diff --git a/Makefile b/Makefile
index 7c6303c..24181c7 100644
--- a/Makefile
+++ b/Makefile
@@ -63,7 +63,7 @@ endef
 %.post.fst: rtl/%.post.vvp tb/%.py FORCE
 	$(run-vvp)
 
-MODULES := pcs pmd nrzi_encode nrzi_decode scramble descramble mdio mdio_io mii_io_rx
+MODULES := pcs pmd nrzi_encode nrzi_decode scramble descramble mdio mdio_io mii_io_rx mii_io_tx
 
 .PHONY: test
 test: $(addsuffix .fst,$(MODULES)) $(addsuffix .post.fst,$(MODULES))
diff --git a/rtl/mii_io_tx.v b/rtl/mii_io_tx.v
new file mode 100644
index 0000000..79eaa99
--- /dev/null
+++ b/rtl/mii_io_tx.v
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: AGPL-3.0-Only
+/*
+ * Copyright (C) 2022 Sean Anderson <seanga2@gmail.com>
+ */
+
+`include "common.vh"
+`include "io.vh"
+
+module mii_io_tx (
+	/* On-chip */
+	input clk,
+	output reg ce,
+	output reg enable,
+	output reg err,
+	output reg [3:0] data,
+
+	/* Off-chip */
+	output reg tx_clk,
+	input tx_en,
+	input tx_er,
+	input [3:0] txd
+);
+
+	reg ce_next;
+	reg tx_clk_p_next, tx_clk_n, tx_clk_n_next;
+	reg [2:0] counter, counter_next;
+	/* I have no idea why we need to use initial... */
+	initial counter = 4;
+
+	always @(*) begin
+		tx_clk_p_next = 0;
+		tx_clk_n_next = 0;
+		ce_next = 0;
+		counter_next = counter - 1;
+		case (counter)
+		4, 3: begin
+			tx_clk_p_next = 1;
+			tx_clk_n_next = 1;
+		end
+		2: tx_clk_p_next = 1;
+		1: ;
+		0: begin
+			ce_next = 1;
+			counter_next = 4;
+		end
+		endcase
+	end
+
+	always @(posedge clk) begin
+		counter <= counter_next;
+		tx_clk_n <= tx_clk_n_next;
+		ce <= ce_next;
+	end
+
+`ifdef SYNTHESIS
+	SB_IO #(
+		.PIN_TYPE(`PIN_OUTPUT_ALWAYS | `PIN_OUTPUT_DDR)
+	) tx_clk_pin (
+		.PACKAGE_PIN(tx_clk),
+		.OUTPUT_CLK(clk),
+		.D_OUT_0(tx_clk_p_next),
+		.D_OUT_1(tx_clk_n)
+	);
+
+	SB_IO #(
+		.PIN_TYPE(`PIN_INPUT_REGISTERED)
+	) tx_en_pin (
+		.PACKAGE_PIN(tx_en),
+		.CLOCK_ENABLE(ce_next),
+		.INPUT_CLK(clk),
+		.D_IN_0(enable)
+	);
+
+	SB_IO #(
+		.PIN_TYPE(`PIN_INPUT_REGISTERED)
+	) tx_er_pin (
+		.PACKAGE_PIN(tx_er),
+		.CLOCK_ENABLE(ce_next),
+		.INPUT_CLK(clk),
+		.D_IN_0(err)
+	);
+
+	genvar i;
+	generate for (i = 0; i < 4; i = i + 1) begin
+		SB_IO #(
+			.PIN_TYPE(`PIN_INPUT_REGISTERED)
+		) txd_pin (
+			.PACKAGE_PIN(txd[i]),
+			.CLOCK_ENABLE(ce_next),
+			.INPUT_CLK(clk),
+			.D_IN_0(data[i])
+		);
+	end
+	endgenerate
+`else
+	always @(posedge clk) begin
+		tx_clk <= tx_clk_p_next;
+		if (ce_next) begin
+			enable <= tx_en;
+			err <= tx_er;
+			data <= txd;
+		end
+	end
+
+	always @(negedge clk)
+		tx_clk <= tx_clk_n;
+`endif
+
+	`DUMP(0)
+
+endmodule
diff --git a/tb/mii_io_tx.py b/tb/mii_io_tx.py
new file mode 100644
index 0000000..2fef5d8
--- /dev/null
+++ b/tb/mii_io_tx.py
@@ -0,0 +1,51 @@
+# SPDX-License-Identifier: AGPL-3.0-Only
+# Copyright (C) 2022 Sean Anderson <seanga2@gmail.com>
+
+import random
+
+import cocotb
+from cocotb.clock import Clock
+from cocotb.triggers import ClockCycles, Edge, FallingEdge, First, RisingEdge, Timer
+from cocotb.types import LogicArray
+
+from .util import ClockEnable
+
+@cocotb.test(timeout_time=500, timeout_unit='ns')
+async def test_io(io):
+    io.tx_en.value = LogicArray('X')
+    io.tx_er.value = LogicArray('X')
+    io.txd.value = LogicArray('X' * 4)
+    await Timer(1)
+    await cocotb.start(Clock(io.clk, 8, units='ns').start())
+
+    async def send_datum(enable, err, data):
+        await RisingEdge(io.tx_clk)
+        io.tx_en.value = LogicArray('X')
+        io.tx_er.value = LogicArray('X')
+        io.txd.value = LogicArray('X' * 4)
+        await Timer(25, 'ns')
+        io.tx_en.value = enable
+        io.tx_er.value = err
+        io.txd.value = data
+
+    async def send_data():
+        await send_datum(0, 1, 1)
+        await send_datum(1, 0, 2)
+        await send_datum(0, 1, 3)
+        await send_datum(1, 0, 4)
+        await send_datum(0, 1, 5)
+    await cocotb.start(send_data())
+
+    async def recv_datum(enable, err, data):
+        await RisingEdge(io.ce)
+        await RisingEdge(io.clk)
+        assert io.ce.value
+        assert io.enable.value == enable
+        assert io.err.value == err
+        assert io.data.value == data
+
+    await recv_datum(0, 1, 1)
+    await recv_datum(1, 0, 2)
+    await recv_datum(0, 1, 3)
+    await recv_datum(1, 0, 4)
+    await recv_datum(0, 1, 5)