Initial SPI Slave implementation and examples (#2234)

* Initial SPI Slave implementation and examples

* Update style and info

libraries/SPISlave/examples/SPISlave_SafeMaster/SPISlave_SafeMaster.ino

@@ -0,0 +1,124 @@
+/*
+    SPI Safe Master Demo Sketch
+    Connect the SPI Master device to the following pins on the esp8266:
+
+    GPIO    NodeMCU   Name  |   Uno
+   ===================================
+     15       D8       SS   |   D10
+     13       D7      MOSI  |   D11
+     12       D6      MISO  |   D12
+     14       D5      SCK   |   D13
+
+    Note: If the ESP is booting at a moment when the SPI Master has the Select line HIGH (deselected)
+    the ESP8266 WILL FAIL to boot!
+    This sketch tries to go around this issue by only pulsing the Slave Select line to reset the command
+    and keeping the line LOW all other time.
+
+*/
+#include <SPI.h>
+
+class ESPSafeMaster
+{
+private:
+    uint8_t _ss_pin;
+    void _pulseSS()
+    {
+        digitalWrite(_ss_pin, HIGH);
+        delayMicroseconds(5);
+        digitalWrite(_ss_pin, LOW);
+    }
+public:
+    ESPSafeMaster(uint8_t pin):_ss_pin(pin) {}
+    void begin()
+    {
+        pinMode(_ss_pin, OUTPUT);
+        _pulseSS();
+    }
+
+    uint32_t readStatus()
+    {
+        _pulseSS();
+        SPI.transfer(0x04);
+        uint32_t status = (SPI.transfer(0) | ((uint32_t)(SPI.transfer(0)) << 8) | ((uint32_t)(SPI.transfer(0)) << 16) | ((uint32_t)(SPI.transfer(0)) << 24));
+        _pulseSS();
+        return status;
+    }
+
+    void writeStatus(uint32_t status)
+    {
+        _pulseSS();
+        SPI.transfer(0x01);
+        SPI.transfer(status & 0xFF);
+        SPI.transfer((status >> 8) & 0xFF);
+        SPI.transfer((status >> 16) & 0xFF);
+        SPI.transfer((status >> 24) & 0xFF);
+        _pulseSS();
+    }
+
+    void readData(uint8_t * data)
+    {
+        _pulseSS();
+        SPI.transfer(0x03);
+        SPI.transfer(0x00);
+        for(uint8_t i=0; i<32; i++) {
+            data[i] = SPI.transfer(0);
+        }
+        _pulseSS();
+    }
+
+    void writeData(uint8_t * data, size_t len)
+    {
+        uint8_t i=0;
+        _pulseSS();
+        SPI.transfer(0x02);
+        SPI.transfer(0x00);
+        while(len-- && i < 32) {
+            SPI.transfer(data[i++]);
+        }
+        while(i++ < 32) {
+            SPI.transfer(0);
+        }
+        _pulseSS();
+    }
+
+    String readData()
+    {
+        char data[33];
+        data[32] = 0;
+        readData((uint8_t *)data);
+        return String(data);
+    }
+
+    void writeData(const char * data)
+    {
+        writeData((uint8_t *)data, strlen(data));
+    }
+};
+
+ESPSafeMaster esp(SS);
+
+void send(const char * message)
+{
+    Serial.print("Master: ");
+    Serial.println(message);
+    esp.writeData(message);
+    delay(10);
+    Serial.print("Slave: ");
+    Serial.println(esp.readData());
+    Serial.println();
+}
+
+void setup()
+{
+    Serial.begin(115200);
+    SPI.begin();
+    esp.begin();
+    delay(1000);
+    send("Hello Slave!");
+}
+
+void loop()
+{
+    delay(1000);
+    send("Are you alive?");
+}