spi: enable both LSB first and MSB first transfers

Enable both LSB first and MSB first transfers: add
function set_bit_order to select configuration.
By default MSB first configuration is used.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>

src/spi.rs

@@ -10,6 +10,19 @@ pub enum Error {
     NoData,
 }
 
+#[derive(Debug)]
+pub enum BitOrder {
+    MSBFirst,
+    LSBFirst,
+}
+
+impl Default for BitOrder {
+    /// Default bit order: MSB first
+    fn default() -> Self {
+        BitOrder::MSBFirst
+    }
+}
+
 /// A Full-Duplex SPI implementation, takes 3 pins, and a timer running at 2x
 /// the desired SPI frequency.
 pub struct SPI<Miso, Mosi, Sck, Timer>
@@ -25,6 +38,7 @@ where
     sck: Sck,
     timer: Timer,
     read_val: Option<u8>, 
+    bit_order: BitOrder,
 }
 
 impl <Miso, Mosi, Sck, Timer> SPI<Miso, Mosi, Sck, Timer>
@@ -48,9 +62,14 @@ where
             sck: sck,
             timer: timer,
             read_val: None,
+            bit_order: BitOrder::default(),
         }
     }
 
+    pub fn set_bit_order(&mut self, order: BitOrder) {
+        self.bit_order = order;
+    }
+
     fn read_bit(&mut self) {
         if self.miso.is_high() {
             self.read_val = Some((self.read_val.unwrap_or(0) << 1) | 1);
@@ -77,14 +96,18 @@ where
     }
 
     fn send(&mut self, byte: u8) -> nb::Result<(), Self::Error> {
-        let mut data_out = byte;
-        for _bit in 0..8 {
-            let out_bit = (data_out >> 7) & 1;
+        for bit in 0..8 {
+            let out_bit = match self.bit_order {
+                BitOrder::MSBFirst => (byte >> (7 - bit)) & 0b1,
+                BitOrder::LSBFirst => (byte >> bit) & 0b1,
+            };
+
             if out_bit == 1 {
                 self.mosi.set_high(); 
             } else {
                 self.mosi.set_low();
             }
+
             if self.mode.phase == CaptureOnFirstTransition {
                 if self.mode.polarity == IdleLow {
                     block!(self.timer.wait()).ok(); 
@@ -114,7 +137,6 @@ where
                     block!(self.timer.wait()).ok(); 
                 }
             }
-            data_out <<= 1;
         }
         Ok(())
     }