Make I2C transactions continuous according to specification (#741)

hal/CHANGELOG.md

@@ -1,5 +1,6 @@
 # Unreleased Changes
 
+- Fix I2C transaction to be as continuous as possible according to `embedded-hal` specification
 - Allow configuring USB clock with `GenericClockController` on atsamd11
 - fix samd51j not having i2s support
 - remove i2s functionality for samd51g since it does not have it

hal/src/sercom/i2c.rs

@@ -370,11 +370,27 @@ impl<C: AnyConfig> I2c<C> {
         self.config.as_mut().registers.do_write(addr, bytes)
     }
 
+    /// Continue a write operation that was issued before with
+    /// [`do_write`](Self::do_write) or [`continue_write`](Self::continue_write)
+    /// without a repeated start condition in between
+    #[inline]
+    fn continue_write(&mut self, bytes: &[u8]) -> Result<(), Error> {
+        self.config.as_mut().registers.continue_write(bytes)
+    }
+
     #[inline]
     fn do_read(&mut self, addr: u8, bytes: &mut [u8]) -> Result<(), Error> {
         self.config.as_mut().registers.do_read(addr, bytes)
     }
 
+    /// Continue a read operation that was issued before with
+    /// [`do_read`](Self::do_read) or [`continue_read`](Self::continue_read)
+    /// without a repeated start condition in between
+    #[inline]
+    fn continue_read(&mut self, bytes: &mut [u8]) -> Result<(), Error> {
+        self.config.as_mut().registers.continue_read(bytes)
+    }
+
     #[inline]
     fn do_write_read(&mut self, addr: u8, bytes: &[u8], buffer: &mut [u8]) -> Result<(), Error> {
         self.config

hal/src/sercom/i2c/impl_ehal.rs

@@ -25,19 +25,40 @@ impl<C: AnyConfig> i2c::I2c for I2c<C> {
         address: u8,
         operations: &mut [i2c::Operation<'_>],
     ) -> Result<(), Self::Error> {
+        /// Helper type for keeping track of the type of operation that was
+        /// executed last
+        #[derive(Clone, Copy)]
+        enum Operation {
+            Read,
+            Write,
+        }
+
+        // Keep track of the last executed operation type. The method
+        // specification demands, that no repeated start condition is sent
+        // between adjacent operations of the same type.
+        let mut last_op = None;
         for op in operations {
             match op {
                 i2c::Operation::Read(buf) => {
-                    self.do_read(address, buf)?;
-                    self.cmd_stop();
+                    if let Some(Operation::Read) = last_op {
+                        self.continue_read(buf)?;
+                    } else {
+                        self.do_read(address, buf)?;
+                        last_op = Some(Operation::Read);
+                    }
                 }
                 i2c::Operation::Write(bytes) => {
-                    self.do_write(address, bytes)?;
-                    self.cmd_stop();
+                    if let Some(Operation::Write) = last_op {
+                        self.continue_write(bytes)?;
+                    } else {
+                        self.do_write(address, bytes)?;
+                        last_op = Some(Operation::Write);
+                    }
                 }
             }
         }
 
+        self.cmd_stop();
         Ok(())
     }
 

hal/src/sercom/i2c/reg.rs

@@ -401,12 +401,28 @@ impl<S: Sercom> Registers<S> {
         self.send_bytes(bytes)
     }
 
+    /// Continue a write operation that was issued before with
+    /// [`do_write`](Self::do_write) or [`continue_write`](Self::continue_write)
+    /// without a repeated start condition in between
+    #[inline]
+    pub(super) fn continue_write(&mut self, bytes: &[u8]) -> Result<(), Error> {
+        self.send_bytes(bytes)
+    }
+
     #[inline]
     pub(super) fn do_read(&mut self, addr: u8, buffer: &mut [u8]) -> Result<(), Error> {
         self.start_read_blocking(addr)?;
         self.fill_buffer(buffer)
     }
 
+    /// Continue a read operation that was issued before with
+    /// [`do_read`](Self::do_read) or [`continue_read`](Self::continue_read)
+    /// without a repeated start condition in between
+    #[inline]
+    pub(super) fn continue_read(&mut self, buffer: &mut [u8]) -> Result<(), Error> {
+        self.fill_buffer(buffer)
+    }
+
     #[inline]
     pub(super) fn do_write_read(
         &mut self,