Merge pull request #5565 from LMESTM/fix_slave_3wires

STM32: SPI 3 wires mode not supported in SPI slave

Author: 0xc0170

targets/TARGET_STM/stm_spi_api.c

@@ -29,6 +29,7 @@
  */
 #include "mbed_assert.h"
 #include "mbed_error.h"
+#include "mbed_debug.h"
 #include "spi_api.h"
 
 #if DEVICE_SPI
@@ -286,6 +287,16 @@ void spi_format(spi_t *obj, int bits, int mode, int slave)
 
     handle->Init.Mode = (slave) ? SPI_MODE_SLAVE : SPI_MODE_MASTER;
 
+    if (slave && (handle->Init.Direction == SPI_DIRECTION_1LINE)) {
+        /*  SPI slave implemtation in MBED does not support the 3 wires SPI.
+         *  (e.g. when MISO is not connected). So we're forcing slave in
+         *  2LINES mode. As MISO is not connected, slave will only read
+         *  from master, and cannot write to it. Inform user.
+         */
+        debug("3 wires SPI slave not supported - slave will only read\r\n");
+        handle->Init.Direction = SPI_DIRECTION_2LINES;
+    }
+
     init_spi(obj);
 }