Make RF24_POWERUP_DELAY configurable in RF24_config.h (#690)

* available() could be slightly faster #631

* allow skipping delay(5) in powerUp()

* allow skipping delay(5) in powerUp()

* Revert "allow skipping delay(5) in powerUp()"

This reverts commit ffd7524.

* Revert "allow skipping delay(5) in powerUp()"

This reverts commit ffd7524.

* add RF24_POWERUP_DELAY_MS to RF24_config.h

* delayMicroseconds()

* update doc comment

RF24.cpp

@@ -765,7 +765,7 @@ void RF24::powerUp(void)
         // For nRF24L01+ to go from power down mode to TX or RX mode it must first pass through stand-by mode.
         // There must be a delay of Tpd2stby (see Table 16.) after the nRF24L01+ leaves power down mode before
         // the CEis set high. - Tpd2stby can be up to 5ms per the 1.0 datasheet
-        delay(5);
+        delayMicroseconds(RF24_POWERUP_DELAY);
     }
 }
 

RF24_config.h

@@ -18,13 +18,22 @@
 #ifndef __RF24_CONFIG_H__
 #define __RF24_CONFIG_H__
 
-/*** USER DEFINES:    ***/    
+/*** USER DEFINES:    ***/
 #define FAILURE_HANDLING
 //#define SERIAL_DEBUG
 //#define MINIMAL
 //#define SPI_UART    // Requires library from https://github.com/TMRh20/Sketches/tree/master/SPI_UART
 //#define SOFTSPI     // Requires library from https://github.com/greiman/DigitalIO
-
+
+/**
+ * User access to internally used delay time (in microseconds) during RF24::powerUp()
+ * @warning This default value compensates for all supported hardware. Only adjust this if you
+ * know your radio's hardware is, in fact, genuine and reliable.
+ */
+#if !defined(RF24_POWERUP_DELAY)
+#define RF24_POWERUP_DELAY	5000
+#endif
+
 /**********************/
 #define rf24_max(a,b) (a>b?a:b)
 #define rf24_min(a,b) (a<b?a:b)