Redesiging SpiDispatcher to conform the write_then_read API of Adafruit_BusIO.

* All data returned by spi.transfer() before the end of the write buffer are directly written into the write buffer.
* All data returned by spi.transer() after the end of the write buffer are written into the read buffer.
* After exhaustion of the write buffer only the fill_value is being sent.

Author: aentinger

examples/Threadsafe_SPI/Threadsafe_SPI.ino

@@ -60,22 +60,17 @@ void loop()
 
 byte bmp388_read_reg(byte const reg_addr)
 {
-  byte const write_buf[3] =
-  {
-    static_cast<byte>(0x80 | reg_addr), /* REG_ADDR, if MSBit is set -> READ access */
-    0,                                  /* Dummy byte.                              */
-    0                                   /* REG_VAL is output on SDO                 */
-  };
-  byte read_buf[3] = {0};
+  /* REG_ADDR | DUMMY_BYTE | REG_VAL is on SDO */
+  byte read_write_buf[] = {static_cast<byte>(0x80 | reg_addr), 0, 0};
 
-  IoRequest req(write_buf, sizeof(write_buf), read_buf, sizeof(read_buf));
+  IoRequest req(read_write_buf, sizeof(read_write_buf), nullptr, 0);
   IoResponse rsp = bmp388.transfer(req);
 
   /* Do other stuff */
 
   rsp->wait();
 
-  return read_buf[2];
+  return read_write_buf[2];
 }
 
 void bmp388_thread_func()

examples/Threadsafe_SPI_BusIO/Threadsafe_SPI_BusIO.ino

@@ -60,17 +60,12 @@ void loop()
 
 byte bmp388_read_reg(byte const reg_addr)
 {
-  byte const write_buf[3] =
-  {
-    static_cast<byte>(0x80 | reg_addr), /* REG_ADDR, if MSBit is set -> READ access */
-    0,                                  /* Dummy byte.                              */
-    0                                   /* REG_VAL is output on SDO                 */
-  };
-  byte read_buf[3] = {0};
-
-  bmp388.spi().write_then_read(write_buf, sizeof(write_buf), read_buf, sizeof(read_buf));
+  /* REG_ADDR | DUMMY_BYTE | REG_VAL is on SDO */
+  byte write_buf[2] = {static_cast<byte>(0x80 | reg_addr), 0};
+  byte read_buf = 0;
 
-  return read_buf[2];
+  bmp388.spi().write_then_read(write_buf, sizeof(write_buf), &read_buf, sizeof(read_buf));
+  return read_buf;
 }
 
 void bmp388_thread_func()

examples/Threadsafe_Wire_BusIO/Threadsafe_Wire_BusIO.ino

@@ -55,7 +55,7 @@ void loop()
  * FUNCTION DEFINITION
  **************************************************************************************/
 
-byte lsm6dsox_read_reg(byte const reg_addr)
+byte lsm6dsox_read_reg(byte reg_addr)
 {
   byte read_buf = 0;
   lsm6dsox.wire().write_then_read(&reg_addr, 1, &read_buf, 1);

src/IoTransaction.h

@@ -41,18 +41,18 @@ class IoRequest
 {
 public:
 
-  IoRequest(byte const * const write_buf_, size_t const bytes_to_write_, byte * read_buf_, size_t const bytes_to_read_)
+  IoRequest(byte * write_buf_, size_t const bytes_to_write_, byte * read_buf_, size_t const bytes_to_read_)
   : write_buf{write_buf_}
   , bytes_to_write{bytes_to_write_}
   , read_buf{read_buf_}
   , bytes_to_read{bytes_to_read_}
   { }
 
-  IoRequest(byte const & write_buf_, byte & read_buf_)
+  IoRequest(byte & write_buf_, byte & read_buf_)
   : IoRequest{&write_buf_, 1, &read_buf_, 1}
   { }
 
-  byte const * const write_buf{nullptr};
+  byte * write_buf{nullptr};
   size_t const bytes_to_write{0};
   byte * read_buf{nullptr};
   size_t const bytes_to_read{0};

src/spi/SpiBusDevice.cpp

@@ -52,15 +52,15 @@ bool SpiBusDevice::read(uint8_t * buffer, size_t len, uint8_t sendvalue)
   return true;
 }
 
-bool SpiBusDevice::write(const uint8_t * buffer, size_t len)
+bool SpiBusDevice::write(uint8_t * buffer, size_t len)
 {
   IoRequest req(buffer, len, nullptr, 0);
   IoResponse rsp = SpiDispatcher::instance().dispatch(&req, &_config);
   rsp->wait();
   return true;
 }
 
-bool SpiBusDevice::write_then_read(const uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, uint8_t sendvalue)
+bool SpiBusDevice::write_then_read(uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, uint8_t sendvalue)
 {
   SpiBusDeviceConfig config(_config.spi(), _config.settings(), _config.select_func(), _config.deselect_func(), sendvalue);
   IoRequest req(write_buffer, write_len, read_buffer, read_len);

src/spi/SpiBusDevice.h

@@ -43,8 +43,8 @@ class SpiBusDevice : public BusDeviceBase
 
 
   bool read(uint8_t * buffer, size_t len, uint8_t sendvalue = 0xFF);
-  bool write(const uint8_t * buffer, size_t len);
-  bool write_then_read(const uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, uint8_t sendvalue = 0xFF);
+  bool write(uint8_t * buffer, size_t len);
+  bool write_then_read(uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, uint8_t sendvalue = 0xFF);
 
 
 private:

src/spi/SpiDispatcher.cpp

@@ -140,24 +140,30 @@ void SpiDispatcher::processSpiIoRequest(SpiIoTransaction * spi_io_transaction)
 
   config->spi().beginTransaction(config->settings());
 
-  size_t bytes_received = 0,
-         bytes_sent = 0;
-  for(;
-      bytes_received < std::max(io_request->bytes_to_read, io_request->bytes_to_write);
-      bytes_received++, bytes_sent++)
+  /* In a first step transmit the complete write buffer and
+   * write back the receive data directly into the write buffer  
+   */
+  size_t bytes_sent = 0;
+  for(; bytes_sent < io_request->bytes_to_write; bytes_sent++)
   {
-    byte tx_byte = 0;
+    uint8_t const tx_byte = io_request->write_buf[bytes_sent];
+    uint8_t const rx_byte = config->spi().transfer(tx_byte);
+
+    io_request->write_buf[bytes_sent] = rx_byte;
+  }
 
-    if (io_request->write_buf && (bytes_sent < io_request->bytes_to_write))
-      tx_byte = io_request->write_buf[bytes_sent];
-    else
-      tx_byte = config->fill_symbol();
+  /* In a second step, transmit the fill symbol and write the
+   * received data into the read buffer.
+   */
+  size_t bytes_received = 0;
+  for(; bytes_received < io_request->bytes_to_read; bytes_received++)
+  {
+    uint8_t const tx_byte = config->fill_symbol();
+    uint8_t const rx_byte = config->spi().transfer(tx_byte);
 
-    byte const rx_byte = config->spi().transfer(tx_byte);
+    io_request->read_buf[bytes_received] = rx_byte;
+  }
 
-    if (io_request->read_buf && (bytes_received < io_request->bytes_to_read))
-      io_request->read_buf[bytes_received] = rx_byte;
-   }
   config->spi().endTransaction();
 
   config->deselect();

src/wire/WireBusDevice.cpp

@@ -52,7 +52,7 @@ bool WireBusDevice::read(uint8_t * buffer, size_t len, bool stop)
   return true;
 }
 
-bool WireBusDevice::write(const uint8_t * buffer, size_t len, bool stop)
+bool WireBusDevice::write(uint8_t * buffer, size_t len, bool stop)
 {
   bool const restart = !stop;
   WireBusDeviceConfig config(_config.wire(), _config.slave_addr(), restart, _config.stop());
@@ -62,7 +62,7 @@ bool WireBusDevice::write(const uint8_t * buffer, size_t len, bool stop)
   return true;
 }
 
-bool WireBusDevice::write_then_read(const uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, bool stop)
+bool WireBusDevice::write_then_read(uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, bool stop)
 {
   /* Copy the Wire parameters from the device and modify only those
    * which can be modified via the parameters of this function.

src/wire/WireBusDevice.h

@@ -43,8 +43,8 @@ class WireBusDevice : public BusDeviceBase
 
 
   bool read(uint8_t * buffer, size_t len, bool stop = true);
-  bool write(const uint8_t * buffer, size_t len, bool stop = true);
-  bool write_then_read(const uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, bool stop = false);
+  bool write(uint8_t * buffer, size_t len, bool stop = true);
+  bool write_then_read(uint8_t * write_buffer, size_t write_len, uint8_t * read_buffer, size_t read_len, bool stop = false);
 
 
 private: