nrf24l01: Optimize status reading.

The value of the STATUS register is always transmitted by the chip when
reading any command.  So a R_REGISTER command and the turnaround time can
be spared by issuing a NOP command instead.

This implementation suggested by the datasheet.

This operation is compatible with both nRF24L01 and nRF24L01+.

Signed-off-by: Marcell Pünkösd <punkosdmarcell@rocketmail.com>

micropython/drivers/radio/nrf24l01/nrf24l01.py

@@ -130,6 +130,13 @@ class NRF24L01:
         self.cs(1)
         return ret
 
+    def read_status(self):
+        self.cs(0)
+        # STATUS register is always shifted during command transmit
+        self.spi.readinto(self.buf, NOP)
+        self.cs(1)
+        return self.buf[0]
+
     def flush_rx(self):
         self.cs(0)
         self.spi.readinto(self.buf, FLUSH_RX)
@@ -250,7 +257,8 @@ class NRF24L01:
 
     # returns None if send still in progress, 1 for success, 2 for fail
     def send_done(self):
-        if not (self.reg_read(STATUS) & (TX_DS | MAX_RT)):
+        status = self.read_status()
+        if not (status & (TX_DS | MAX_RT)):
             return None  # tx not finished
 
         # either finished or failed: get and clear status flags, power down