diff --git a/Cargo.lock b/Cargo.lock
index ce740b9..dc289e6 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,15 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "embedded-hal"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "361a90feb7004eca4019fb28352a9465666b24f840f5c3cddf0ff13920590b89"
+
 [[package]]
 name = "zel-tca8418"
 version = "0.1.0"
+dependencies = [
+ "embedded-hal",
+]
diff --git a/Cargo.toml b/Cargo.toml
index d9191de..210a175 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,3 +4,4 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+embedded-hal = "1.0.0"
diff --git a/src/lib.rs b/src/lib.rs
index 1ef6109..69650c1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,2 +1,159 @@
 #![no_std]
 
+use embedded_hal::i2c::SevenBitAddress;
+
+pub const I2C_ADDR: SevenBitAddress = 0b0_0110100;
+
+pub struct TCA8418<I2C> {
+    i2c: I2C,
+}
+
+#[derive(Clone, Copy)]
+pub struct MatrixConfig {
+    rows: u8,
+    columns: u16,
+}
+
+impl MatrixConfig {
+    /// A new config where the matrix has no rows or columns and all of the pins are
+    /// allocated for GPIO.
+    pub const fn new_empty() -> Self {
+        MatrixConfig {
+            rows: 0,
+            columns: 0,
+        }
+    }
+
+    /// Add a row to the keyboard matrix, meaning its pin will not be available for GPIO.
+    ///
+    /// `row_number` must be in the range `0..=7`
+    pub const fn with_row(self, row_number: u8) -> Self {
+        if row_number >= 8 {
+            panic!("row_number out of bounds");
+        }
+        MatrixConfig {
+            rows: self.rows | (1 << row_number),
+            columns: self.columns,
+        }
+    }
+
+    /// Add a column to the keyboard matrix, meaning its pin will not be available for GPIO.
+    ///
+    /// `column_number` must be in the range `0..=9`
+    pub const fn with_column(self, column_number: u8) -> Self {
+        if column_number >= 10 {
+            panic!("column_number out of bounds");
+        }
+        MatrixConfig {
+            rows: self.rows,
+            columns: self.columns | (1 << column_number),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct InterruptConfig(u8);
+
+impl InterruptConfig {
+    /// A new interrupt with the following configuration:
+    /// - GPI events are tracked when keypad is locked
+    /// - Overflow data is lost
+    /// - Processor interrupt remains asserted (or low) if host tries to clear
+    ///   interrupt while there is still a pending key press, key release or
+    ///   GPI interrupt
+    /// - ~INT is not asserted if the FIFO overflows
+    /// - ~INT is not asserted after a correct unlock key sequence
+    /// - ~INT is not asserted for a change on a GPI
+    /// - ~INT is not asserted when a key event occurs
+    pub const fn new() -> Self {
+        Self(0)
+    }
+
+    pub const fn without_gpi_event_tracking_when_locked(self) -> Self {
+        InterruptConfig(self.0 | 0b0100_0000)
+    }
+
+    pub const fn with_overflow_data_shifting(self) -> Self {
+        InterruptConfig(self.0 | 0b0010_0000)
+    }
+
+    pub const fn with_temporary_interrupt_deassertion(self) -> Self {
+        InterruptConfig(self.0 | 0b0001_0000)
+    }
+}
+
+#[derive(Clone, Copy)]
+pub struct KeySet(u128);
+
+impl KeySet {
+    fn contains(&self, key_number: u8) -> bool {
+        (self.0 & (1 << key_number)) != 0
+    }
+}
+
+impl core::ops::Not for KeySet {
+    type Output = KeySet;
+
+    fn not(self) -> Self::Output {
+        KeySet(!self.0)
+    }
+}
+
+pub struct KeySetIter {
+    key_set: KeySet,
+    shift: u8,
+}
+
+impl core::iter::Iterator for KeySetIter {
+    type Item = u8;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let next_shift = self.key_set.0.trailing_zeros() as u8 + 1;
+        if next_shift >= 128 {
+            return None;
+        }
+
+        self.key_set.0 >>= next_shift;
+        self.shift += next_shift;
+
+        Some(self.shift - 1)
+    }
+}
+
+impl core::iter::IntoIterator for KeySet {
+    type Item = u8;
+
+    type IntoIter = KeySetIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        KeySetIter {
+            key_set: self,
+            shift: 0
+        }
+    }
+}
+
+impl<I2C: embedded_hal::i2c::I2c> TCA8418<I2C> {
+    pub fn new(
+        i2c: I2C,
+        matrix_config: MatrixConfig,
+        interrupt_config: InterruptConfig,
+    ) -> Self {
+        let mut self_ = Self { i2c };
+        self_.init(matrix_config, interrupt_config);
+        self_
+    }
+
+    fn init(&mut self, matrix_config: MatrixConfig, interrupt_config: InterruptConfig) {
+        self.i2c.write(I2C_ADDR, &[0x1D, matrix_config.rows]);
+        self.i2c
+            .write(I2C_ADDR, &[0x1E, matrix_config.columns as u8]);
+        self.i2c
+            .write(I2C_ADDR, &[0x1F, (matrix_config.columns >> 8) as u8]);
+        self.i2c.write(I2C_ADDR, &[0x01, interrupt_config.0]);
+        self.i2c.write(I2C_ADDR, &[0x0F, 0]);
+        self.i2c.write(I2C_ADDR, &[0x10, 0]);
+        self.i2c.write(I2C_ADDR, &[0x0E, 0]);
+    }
+}
+