Fixes for SPI slave implementation (#336)

* Start on SPI slave comms test

* Free pins in SPI and SPISlave destructor

* Improve LPC1768 readable()

drivers/include/drivers/SPISlave.h

@@ -33,7 +33,7 @@ namespace mbed {
 
 /** A SPI slave, used for communicating with a SPI master device.
  *
- * The default format is set to 8 bits, mode 0 and a clock frequency of 1MHz.
+ * The default format is set to 8 bits, mode 0.
  *
  * @note Synchronization level: Not protected
  *
@@ -80,6 +80,11 @@ public:
     SPISlave(const spi_pinmap_t &pinmap);
     SPISlave(const spi_pinmap_t &&) = delete; // prevent passing of temporary objects
 
+    /**
+     * @brief Destructor.  Frees the SPI peripheral so it can be used elsewhere.
+     */
+    ~SPISlave();
+
     /** Configure the data transmission format.
      *
      *  @param bits Number of bits per SPI frame (4 - 16).
@@ -96,12 +101,6 @@ public:
      */
     void format(int bits, int mode = 0);
 
-    /** Set the SPI bus clock frequency.
-     *
-     *  @param hz Clock frequency in hz (default = 1MHz).
-     */
-    void frequency(int hz = 1000000);
-
     /** Polls the SPI to see if data has been received.
      *
      *  @return Presence of received data.
@@ -133,8 +132,6 @@ protected:
     int _bits;
     /* Clock phase and polarity */
     int _mode;
-    /* Clock frequency */
-    int _hz;
 
 #endif //!defined(DOXYGEN_ONLY)
 };

drivers/source/SPISlave.cpp

@@ -24,23 +24,34 @@ namespace mbed {
 SPISlave::SPISlave(PinName mosi, PinName miso, PinName sclk, PinName ssel) :
     _spi(),
     _bits(8),
-    _mode(0),
-    _hz(1000000)
+    _mode(0)
 {
     spi_init(&_spi, mosi, miso, sclk, ssel);
     spi_format(&_spi, _bits, _mode, 1);
-    spi_frequency(&_spi, _hz);
+
+    // For legacy compatibility, tell the HAL to set the frequency to 1MHz.
+    // This is done even though it does not make sense to set the frequency of a slave device;
+    // it has to run at whatever SCLK frequency the master supplies.
+    spi_frequency(&_spi, 1000000);
 }
 
 SPISlave::SPISlave(const spi_pinmap_t &pinmap) :
     _spi(),
     _bits(8),
-    _mode(0),
-    _hz(1000000)
+    _mode(0)
 {
     spi_init_direct(&_spi, &pinmap);
     spi_format(&_spi, _bits, _mode, 1);
-    spi_frequency(&_spi, _hz);
+
+    // For legacy compatibility, tell the HAL to set the frequency to 1MHz.
+    // This is done even though it does not make sense to set the frequency of a slave device;
+    // it has to run at whatever SCLK frequency the master supplies.
+    spi_frequency(&_spi, 1000000);
+}
+
+SPISlave::~SPISlave()
+{
+    spi_free(&_spi);
 }
 
 void SPISlave::format(int bits, int mode)
@@ -50,12 +61,6 @@ void SPISlave::format(int bits, int mode)
     spi_format(&_spi, _bits, _mode, 1);
 }
 
-void SPISlave::frequency(int hz)
-{
-    _hz = hz;
-    spi_frequency(&_spi, _hz);
-}
-
 int SPISlave::receive(void)
 {
     return (spi_slave_receive(&_spi));

targets/TARGET_NXP/TARGET_LPC176X/objects.h

@@ -71,6 +71,12 @@ struct i2c_s {
 struct spi_s {
     LPC_SSP_TypeDef *spi;
     uint8_t bits_per_word;
+
+    // Pin names, so that we can unmap them when the SPI is freed
+    PinName mosi_pin;
+    PinName miso_pin;
+    PinName sclk_pin;
+    PinName ssel_pin;
 };
 
 struct flash_s {

targets/TARGET_NXP/TARGET_LPC176X/spi_api.c

@@ -34,18 +34,30 @@ void spi_init_direct(spi_t *obj, const spi_pinmap_t *pinmap) {
         case SPI_1: LPC_SC->PCONP |= 1 << 10; break;
     }
 
-    // pin out the spi pins
-    pin_function(pinmap->mosi_pin, pinmap->mosi_function);
-    pin_mode(pinmap->mosi_pin, PullNone);
-    pin_function(pinmap->miso_pin, pinmap->miso_function);
-    pin_mode(pinmap->miso_pin, PullNone);
+    // Pin out the spi pins.
+    // Note: SCLK is the only pin that is always required.
+    if(pinmap->mosi_pin != NC)
+    {
+        pin_function(pinmap->mosi_pin, pinmap->mosi_function);
+        pin_mode(pinmap->mosi_pin, PullNone);
+    }
+    if(pinmap->miso_pin != NC)
+    {
+        pin_function(pinmap->miso_pin, pinmap->miso_function);
+        pin_mode(pinmap->miso_pin, PullNone);
+    }
     pin_function(pinmap->sclk_pin, pinmap->sclk_function);
     pin_mode(pinmap->sclk_pin, PullNone);
-
     if (pinmap->ssel_pin != NC) {
         pin_function(pinmap->ssel_pin, pinmap->ssel_function);
         pin_mode(pinmap->ssel_pin, PullNone);
     }
+
+    // Save pins
+    obj->mosi_pin = pinmap->mosi_pin;
+    obj->miso_pin = pinmap->miso_pin;
+    obj->sclk_pin = pinmap->sclk_pin;
+    obj->ssel_pin = pinmap->ssel_pin;
 }
 
 SPIName spi_get_peripheral_name(PinName mosi, PinName miso, PinName sclk)
@@ -80,7 +92,14 @@ void spi_init(spi_t *obj, PinName mosi, PinName miso, PinName sclk, PinName ssel
     spi_init_direct(obj, &pinmap);
 }
 
-void spi_free(spi_t *obj) {}
+void spi_free(spi_t *obj)
+{
+    // Reset all pins to GPIO function
+    if(obj->mosi_pin != NC) pin_function(obj->mosi_pin, 0);
+    if(obj->miso_pin != NC) pin_function(obj->miso_pin, 0);
+    pin_function(obj->sclk_pin, 0);
+    if(obj->ssel_pin != NC) pin_function(obj->ssel_pin, 0);
+}
 
 void spi_format(spi_t *obj, int bits, int mode, int slave) {
     ssp_disable(obj);
@@ -233,7 +252,7 @@ int spi_master_block_write(spi_t *obj, const char *tx_buffer, int tx_length,
 }
 
 int spi_slave_receive(spi_t *obj) {
-    return (ssp_readable(obj) && !ssp_busy(obj)) ? (1) : (0);
+    return ssp_readable(obj);
 }
 
 int spi_slave_read(spi_t *obj) {