New test for SPI loop transfer

This sketch is testing the SPI Tx to Rx transfer
when the SPI MISO is connected to the SPI MOSI pin
of the hardware board.

Signed-off-by: Francois Ramu <francois.ramu@st.com>

Author: FRASTM

examples/NonReg/SPI_loop/SPI_loop/SPI_loop.ino

@@ -0,0 +1,71 @@
+/*
+  SPI_loopback
+
+  Test the loopback transfer on SPI serial link.
+  A test string is sent 9 times and compared to the Rx frame.
+  when the MISO pin is physically connected to the MOSI pin of the SPI bus
+  Default test the SPI_A link.
+  Led remains ON when test ends successfully (off in case of error).
+  To test the transfer on the SPI_B , just define SPI_B pins and connect,
+  as follows:
+    int MOSI_PIN = 22; // to be connected to the B_MISO
+    int SCK_PIN = 23;
+    int CS_PIN = 24;
+    int MISO_PIN = 25; // to be connected to the B_MOSI
+  and configure as follows:
+    SPI.setMISO(MISO_PIN);
+    SPI.setMOSI(MOSI_PIN);
+    SPI.setSCLK(SCK_PIN);
+    SPI.setSSEL(CS_PIN);
+
+  This example code is in the public domain.
+
+*/
+#include "SPI.h"
+
+#define MOSI_PIN  PIN_SPI_MOSI
+#define SCK_PIN   PIN_SPI_SCK
+#define CS_PIN    PIN_SPI_SS
+#define MISO_PIN  PIN_SPI_MOSI
+
+#define BUF_SIZE 36
+uint8_t buffer_tx[] = "Thequickbrownfoxjumpsoverthelazydog\0";
+uint8_t buffer_rx[BUF_SIZE] = {};
+
+/* SPI transfer loop nb */
+#define TEST_LOOP_NB 9
+
+void setup() {
+  uint8_t tested_ok = 0; // test result
+
+  /* the SPI pin configuration is done by the SPI.begin */
+   SPI.begin(CS_PIN); // waking up SPI bus
+  // SPI.setClockDivider(CS_PIN, 256);     // setting a slower clock
+
+  pinMode(LED_BUILTIN, OUTPUT);   // prepare led for status
+  digitalWrite(LED_BUILTIN, LOW); // start with led off
+
+  for (uint8_t received = 0; received < TEST_LOOP_NB; received++) {
+    SPI.transfer(CS_PIN, buffer_tx, buffer_rx, BUF_SIZE, SPI_LAST);
+    delay(100);
+    /* compare what is received to what was sent */
+    if (!memcmp(buffer_tx, buffer_rx, BUF_SIZE)) {
+      /* this transfer passed */
+      tested_ok++;
+    }
+    memset(buffer_rx, 0, BUF_SIZE);
+  }
+  /* display test result */
+  if (tested_ok == TEST_LOOP_NB) {
+      /* success */
+      digitalWrite(LED_BUILTIN, HIGH);
+  } else {
+      /* error */
+      digitalWrite(LED_BUILTIN, LOW);
+  }
+}
+
+void loop() {
+  /* nothing special */
+}
+