libraries/SPI/src/SPI.h: SPIClass: add method to get SS pin number (#5788)

* SPI.h add new call to return a SS pin number used.

As code example states, the SS pin needs to be explicitly set for output for SPI to work, but the pin number have to be coded in addition to the SPI logic in the library, which means this duplicates code. It is much better to just be able to get the pin number from library itself.

* Update SPI_Multiple_Buses.ino to use new pinSS method

Simplify the example case, to show usage of pinSS method. This also simplifies the example, removing duplicated code.

libraries/SPI/examples/SPI_Multiple_Buses/SPI_Multiple_Buses.ino

@@ -76,36 +76,24 @@ void setup() {
 
   //set up slave select pins as outputs as the Arduino API
   //doesn't handle automatically pulling SS low
-  pinMode(VSPI_SS, OUTPUT); //VSPI SS
-  pinMode(HSPI_SS, OUTPUT); //HSPI SS
+  pinMode(vspi->pinSS(), OUTPUT); //VSPI SS
+  pinMode(hspi->pinSS(), OUTPUT); //HSPI SS
 
 }
 
 // the loop function runs over and over again until power down or reset
 void loop() {
   //use the SPI buses
-  vspiCommand();
-  hspiCommand();
+  spiCommand(vspi, 0b01010101); // junk data to illustrate usage
+  spiCommand(hspi, 0b11001100);
   delay(100);
 }
 
-void vspiCommand() {
-  byte data = 0b01010101; // junk data to illustrate usage
-
+void spiCommand(SPIClass *spi, byte data) {
   //use it as you would the regular arduino SPI API
-  vspi->beginTransaction(SPISettings(spiClk, MSBFIRST, SPI_MODE0));
-  digitalWrite(VSPI_SS, LOW); //pull SS slow to prep other end for transfer
-  vspi->transfer(data);  
-  digitalWrite(VSPI_SS, HIGH); //pull ss high to signify end of data transfer
-  vspi->endTransaction();
-}
-
-void hspiCommand() {
-  byte stuff = 0b11001100;
-  
-  hspi->beginTransaction(SPISettings(spiClk, MSBFIRST, SPI_MODE0));
-  digitalWrite(HSPI_SS, LOW);
-  hspi->transfer(stuff);
-  digitalWrite(HSPI_SS, HIGH);
-  hspi->endTransaction();
+  spi->beginTransaction(SPISettings(spiClk, MSBFIRST, SPI_MODE0));
+  digitalWrite(spi->pinSS(), LOW); //pull SS slow to prep other end for transfer
+  spi->transfer(data);
+  digitalWrite(spi->pinSS(), HIGH); //pull ss high to signify end of data transfer
+  spi->endTransaction();
 }

libraries/SPI/src/SPI.h

@@ -83,6 +83,7 @@ public:
     void writePattern(const uint8_t * data, uint8_t size, uint32_t repeat);
 
     spi_t * bus(){ return _spi; }
+    int8_t pinSS() { return _ss; }
 };
 
 extern SPIClass SPI;