implement software framing for BPSK modulation

src/modulations/bpsk.rs

@@ -32,6 +32,200 @@ impl Bitrate {
     }
 }
 
+#[derive(Clone, Copy, defmt::Format)]
+pub enum CrcType {
+    None,
+    /// Using a common 0x07 polynomial
+    Crc8,
+    /// CRC-16 CCITT using 0x1021 polynomial
+    Crc16,
+}
+
+impl CrcType {
+    fn compute(self, data: &[u8]) -> (u16, usize) {
+        match self {
+            CrcType::None => (0, 0),
+            CrcType::Crc8 => {
+                let mut crc: u8 = 0x00;
+                for &byte in data {
+                    crc ^= byte;
+                    for _ in 0..8 {
+                        if crc & 0x80 != 0 {
+                            crc = (crc << 1) ^ 0x07;
+                        } else {
+                            crc <<= 1;
+                        }
+                    }
+                }
+                (crc as u16, 1)
+            }
+            CrcType::Crc16 => {
+                let mut crc: u16 = 0xFFFF;
+                for &byte in data {
+                    crc ^= (byte as u16) << 8;
+                    for _ in 0..8 {
+                        if crc & 0x8000 != 0 {
+                            crc = (crc << 1) ^ 0x1021;
+                        } else {
+                            crc <<= 1;
+                        }
+                    }
+                }
+                (crc, 2)
+            }
+        }
+    }
+
+    fn write(self, crc: u16, buf: &mut [u8]) {
+        match self {
+            CrcType::None => {}
+            CrcType::Crc8 => {
+                buf[0] = crc as u8;
+            }
+            CrcType::Crc16 => {
+                buf[0] = (crc >> 8) as u8;
+                buf[1] = crc as u8;
+            }
+        }
+    }
+}
+
+#[derive(Clone, Copy, defmt::Format)]
+pub enum Whitening {
+    None,
+    Ccitt,
+}
+
+impl Whitening {
+    fn apply(self, seed: u16, data: &mut [u8]) {
+        match self {
+            Whitening::None => return,
+            Whitening::Ccitt => {}
+        }
+
+        // Calculate CCITT whitening using x^9 + x^4 + 1 polynomial and LFSR
+        let mut lfsr: u16 = seed & 0x1FF;
+        for byte in data.iter_mut() {
+            let mut mask = 0u8;
+            for bit in 0..8 {
+                let feedback = ((lfsr >> 8) ^ (lfsr >> 3)) & 1;
+                lfsr = ((lfsr << 1) | feedback) & 0x1FF;
+                mask |= (feedback as u8) << (7 - bit);
+            }
+            *byte ^= mask;
+        }
+    }
+}
+
+#[derive(Clone, Copy, defmt::Format)]
+pub enum BpskPacket {
+    /// No framing, just send raw data
+    Raw,
+    /// Use a configurable framing
+    Framing {
+        /// Length of the preamble (0xAA) in bytes
+        preamble_len: usize,
+        /// Synchronization word (max 32 bytes)
+        sync_word: [u8; 32],
+        /// Sync word length
+        sync_word_len: usize,
+        /// Enable/disable reporting length in the packet
+        include_len: bool,
+        /// CRC size (0, 1 or 2 bytes)
+        crc_type: CrcType,
+        /// Whitening algorithm
+        whitening: Whitening,
+        /// Whitening LFSR seed (9-bit, 0x000..0x1FF)
+        whitening_seed: u16,
+    },
+}
+
+impl BpskPacket {
+    pub fn default() -> Self {
+        Self::Framing {
+            preamble_len: 32,
+            // Baker-13 code (2 bytes)
+            sync_word: [0x1F, 0x35, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00],
+            sync_word_len: 2,
+            include_len: true,
+            crc_type: CrcType::Crc16,
+            whitening: Whitening::Ccitt,
+            whitening_seed: 0x1FF,
+        }
+    }
+
+    fn to_bytes(self, payload: &[u8], buf: &mut [u8]) -> Result<u8, RadioError> {
+        match self {
+            BpskPacket::Raw => {
+                // Simple copy operation, no modifications made
+                buf[..payload.len()].copy_from_slice(payload);
+                payload.len().try_into().map_err(|_| RadioError::PayloadTooLarge)
+            }
+            BpskPacket::Framing {
+                preamble_len,
+                sync_word,
+                sync_word_len,
+                include_len,
+                crc_type,
+                whitening,
+                whitening_seed,
+            } => {
+                let len_field_size = if include_len { 1 } else { 0 };
+                let crc_size = match crc_type {
+                    CrcType::None => 0,
+                    CrcType::Crc8 => 1,
+                    CrcType::Crc16 => 2,
+                };
+                // Validate packet length
+                let total = preamble_len + sync_word_len + len_field_size + payload.len() + crc_size;
+
+                if total > buf.len() {
+                    return Err(RadioError::PayloadTooLarge);
+                }
+
+                // Keeps track of the current position in the buffer
+                let mut pos = 0;
+
+                // Write preamble which consists of 0xAA symbols
+                buf[pos..pos + preamble_len].fill(0xAA);
+                pos += preamble_len;
+
+                // Write sync word
+                buf[pos..pos + sync_word_len].copy_from_slice(&sync_word[..sync_word_len]);
+                pos += sync_word_len;
+
+                // Actual payload starts here
+                let data_start = pos;
+
+                // If enabled in the config, write length info
+                if include_len {
+                    let payload_len: u8 = payload.len().try_into().map_err(|_| RadioError::PayloadTooLarge)?;
+                    buf[pos] = payload_len;
+                    pos += 1;
+                }
+
+                // Copy the original payload itself
+                buf[pos..pos + payload.len()].copy_from_slice(payload);
+                pos += payload.len();
+
+                // Compute CRC before the whitening
+                let (crc, crc_len) = crc_type.compute(&buf[data_start..pos]);
+                crc_type.write(crc, &mut buf[pos..]);
+                pos += crc_len;
+
+                // Apply whitening
+                whitening.apply(whitening_seed, &mut buf[data_start..pos]);
+
+                // Additional validation - if buffer position can't fit in u8, it's invalid
+                pos.try_into().map_err(|_| RadioError::PayloadTooLarge)
+            }
+        }
+    }
+}
+
 #[derive(Clone, Copy, defmt::Format)]
 pub struct BpskConfig {
     pub frequency: u32,
@@ -39,6 +233,7 @@ pub struct BpskConfig {
     pub pa: PaSelection,
     pub power_dbm: i8,
     pub ramp: RampTime,
+    pub packet: BpskPacket,
 }
 
 impl Default for BpskConfig {
@@ -49,6 +244,7 @@ impl Default for BpskConfig {
             pa: PaSelection::LowPower,
             power_dbm: 14,
             ramp: RampTime::Us40,
+            packet: BpskPacket::default(),
         }
     }
 }
@@ -123,16 +319,16 @@ impl<SPI: SpiDevice, TX: OutputPin, RX: OutputPin, EN: OutputPin> Transmit
     for BpskRadio<'_, SPI, TX, RX, EN>
 {
     async fn tx(&mut self, data: &[u8]) -> Result<(), RadioError> {
-        if data.len() > 255 {
-            return Err(RadioError::PayloadTooLarge);
-        }
+        let mut buf = [0u8; 255];
+        // Convert buffer to packet with chosen framing
+        let len = self.config.packet.to_bytes(data, &mut buf)?;
 
         // Write payload to radio buffer
         self.radio.set_buffer_base(0x00, 0x00).await?;
-        self.radio.write_buffer(0x00, data).await?;
+        self.radio.write_buffer(0x00, &buf[..len as usize]).await?;
 
         // Update packet params with actual payload length
-        self.update_payload_len(data.len() as u8).await?;
+        self.update_payload_len(len).await?;
 
         // Clear any stale IRQ flags before starting TX
         self.radio.clear_irq(irq::ALL).await?;