diff --git a/src/ntag5/mod.rs b/src/ntag5/mod.rs
index 0de14d8..e08c096 100644
--- a/src/ntag5/mod.rs
+++ b/src/ntag5/mod.rs
@@ -9,6 +9,8 @@
 /// - READ REGISTER: write [BL_AD1, BL_AD0, REGA], read 1 byte
 /// - WRITE REGISTER: write [BL_AD1, BL_AD0, REGA, MASK, REGDATA]
 
+pub mod sram;
+
 use embedded_hal::i2c::I2c;
 
 pub const DEFAULT_ADDRESS: u8 = 0x54;
diff --git a/src/ntag5/sram.rs b/src/ntag5/sram.rs
new file mode 100644
index 0000000..496801c
--- /dev/null
+++ b/src/ntag5/sram.rs
@@ -0,0 +1,553 @@
+//! SRAM mailbox command/response protocol for NFC<->MCU communication.
+//!
+//! The phone writes a command packet to NTAG5Link SRAM via NFC.
+//! The MCU reads it over I2C, processes the command, and writes
+//! a response packet back to SRAM for the phone to read.
+
+use crate::ntag5::{self, Ntag5Link, SRAM_SIZE};
+use crate::pattern;
+use embedded_hal::i2c::I2c;
+
+// ---------------------------------------------------------------------------
+// Constants
+// ---------------------------------------------------------------------------
+
+/// Firmware version reported in GET_STATUS response.
+pub const FIRMWARE_VERSION: u8 = 0x01;
+
+// Command IDs (phone -> MCU)
+pub const CMD_WRITE_PATTERN: u8 = 0x01;
+pub const CMD_GET_STATUS: u8 = 0x02;
+pub const CMD_SET_ACTIVE: u8 = 0x03;
+pub const CMD_SYNC_START: u8 = 0x05;
+pub const CMD_SYNC_END: u8 = 0x06;
+pub const CMD_READ_LIBRARY: u8 = 0x07;
+pub const CMD_READ_NEXT: u8 = 0x08;
+
+// Response marker
+pub const RESPONSE_MARKER: u8 = 0xFF;
+
+// Status codes
+pub const STATUS_OK: u8 = 0x00;
+pub const STATUS_BAD_CRC: u8 = 0x01;
+pub const STATUS_BAD_CMD: u8 = 0x02;
+pub const STATUS_EEPROM_FAIL: u8 = 0x03;
+pub const STATUS_INVALID_INDEX: u8 = 0x04;
+
+/// Command header size (cmd + seq + payload_len_u16 + crc_u16).
+pub const CMD_HEADER_SIZE: usize = 6;
+
+/// Response header size (marker + seq + status + payload_len + crc_u16).
+pub const RSP_HEADER_SIZE: usize = 6;
+
+// ---------------------------------------------------------------------------
+// Mailbox state
+// ---------------------------------------------------------------------------
+
+/// Persistent state for the mailbox protocol across commands.
+pub struct MailboxState {
+    /// True while a SYNC_START..SYNC_END sequence is in progress.
+    pub syncing: bool,
+    /// Pattern count supplied by SYNC_START.
+    pub sync_count: u8,
+    /// Current (mA setting) supplied by SYNC_START.
+    pub sync_current: u8,
+    /// LED mode supplied by SYNC_START.
+    pub sync_mode: u8,
+    /// Next pattern index for READ_NEXT.
+    pub read_index: u8,
+    /// Total patterns available for READ_NEXT iteration.
+    pub read_count: u8,
+}
+
+impl MailboxState {
+    pub fn new() -> Self {
+        Self {
+            syncing: false,
+            sync_count: 0,
+            sync_current: 0,
+            sync_mode: 0,
+            read_index: 0,
+            read_count: 0,
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// CRC helpers
+// ---------------------------------------------------------------------------
+
+/// Compute CRC-16/CCITT-FALSE over the given data, starting from an
+/// initial CRC value. This allows continuing a CRC computation across
+/// multiple buffers.
+fn crc16_continue(init: u16, data: &[u8]) -> u16 {
+    let mut crc = init;
+    for &b in data {
+        crc ^= (b as u16) << 8;
+        for _ in 0..8 {
+            if crc & 0x8000 != 0 {
+                crc = (crc << 1) ^ 0x1021;
+            } else {
+                crc <<= 1;
+            }
+            crc &= 0xFFFF;
+        }
+    }
+    crc
+}
+
+// ---------------------------------------------------------------------------
+// Protocol parse / build
+// ---------------------------------------------------------------------------
+
+/// Parse the 6-byte command header. Returns (cmd, seq, payload_len) or None
+/// if the buffer is too short or the command ID is out of range.
+fn parse_header(buf: &[u8]) -> Option<(u8, u8, u16)> {
+    if buf.len() < CMD_HEADER_SIZE {
+        return None;
+    }
+    let cmd = buf[0];
+    if cmd == 0 || cmd >= 0x80 {
+        return None;
+    }
+    let seq = buf[1];
+    let payload_len = (buf[2] as u16) | ((buf[3] as u16) << 8); // LE
+    Some((cmd, seq, payload_len))
+}
+
+/// Verify CRC over the command packet.
+/// CRC is computed over bytes 0-3 (header minus CRC) + payload, then
+/// compared to the big-endian CRC in bytes 4-5.
+fn verify_crc(buf: &[u8], payload_len: u16) -> bool {
+    let total = CMD_HEADER_SIZE + payload_len as usize;
+    if buf.len() < total {
+        return false;
+    }
+    let expected = ((buf[4] as u16) << 8) | buf[5] as u16;
+    let crc = crc16_continue(0xFFFF, &buf[0..4]);
+    let crc = crc16_continue(crc, &buf[CMD_HEADER_SIZE..total]);
+    crc == expected
+}
+
+/// Build a response packet into `buf`. Returns the total number of bytes
+/// written (padded to a multiple of 4 for SRAM block alignment).
+fn build_response(buf: &mut [u8], seq: u8, status: u8, payload: &[u8]) -> usize {
+    let plen = payload.len();
+    buf[0] = RESPONSE_MARKER;
+    buf[1] = seq;
+    buf[2] = status;
+    buf[3] = plen as u8;
+    // CRC placeholder at [4..6], computed below
+    // Copy payload
+    buf[RSP_HEADER_SIZE..RSP_HEADER_SIZE + plen].copy_from_slice(payload);
+    // Compute CRC over bytes 0-3 + payload
+    let crc = crc16_continue(0xFFFF, &buf[0..4]);
+    let crc = crc16_continue(crc, payload);
+    buf[4] = (crc >> 8) as u8;
+    buf[5] = crc as u8;
+    // Pad total length to multiple of 4
+    let raw_len = RSP_HEADER_SIZE + plen;
+    let padded = (raw_len + 3) & !3;
+    // Zero padding bytes
+    for b in buf[raw_len..padded].iter_mut() {
+        *b = 0;
+    }
+    padded
+}
+
+// ---------------------------------------------------------------------------
+// EEPROM helpers
+// ---------------------------------------------------------------------------
+
+/// Block address for pattern N in the EEPROM library.
+fn pattern_block(index: u8) -> u16 {
+    pattern::LIBRARY_BASE_BLOCK + 4 + (index as u16) * 28
+}
+
+/// Read the XBLK library header from EEPROM.
+/// Returns None if magic/version don't match.
+fn read_library_header<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+) -> Result<Option<pattern::LibraryHeader>, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    let mut hdr_buf = [0u8; pattern::HEADER_SIZE];
+    ntag.read_memory(pattern::LIBRARY_BASE_BLOCK, &mut hdr_buf)?;
+    Ok(pattern::parse_header(&hdr_buf))
+}
+
+/// Write the 16-byte header to EEPROM (4 blocks starting at LIBRARY_BASE_BLOCK).
+fn write_header_to_eeprom<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    hdr: &[u8; pattern::HEADER_SIZE],
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<(), ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    for i in 0..4u16 {
+        let mut chunk = [0u8; 4];
+        let off = (i as usize) * 4;
+        chunk.copy_from_slice(&hdr[off..off + 4]);
+        ntag.write_verify_block(pattern::LIBRARY_BASE_BLOCK + i, &chunk, delay)?;
+    }
+    Ok(())
+}
+
+/// Read all pattern data from EEPROM for the given count (for CRC computation).
+/// Returns the number of bytes read into `all_data`.
+fn read_all_pattern_data<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    count: u8,
+    all_data: &mut [u8],
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    let total = count as usize * pattern::PATTERN_ENTRY_SIZE;
+    // Read in chunks — read_memory can handle arbitrary lengths
+    // but we read per-pattern for clarity
+    for i in 0..count as usize {
+        let block = pattern_block(i as u8);
+        let off = i * pattern::PATTERN_ENTRY_SIZE;
+        ntag.read_memory(block, &mut all_data[off..off + pattern::PATTERN_ENTRY_SIZE])?;
+    }
+    Ok(total)
+}
+
+/// Recalculate the header CRC based on current EEPROM pattern data,
+/// then write the updated header back.
+fn recalculate_header_crc<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    count: u8,
+    active: u8,
+    current: u8,
+    led_mode: u8,
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<(), ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    // Read all pattern data for CRC
+    let mut all_data = [0u8; pattern::MAX_PATTERNS * pattern::PATTERN_ENTRY_SIZE];
+    let data_len = read_all_pattern_data(ntag, count, &mut all_data)?;
+
+    let mut hdr = [0u8; pattern::HEADER_SIZE];
+    pattern::serialize_header(count, active, current, led_mode, &all_data[..data_len], &mut hdr);
+    write_header_to_eeprom(ntag, &hdr, delay)
+}
+
+/// After a standalone WRITE_PATTERN (not during sync), re-read the header
+/// and recalculate its CRC to account for the new pattern data.
+fn update_header_after_write<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<(), ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    let hdr = match read_library_header(ntag)? {
+        Some(h) => h,
+        None => return Ok(()), // no valid header, nothing to update
+    };
+    recalculate_header_crc(
+        ntag,
+        hdr.pattern_count,
+        hdr.active_index,
+        hdr.current,
+        hdr.led_mode,
+        delay,
+    )
+}
+
+// ---------------------------------------------------------------------------
+// Command handlers
+// ---------------------------------------------------------------------------
+
+/// GET_STATUS (0x02): Return firmware version + library summary.
+fn handle_get_status<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    seq: u8,
+    rsp_buf: &mut [u8],
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    let mut payload = [0u8; 5];
+    payload[0] = FIRMWARE_VERSION;
+    match read_library_header(ntag)? {
+        Some(h) => {
+            payload[1] = h.pattern_count;
+            payload[2] = h.active_index;
+            payload[3] = h.current;
+            payload[4] = h.led_mode;
+        }
+        None => {
+            // No valid header — return zeros
+        }
+    }
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &payload))
+}
+
+/// WRITE_PATTERN (0x01): Write a 112-byte pattern entry to EEPROM.
+fn handle_write_pattern<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    state: &MailboxState,
+    seq: u8,
+    payload: &[u8],
+    rsp_buf: &mut [u8],
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    if payload.len() < 1 + pattern::PATTERN_ENTRY_SIZE {
+        return Ok(build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]));
+    }
+    let index = payload[0];
+    if index as usize >= pattern::MAX_PATTERNS {
+        return Ok(build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]));
+    }
+
+    // Write 112 bytes = 28 blocks
+    let base_block = pattern_block(index);
+    let pattern_data = &payload[1..1 + pattern::PATTERN_ENTRY_SIZE];
+    for i in 0..28u16 {
+        let off = (i as usize) * 4;
+        let mut chunk = [0u8; 4];
+        chunk.copy_from_slice(&pattern_data[off..off + 4]);
+        match ntag.write_verify_block(base_block + i, &chunk, delay) {
+            Ok(()) => {}
+            Err(ntag5::Error::VerifyFailed) => {
+                return Ok(build_response(rsp_buf, seq, STATUS_EEPROM_FAIL, &[]));
+            }
+            Err(e) => return Err(e),
+        }
+    }
+
+    // If not syncing, update header CRC to reflect changed pattern data
+    if !state.syncing {
+        match update_header_after_write(ntag, delay) {
+            Ok(()) => {}
+            Err(ntag5::Error::VerifyFailed) => {
+                return Ok(build_response(rsp_buf, seq, STATUS_EEPROM_FAIL, &[]));
+            }
+            Err(e) => return Err(e),
+        }
+    }
+
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &[]))
+}
+
+/// SET_ACTIVE (0x03): Change the active pattern index in the EEPROM header.
+fn handle_set_active<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    seq: u8,
+    payload: &[u8],
+    rsp_buf: &mut [u8],
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    if payload.is_empty() {
+        return Ok(build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]));
+    }
+    let index = payload[0];
+
+    let hdr = match read_library_header(ntag)? {
+        Some(h) => h,
+        None => return Ok(build_response(rsp_buf, seq, STATUS_EEPROM_FAIL, &[])),
+    };
+
+    if index >= hdr.pattern_count {
+        return Ok(build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]));
+    }
+
+    // Recalculate header with new active index
+    match recalculate_header_crc(ntag, hdr.pattern_count, index, hdr.current, hdr.led_mode, delay)
+    {
+        Ok(()) => {}
+        Err(ntag5::Error::VerifyFailed) => {
+            return Ok(build_response(rsp_buf, seq, STATUS_EEPROM_FAIL, &[]));
+        }
+        Err(e) => return Err(e),
+    }
+
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &[]))
+}
+
+/// SYNC_START (0x05): Begin a bulk pattern upload session.
+fn handle_sync_start(
+    state: &mut MailboxState,
+    seq: u8,
+    payload: &[u8],
+    rsp_buf: &mut [u8],
+) -> usize {
+    if payload.len() < 3 {
+        return build_response(rsp_buf, seq, STATUS_BAD_CMD, &[]);
+    }
+    let count = payload[0];
+    if count == 0 || count as usize > pattern::MAX_PATTERNS {
+        return build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]);
+    }
+    state.syncing = true;
+    state.sync_count = count;
+    state.sync_current = payload[1];
+    state.sync_mode = payload[2];
+    build_response(rsp_buf, seq, STATUS_OK, &[])
+}
+
+/// SYNC_END (0x06): Finalize bulk upload — write the XBLK header with CRC.
+fn handle_sync_end<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    state: &mut MailboxState,
+    seq: u8,
+    rsp_buf: &mut [u8],
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    if !state.syncing {
+        return Ok(build_response(rsp_buf, seq, STATUS_BAD_CMD, &[]));
+    }
+
+    // Active index defaults to 0
+    match recalculate_header_crc(
+        ntag,
+        state.sync_count,
+        0, // active_index = 0
+        state.sync_current,
+        state.sync_mode,
+        delay,
+    ) {
+        Ok(()) => {}
+        Err(ntag5::Error::VerifyFailed) => {
+            state.syncing = false;
+            return Ok(build_response(rsp_buf, seq, STATUS_EEPROM_FAIL, &[]));
+        }
+        Err(e) => {
+            state.syncing = false;
+            return Err(e);
+        }
+    }
+
+    state.syncing = false;
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &[]))
+}
+
+/// READ_LIBRARY (0x07): Return library summary and prepare for READ_NEXT.
+fn handle_read_library<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    state: &mut MailboxState,
+    seq: u8,
+    rsp_buf: &mut [u8],
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    let mut payload = [0u8; 4];
+    match read_library_header(ntag)? {
+        Some(h) => {
+            payload[0] = h.pattern_count;
+            payload[1] = h.active_index;
+            payload[2] = h.current;
+            payload[3] = h.led_mode;
+            state.read_index = 0;
+            state.read_count = h.pattern_count;
+        }
+        None => {
+            state.read_index = 0;
+            state.read_count = 0;
+        }
+    }
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &payload))
+}
+
+/// READ_NEXT (0x08): Return the next pattern entry (112 bytes).
+fn handle_read_next<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    state: &mut MailboxState,
+    seq: u8,
+    rsp_buf: &mut [u8],
+) -> Result<usize, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    if state.read_index >= state.read_count {
+        return Ok(build_response(rsp_buf, seq, STATUS_INVALID_INDEX, &[]));
+    }
+
+    let mut entry = [0u8; pattern::PATTERN_ENTRY_SIZE];
+    let block = pattern_block(state.read_index);
+    ntag.read_memory(block, &mut entry)?;
+    state.read_index += 1;
+    Ok(build_response(rsp_buf, seq, STATUS_OK, &entry))
+}
+
+// ---------------------------------------------------------------------------
+// Main entry point
+// ---------------------------------------------------------------------------
+
+/// Read SRAM, dispatch the command, and write the response back.
+///
+/// Returns `Ok(true)` if a command was processed, `Ok(false)` if no valid
+/// command was found in SRAM (e.g., empty buffer, bad header, bad CRC).
+///
+/// I2C errors propagate as `Err`. EEPROM verify failures are reported via
+/// a STATUS_EEPROM_FAIL response (not as Err).
+pub fn process_command<I2C, E>(
+    ntag: &mut Ntag5Link<I2C>,
+    state: &mut MailboxState,
+    delay: &mut impl embedded_hal::delay::DelayNs,
+) -> Result<bool, ntag5::Error<E>>
+where
+    I2C: I2c<Error = E>,
+{
+    // Read full SRAM
+    let mut sram = [0u8; SRAM_SIZE];
+    ntag.read_sram(&mut sram)?;
+
+    // Parse header
+    let (cmd, seq, payload_len) = match parse_header(&sram) {
+        Some(h) => h,
+        None => return Ok(false),
+    };
+
+    // Bounds check
+    let total = CMD_HEADER_SIZE + payload_len as usize;
+    if total > SRAM_SIZE {
+        return Ok(false);
+    }
+
+    // Verify CRC
+    if !verify_crc(&sram, payload_len) {
+        // Write BAD_CRC response
+        let mut rsp = [0u8; SRAM_SIZE];
+        let len = build_response(&mut rsp, seq, STATUS_BAD_CRC, &[]);
+        ntag.write_sram_blocks(0, &rsp[..len])?;
+        return Ok(true);
+    }
+
+    let payload = &sram[CMD_HEADER_SIZE..total];
+
+    // Dispatch
+    let mut rsp = [0u8; SRAM_SIZE];
+    let rsp_len = match cmd {
+        CMD_GET_STATUS => handle_get_status(ntag, seq, &mut rsp)?,
+        CMD_WRITE_PATTERN => {
+            handle_write_pattern(ntag, state, seq, payload, &mut rsp, delay)?
+        }
+        CMD_SET_ACTIVE => handle_set_active(ntag, seq, payload, &mut rsp, delay)?,
+        CMD_SYNC_START => handle_sync_start(state, seq, payload, &mut rsp),
+        CMD_SYNC_END => handle_sync_end(ntag, state, seq, &mut rsp, delay)?,
+        CMD_READ_LIBRARY => handle_read_library(ntag, state, seq, &mut rsp)?,
+        CMD_READ_NEXT => handle_read_next(ntag, state, seq, &mut rsp)?,
+        _ => build_response(&mut rsp, seq, STATUS_BAD_CMD, &[]),
+    };
+
+    ntag.write_sram_blocks(0, &rsp[..rsp_len])?;
+    Ok(true)
+}