Add UART transmit module

I join everyone and their mother in creating my own UART. 8n1 only, and 2
baud rates. Accepts AXI-stream.

Signed-off-by: Sean Anderson <seanga2@gmail.com>

Makefile

@@ -142,6 +142,7 @@ MODULES += phy_core
 MODULES += pmd_dp83223
 MODULES += pmd_dp83223_rx
 MODULES += scramble
+MODULES += uart_tx
 MODULES += wb_mux
 
 .PHONY: test

README.adoc

@@ -200,6 +200,11 @@ nebulously-specified, and limited-in-number iCE40 PLLs.
 This module implements a scrambler as described in ANSI X3.264-1995 section
 7.1.1.
 
+=== `uart_tx`
+
+A standard UART transmit module, accepting AXI-stream. 8n1 only. Supports
+115,200 and 4,000,000 baud.
+
 === `wb_mux`
 
 This implements a simple Wishbone mux, allowing a single master to access

rtl/uart_tx.v

@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: AGPL-3.0-Only
+/*
+ * Copyright (C) 2022 Sean Anderson <seanga2@gmail.com>
+ *
+ * 8n1@115200; no one uses anything else (and neither do I)
+ */
+
+`include "common.vh"
+
+module uart_tx (
+	input clk,
+
+	input [7:0] data,
+	output reg ready,
+	input valid,
+
+	output reg tx,
+
+	/* Run at 4M for testing */
+	input high_speed
+);
+
+	/* 1085 cycles, for 115200 baud with a 125 MHz clock */
+	parameter SLOW_VALUE	= 11'h78c;
+	/* 31 cycles, for 4M baud with a 125 MHz clock */
+	parameter FAST_VALUE	= 11'h68e;
+
+	reg [7:0] data_last;
+	reg valid_last, ready_next;
+	reg [10:0] lfsr, lfsr_next;
+	reg [3:0] counter, counter_next;
+	reg [8:0] bits, bits_next;
+
+	initial begin
+		ready = 1'b1;
+		valid_last = 1'b0;
+		bits = 9'h1ff;
+	end	
+
+	always @(*) begin
+		tx = bits[0];
+
+		ready_next = ready;
+		counter_next = counter;
+		lfsr_next = { lfsr[9:0], lfsr[10] ^ lfsr[8] };
+		bits_next = bits;
+
+		if (&lfsr) begin
+			if (counter)
+				counter_next = counter - 1;
+			else
+				ready_next = 1;
+			lfsr_next = high_speed ? FAST_VALUE : SLOW_VALUE;
+			bits_next = { 1'b1, bits[8:1] };
+		end
+
+		if (valid_last && ready) begin
+			ready_next = 0;
+			counter_next = 9;
+			lfsr_next = high_speed ? FAST_VALUE : SLOW_VALUE;
+			bits_next = { data_last, 1'b0 };
+		end
+	end
+
+	always @(posedge clk) begin
+		data_last <= data;
+		ready <= ready_next;
+		valid_last <= valid;
+		counter <= counter_next;
+		lfsr <= lfsr_next;
+		bits <= bits_next;
+	end
+
+endmodule

tb/axis_replay_buffer.py

@@ -25,7 +25,8 @@ async def send_packet(signals, packet, ratio=1, last_extra=0):
         else:
             signals['data'].value = val
         signals['valid'].value = 1
-        signals['last'].value = last
+        if 'last' in signals:
+            signals['last'].value = last
         await RisingEdge(signals['clk'])
         while True:
             await FallingEdge(signals['clk'])

tb/uart_tx.py

@@ -0,0 +1,56 @@
+# SPDX-License-Identifier: AGPL-3.0-Only
+# Copyright (C) 2023 Sean Anderson <seanga2@gmail.com>
+
+import cocotb
+from cocotb.binary import BinaryValue
+from cocotb.clock import Clock
+from cocotb.triggers import FallingEdge, Timer
+from cocotb.utils import get_sim_time, get_sim_steps
+
+from .axis_replay_buffer import send_packet
+
+BAUD = 4e6
+BIT_STEPS = get_sim_steps(1 / BAUD, 'sec', round_mode='round')
+
+@cocotb.test(timeout_time=1, timeout_unit='ms')
+async def test_tx(uart):
+    uart.clk.value = BinaryValue('Z')
+    uart.valid.value = 0
+    uart.high_speed.value = BAUD == 4e6
+
+    await Timer(1)
+    await cocotb.start(Clock(uart.clk, 8, units='ns').start())
+    await FallingEdge(uart.clk)
+
+    msg = b"Hello"
+
+    await cocotb.start(send_packet({
+        'clk': uart.clk,
+        'data': uart.data,
+        'valid': uart.valid,
+        'ready': uart.ready,
+    }, msg))
+
+    async def getchar():
+        while not uart.tx.value:
+            await FallingEdge(uart.clk)
+        while uart.tx.value:
+            await FallingEdge(uart.clk)
+        await Timer(BIT_STEPS // 2)
+
+        result = 0
+        for _ in range(8):
+            await Timer(BIT_STEPS)
+            result >>= 1
+            result |= 0x80 if uart.tx.value else 0
+
+        return result
+
+    then = get_sim_time()
+    for c in msg:
+        assert c == await getchar()
+    now = get_sim_time()
+
+    expected = BIT_STEPS * (10 * len(msg) - 1.5)
+    actual = now - then
+    assert abs(actual - expected) / expected < 0.01