Add new Non-blocking write example

Author: sandeepmistry

examples/NonBlockingWrite/NonBlockingWrite.ino

@@ -0,0 +1,91 @@
+/*
+  Non-blocking Write
+
+  This example demonstrates how to perform non-blocking writes
+  to a file on a SD card. The file will contain the current millis()
+  value every 10ms. If the SD card is busy, the data will be buffered
+  in order to not block the sketch.
+
+  NOTE: myFile.availableForWrite() will automatically sync the
+        file contents as needed. You may lose some unsynced data
+        still if myFile.sync() or myFile.close() is not called.
+
+  The circuit:
+  - Arduino MKR Zero board
+  - micro SD card attached
+
+  This example code is in the public domain.
+*/
+
+#include <SD.h>
+
+// file name to use for writing
+const char filename[] = "demo.txt";
+
+// File object to represent file
+File txtFile;
+
+// string to buffer output
+String buffer;
+
+unsigned long lastMillis = 0;
+
+void setup() {
+  Serial.begin(9600);
+  while (!Serial);
+
+  // reserve 1kB for String used as a buffer
+  buffer.reserve(1024);
+
+  // set LED pin to output, used to blink when writing
+  pinMode(LED_BUILTIN, OUTPUT);
+
+  // init the SD card
+  if (!SD.begin()) {
+    Serial.println("Card failed, or not present");
+    // don't do anything more:
+    while (1);
+  }
+
+  // If you want to start from an empty file,
+  // uncomment the next line:
+  // SD.remove(filename);
+
+  // try to open the file for writing
+  txtFile = SD.open(filename, FILE_WRITE);
+  if (!txtFile) {
+    Serial.print("error opening ");
+    Serial.println(filename);
+    while (1);
+  }
+
+  // add some new lines to start
+  txtFile.println();
+  txtFile.println("Hello World!");
+}
+
+void loop() {
+  // check if it's been over 10 ms since the last line added
+  unsigned long now = millis();
+  if ((now - lastMillis) >= 10) {
+    // add a new line to the buffer
+    buffer += "Hello ";
+    buffer += now;
+    buffer += "\r\n";
+
+    lastMillis = now;
+  }
+
+  // check if the SD card is available to write data without blocking
+  // and if the buffered data is enough for the full chunk size
+  unsigned int chunkSize = txtFile.availableForWrite();
+  if (chunkSize && buffer.length() >= chunkSize) {
+    // write to file and blink LED
+    digitalWrite(LED_BUILTIN, HIGH);
+    txtFile.write(buffer.c_str(), chunkSize);
+    digitalWrite(LED_BUILTIN, LOW);
+
+    // remove written data from buffer
+    buffer.remove(0, chunkSize);
+  }
+}