modbus
+   https://minimalmodbus.readthedocs.io/en/stable/serialcommunication.html
+*/
 
 // global variable to keep the results from onReceive()
 String uart_buffer = "";
-// a pause of a half second in the UART transmission is considered the end of transmission.
-const uint32_t communicationTimeout_ms = 500;
-
-// Create a mutex for the access to uart_buffer
-// only one task can read/write it at a certain time
-SemaphoreHandle_t uart_buffer_Mutex = NULL;
-
-// UART_RX_IRQ will be executed as soon as data is received by the UART
-// This is a callback function executed from a high priority
-// task created when onReceive() is used
+// The Modbus RTU standard prescribes a silent period corresponding to 3.5 characters between each
+// message, to be able to figure out where one message ends and the next one starts.
+const uint32_t modbusRxTimeoutLimit = 4;
+const uint32_t baudrate = 19200;
+
+// UART_RX_IRQ will be executed as soon as data is received by the UART and an RX Timeout occurs
+// This is a callback function executed from a high priority monitor task
+// All data will be buffered into RX Buffer, which may have its size set to whatever necessary
 void UART0_RX_CB() {
-  // take the mutex, waits forever until loop() finishes its processing
-  if (xSemaphoreTake(uart_buffer_Mutex, portMAX_DELAY)) {
-    uint32_t now = millis();  // tracks timeout
-    while ((millis() - now) < communicationTimeout_ms) {
-      if (UART0.available()) {
-        uart_buffer += (char)UART0.read();
-        now = millis();  // reset the timer
-      }
-    }
-    // releases the mutex for data processing
-    xSemaphoreGive(uart_buffer_Mutex);
+  while (Serial0.available()) {
+    uart_buffer += (char)Serial0.read();
   }
 }
 
 // setup() and loop() are functions executed by a low priority task
 // Therefore, there are 2 tasks running when using onReceive()
 void setup() {
-  UART0.begin(115200);
-
-  // creates a mutex object to control access to uart_buffer
-  uart_buffer_Mutex = xSemaphoreCreateMutex();
-  if (uart_buffer_Mutex == NULL) {
-    log_e("Error creating Mutex. Sketch will fail.");
-    while (true) {
-      UART0.println("Mutex error (NULL). Program halted.");
-      delay(2000);
-    }
-  }
-
-  UART0.onReceive(UART0_RX_CB);  // sets the callback function
-  UART0.println("Send data to UART0 in order to activate the RX callback");
+  // Using Serial0 will work in any case (using or not USB CDC on Boot)
+#if CONFIG_IDF_TARGET_ESP32 || CONFIG_IDF_TARGET_ESP32S2
+  // UART_CLK_SRC_APB will allow higher values of RX Timeout
+  // default for ESP32 and ESP32-S2 is REF_TICK which limits the RX Timeout to 1
+  // setClockSource() must be called before begin()
+  Serial0.setClockSource(UART_CLK_SRC_APB);
+#endif
+  // the amount of data received or waiting to be proessed shall not exceed this limit of 1024 bytes
+  Serial0.setRxBufferSize(1024);  // default is 256 bytes
+  Serial0.begin(baudrate);        // default pins and default mode 8N1 (8 bits data, no parity bit, 1 stopbit)
+  // set RX Timeout based on UART symbols ~ 3.5 symbols of 11 bits (MODBUS standard) ~= 2 ms at 19200
+  Serial0.setRxTimeout(modbusRxTimeoutLimit);  // 4 symbols at 19200 8N1 is about 2.08 ms (40 bits)
+  // sets the callback function that will be executed only after RX Timeout
+  Serial0.onReceive(UART0_RX_CB, true);
+  Serial0.println("Send data using Serial Monitor in order to activate the RX callback");
 }
 
 uint32_t counter = 0;
 void loop() {
+  // String <uart_buffer> is filled by the UART Callback whenever data is received and RX Timeout occurs
   if (uart_buffer.length() > 0) {
-    // signals that the onReceive function shall not change uart_buffer while processing
-    if (xSemaphoreTake(uart_buffer_Mutex, portMAX_DELAY)) {
-      // process the received data from UART0 - example, just print it beside a counter
-      UART0.print("[");
-      UART0.print(counter++);
-      UART0.print("] [");
-      UART0.print(uart_buffer.length());
-      UART0.print(" bytes] ");
-      UART0.println(uart_buffer);
-      uart_buffer = "";  // reset uart_buffer for the next UART reading
-      // releases the mutex for more data to be received
-      xSemaphoreGive(uart_buffer_Mutex);
-    }
+    // process the received data from Serial - example, just print it beside a counter
+    Serial0.print("[");
+    Serial0.print(counter++);
+    Serial0.print("] [");
+    Serial0.print(uart_buffer.length());
+    Serial0.print(" bytes] ");
+    Serial0.println(uart_buffer);
+    uart_buffer = "";  // reset uart_buffer for the next UART reading
   }
-  UART0.println("Sleeping for 1 second...");
-  delay(1000);
+  delay(1);
 }