Reworked the utility/rp2 SPI class and removed static members (#892)

* Removed 'static' keywords in SPI.h such that multiple SPI busses can be used to connect multiple Radios. Also added an example for this use-case
Co-authored-by: Brendan <2bndy5@gmail.com>

docs/pico_sdk.md

@@ -174,3 +174,56 @@ To specify the default SPI pins used at build time, you can use either:
    ```shell
    cmake --build . --config Release -DPICO_DEFAULT_SPI=0 -DPICO_DEFAULT_SPI_SCK_PIN=2 -DPICO_DEFAULT_SPI_TX_PIN=3 -DPICO_DEFAULT_SPI_RX_PIN=4
    ```
+
+## Using Multiple Radios
+
+It is possible to drive multiple nRF24L01 transceivers on a single board. To do this each radio needs dedicated digital output pins for the CE and CSN pins.
+
+If you want to drive each radio with a separate SPI bus, then the following example will demonstrate how to do that.
+
+```cpp
+#include <RF24.h>
+
+// Declare the pin numbers connected to the radios' CE and CSN pins (respectively)
+RF24 radio0(8, 5);   // first radio object
+RF24 radio1(14, 13); // second radio object
+
+// By default, one SPI bus instance is created by the RF24 lib. We'll use this
+// default instance of the `spi0` interface for our first radio, but we want a
+// different SPI bus for the second radio.
+// 
+// So, here we declare a second SPI bus instance:
+SPI my_spi; // we specify the `spi1` bus interface below
+
+bool setupRadios()
+{
+    // Initialize the first radio using the default SPI instance
+    if (!radio0.begin()) {
+        printf("Radio0 hardware is not responding!\n");
+        return false;
+    }
+    // first radio object initialized successfully
+
+    // specify the the second SPI bus interface and corresponding GPIO pins
+    my_spi.begin(spi1, 10, 11, 12); // spi1 bus, SCK, TX, RX
+    if (!radio1.begin(&my_spi)) {
+        printf("Radio1 hardware is not responding!\n");
+        return false;
+    }
+    // second radio object initialized successfully
+
+    return true;
+}
+
+int main()
+{
+    stdio_init_all(); // init necessary IO for the RP2040
+
+    while (!setupRadios()) { // if either radioX.begin() failed
+        sleep_ms(1000); // add 1 second delay for console readability
+        // hold program in infinite attempts to initialize the radios
+    }
+
+    // continue with program as normal ...
+}
+```

utility/rp2/spi.cpp

 #include "spi.h"
 
-spi_inst_t* SPI::_hw_id;
-
 SPI::SPI()
 {
 }

utility/rp2/spi.h

@@ -33,7 +33,7 @@ public:
      * @see begin(spi_inst_t, uint8_t, uint8_t, uint8_t) for using other pins as
      * your SPI bus.
      */
-    static void begin(spi_inst_t* hw_id);
+    void begin(spi_inst_t* hw_id);
 
     /**
      * Start SPI
@@ -47,24 +47,24 @@ public:
      * @see The [Pico SDK has a chart of applicable pins](https://datasheets.raspberrypi.org/pico/raspberry-pi-pico-c-sdk.pdf#%5B%7B%22num%22%3A106%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C115%2C377.118%2Cnull%5D)
      * that can be used for hardware driven SPI transactions.
      */
-    static void begin(spi_inst_t* hw_id, uint8_t _sck, uint8_t _tx, uint8_t _rx);
+    void begin(spi_inst_t* hw_id, uint8_t _sck, uint8_t _tx, uint8_t _rx);
 
-    static uint8_t transfer(uint8_t tx_);
+    uint8_t transfer(uint8_t tx_);
 
-    static void transfernb(const uint8_t* tbuf, uint8_t* rbuf, uint32_t len);
+    void transfernb(const uint8_t* tbuf, uint8_t* rbuf, uint32_t len);
 
-    static void transfern(const uint8_t* buf, uint32_t len);
+    void transfern(const uint8_t* buf, uint32_t len);
 
-    static void beginTransaction(uint32_t _spi_speed);
+    void beginTransaction(uint32_t _spi_speed);
 
     /** deinit the SPI bus (using hw_id passed to begin()) */
-    static void endTransaction();
+    void endTransaction();
 
     virtual ~SPI();
 
 private:
     /** the ID of the hardware driven SPI bus */
-    static spi_inst_t* _hw_id;
+    spi_inst_t* _hw_id;
 };
 
 #endif // RF24_UTILITY_RP2_SPI_H_