diff --git a/mcp4.py b/mcp4.py
index 521918d..c0ba721 100644
--- a/mcp4.py
+++ b/mcp4.py
@@ -71,6 +71,22 @@ class TerminalControl:
         )
 
 
+def command_bytes(address: int, command: int, data: int = 0x0) -> bytearray:
+    """Translate an address, command, and data into bytes to send.
+
+    - Address is a 4-bit memory address.
+    - Command is a 2-bit command code.
+    - Data is 2 bits for increment and decrement operations (ignored), and
+      10 bits for read and write operations.
+
+    """
+    command_byte = address << 4 & 0b11110000 | command << 2 & 0b00001100
+    if command in (0b00, 0b11):
+        # Include data byte for 10 total bits of data
+        return bytearray([command_byte | (0b11 & data >> 8), data & 0xFF])
+    return bytearray([command_byte])
+
+
 class MCP4:
     """MicroPython MCP413X/415X/423X/425X SPI driver"""
 
@@ -88,21 +104,6 @@ class MCP4:
         self.spi = spi
         self.cs = cs
 
-    def _bytes(self, address: int, command: int, data: int = 0x0) -> bytearray:
-        """Translate an address, command, and data into bytes to send.
-
-        - Address is a 4-bit memory address.
-        - Command is a 2-bit command code.
-        - Data is 2 bits for increment and decrement operations (ignored), and
-          10 bits for read and write operations.
-
-        """
-        command_byte = address << 4 & 0b11110000 | command << 2 & 0b00001100
-        if command in (0b00, 0b11):
-            # Include data byte for 10 total bits of data
-            return bytearray([command_byte | (0b11 & data >> 8), data & 0xFF])
-        return bytearray([command_byte])
-
     def _write(self, data: bytearray) -> bytearray:
         """Write data to the SPI interface, returning its output."""
         output = bytearray(len(data))
@@ -112,7 +113,7 @@ class MCP4:
     def do(self, address: int, command: int, data: int = 0x0) -> int:
         """Execute a command on the MCP4, returning its integer result."""
         self.cs(0)
-        output = self._write(self._bytes(address, command, data))
+        output = self._write(command_bytes(address, command, data))
         self.cs(1)
 
         OK = 0b11111110
diff --git a/tests/__init__.py b/tests/__init__.py
new file mode 100644
index 0000000..5f6deed
--- /dev/null
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+from .test_mcp4 import *
diff --git a/tests/test_mcp4.py b/tests/test_mcp4.py
new file mode 100644
index 0000000..bf8c015
--- /dev/null
+++ b/tests/test_mcp4.py
@@ -0,0 +1,31 @@
+import unittest
+
+import mcp4
+
+
+class CommandTests(unittest.TestCase):
+    def assertBitsEqual(self, data: bytearray, expected: str):
+        self.assertEqual(
+            "".join("{:08b}".format(byte) for byte in data),
+            expected.replace(" ", ""),
+        )
+
+    def test_increment(self) -> None:
+        self.assertBitsEqual(mcp4.command_bytes(0b0000, 0b01, 0b00), "0000 01 00")
+        self.assertBitsEqual(mcp4.command_bytes(0b0001, 0b01, 0b00), "0001 01 00")
+
+    def test_decrement(self) -> None:
+        self.assertBitsEqual(mcp4.command_bytes(0b0000, 0b10, 0b00), "0000 10 00")
+        self.assertBitsEqual(mcp4.command_bytes(0b0001, 0b10, 0b00), "0001 10 00")
+
+    def test_read(self) -> None:
+        self.assertBitsEqual(
+            mcp4.command_bytes(0b0000, 0b11, 0b0000000000),
+            "0000 11 00 0000 0000",
+        )
+
+    def test_write(self) -> None:
+        self.assertBitsEqual(
+            mcp4.command_bytes(0b0000, 0b00, 0b0001111111),
+            "0000 00 00 0111 1111",
+        )