Add NTAG5 SRAM read/write and FD pin config for M6 mailbox

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ntag5/mod.rs

@@ -18,6 +18,17 @@ pub const SESSION_CONFIG_REG: u16 = 0x10A1; // CONFIG_0_REG, CONFIG_1_REG, CONFI
 pub const SESSION_EH_CONFIG_REG: u16 = 0x10A7; // EH_CONFIG_REG, RFU
 pub const SESSION_I2C_SLAVE_REG: u16 = 0x10A9; // I2C_SLAVE_ADDR_REG, I2C_SLAVE_CONFIG_REG
 
+// SRAM I2C block address range (64 blocks × 4 bytes = 256 bytes)
+pub const SRAM_BASE_BLOCK: u16 = 0x10F8;
+pub const SRAM_BLOCK_COUNT: u16 = 64;
+pub const SRAM_SIZE: usize = 256;
+
+// Session register for ED/FD pin configuration
+pub const SESSION_ED_FD_PIN_CFG: u16 = 0x10A3;
+
+// FD pin mode: active on SRAM write by RF, cleared on I2C read
+pub const FD_MODE_SRAM_RF_WRITE: u8 = 0x04;
+
 // EEPROM user memory I2C block addresses
 // Block 0 = CC (capability container), blocks 1+ = NDEF data
 pub const EEPROM_BLOCK_0: u16 = 0x0000;
@@ -143,6 +154,47 @@ where
         Ok(buf[0])
     }
 
+    /// Write a single session register byte (WRITE REGISTER command).
+    /// Protocol: write [BL_AD1, BL_AD0, REGA, MASK, REGDATA].
+    /// The MASK selects which bits to modify (1 = modify, 0 = keep).
+    pub fn write_register(&mut self, block: u16, reg_addr: u8, mask: u8, data: u8) -> Result<(), E> {
+        let addr_bytes = block.to_be_bytes();
+        self.i2c.write(
+            self.addr,
+            &[addr_bytes[0], addr_bytes[1], reg_addr, mask, data],
+        )
+    }
+
+    // ---- SRAM access ----
+
+    /// Read N bytes from SRAM starting at byte offset 0.
+    /// SRAM is at I2C blocks 0x10F8-0x10FF (64 blocks × 4 bytes = 256 bytes).
+    /// Always reads from the start of SRAM (block-aligned).
+    pub fn read_sram(&mut self, buf: &mut [u8]) -> Result<(), E> {
+        self.read_memory(SRAM_BASE_BLOCK, buf)
+    }
+
+    /// Write data to SRAM starting at block offset from SRAM base.
+    /// Data length must be a multiple of 4 (NTAG5 block size).
+    pub fn write_sram_blocks(&mut self, block_offset: u16, data: &[u8]) -> Result<(), E> {
+        for (i, chunk) in data.chunks(4).enumerate() {
+            let mut block_data = [0u8; 4];
+            for (j, &b) in chunk.iter().enumerate() {
+                block_data[j] = b;
+            }
+            self.write_memory_block(SRAM_BASE_BLOCK + block_offset + i as u16, &block_data)?;
+        }
+        Ok(())
+    }
+
+    /// Configure FD pin for SRAM-write-by-RF indication.
+    /// FD goes low when phone writes to SRAM, returns high when MCU reads SRAM.
+    pub fn configure_fd_sram_write(&mut self) -> Result<(), E> {
+        // ED_FD_PIN_CFG register index 1 within the session block
+        // Bits 2:0 control FD output mode
+        self.write_register(SESSION_ED_FD_PIN_CFG, 0x01, 0x07, FD_MODE_SRAM_RF_WRITE)
+    }
+
     // ---- Config check ----
 
     /// Read session registers and compare against expected xblink config.