Fix SPI::setClockDivider implementation

As said in Arduino API:
https://www.arduino.cc/en/Reference/SPISetClockDivider

"On the Due, the system clock can be divided by values
from 1 to 255. The default value is 21, which sets the clock
to 4 MHz like other Arduino boards."

So in fact, it should be possible to divide the clock by any value.
Then HAL will use the best prescaler to fit requested Freq.

Moreover, it is not possible to define SPI_CLOCK_DIVx to match
the 16MHz value for avr as SPI frequency depends of system clock
configuration.

Fix #299

Signed-off-by: Frederic.Pillon <[email protected]>

Author: fpistm

libraries/SPI/src/SPI.cpp

@@ -220,16 +220,10 @@ void SPIClass::setDataMode(uint8_t _pin, uint8_t _mode)
 
 /**
   * @brief  Deprecated function.
-  *         Configure the clock speed: 125kHz to 8MHz.
+  *         Configure the clock speed
   * @param  _pin: CS pin associated to a configuration (optional).
-  * @param  _divider: can be one of the following parameters:
-  *         SPI_CLOCK_DIV2    (8MHz)
-  *         SPI_CLOCK_DIV4    (4MHz)
-  *         SPI_CLOCK_DIV8    (2MHz)
-  *         SPI_CLOCK_DIV16   (1MHz)
-  *         SPI_CLOCK_DIV32   (500kHz)
-  *         SPI_CLOCK_DIV64   (250kHz)
-  *         SPI_CLOCK_DIV128  (125kHz)
+  * @param  _divider: the SPI clock can be divided by values from 1 to 255.
+  *         If 0, default SPI speed is used.
   */
 void SPIClass::setClockDivider(uint8_t _pin, uint8_t _divider)
 {
@@ -240,23 +234,11 @@ void SPIClass::setClockDivider(uint8_t _pin, uint8_t _divider)
   if(idx >= NB_SPI_SETTINGS) {
     return;
   }
-
-  /* Get clk freq of the SPI instance */
-  uint32_t spiClkFreq = spi_getClkFreq(&_spi);
-
-  switch(_divider) {
-    case (SPI_CLOCK_DIV2) :
-    case (SPI_CLOCK_DIV4) :
-    case (SPI_CLOCK_DIV8) :
-    case (SPI_CLOCK_DIV16) :
-    case (SPI_CLOCK_DIV32) :
-    case (SPI_CLOCK_DIV64) :
-    case (SPI_CLOCK_DIV128) :
-      spiSettings[idx].clk = spiClkFreq/_divider;
-    break;
-    default:
-      spiSettings[idx].clk = SPI_SPEED_CLOCK_DEFAULT;
-    break;
+  if (_divider == 0) {
+    spiSettings[idx].clk = SPI_SPEED_CLOCK_DEFAULT;
+  } else {
+    /* Get clk freq of the SPI instance and compute it */
+    spiSettings[idx].clk = spi_getClkFreq(&_spi)/_divider;
   }
 
   spi_init(&_spi, spiSettings[idx].clk,

libraries/SPI/src/SPI.h

@@ -22,9 +22,10 @@
 //   - SPISetting(clock, bitOrder, dataMode)
 #define SPI_HAS_TRANSACTION 1
 
-// For compatibility with sketches designed for AVR @ 16 MHz
-// need to go from 64MHz to 16 (/4)
-// This function should not be used in new projects.
+// Compatibility with sketches designed for AVR @ 16 MHz could not
+// be ensured as SPI frequency depends of system clock configuration.
+// user have to use appropriate divider for the SPI clock
+// This function should not be used in new project.
 // Use SPISettings with SPI.beginTransaction() to configure SPI parameters.
 #define SPI_CLOCK_DIV2	 2
 #define SPI_CLOCK_DIV4	 4