Day 11: UART TX — Your First Communication Interface¶

Course: Accelerated HDL for Digital System Design¶

Week 3, Session 11 of 16¶

Student Learning Objectives¶

SLO 11.1: Explain the UART protocol: start bit, 8 data bits, stop bit, baud rate, and framing.
SLO 11.2: Derive baud rate timing from the 25 MHz Go Board clock and implement a baud rate generator.
SLO 11.3: Implement a UART TX module using FSM + PISO shift register + baud generator architecture.
SLO 11.4: Integrate UART TX into a top module that transmits data from the Go Board to a PC terminal.
SLO 11.5: Verify UART TX behavior in simulation before programming hardware.

Pre-Class Video (~50 min)¶

#	Segment	Duration	File
1	UART protocol: start bit, 8 data bits, stop bit, baud rate	14 min	`video/day11_seg1_uart_protocol.mp4`
2	Baud rate generation: deriving precise timing from a fast clock	10 min	`video/day11_seg2_baud_rate.mp4`
3	TX architecture: FSM + PISO shift register + baud generator	16 min	`video/day11_seg3_tx_architecture.mp4`
4	Connecting to a PC: USB-to-serial, terminal emulator setup	10 min	`video/day11_seg4_pc_connection.mp4`

Session Timeline¶

Time	Activity	Duration
0:00	Warm-up: UART protocol quiz, pre-class questions	5 min
0:05	Mini-lecture: UART as FSM+datapath, baud math, live build	30 min
0:35	Lab Exercise 1: UART TX implementation	40 min
1:15	Break	5 min
1:20	Lab Exercise 2: Single character on button press	20 min
1:40	Lab Exercise 3: Transmit "HELLO"	25 min
2:05	Lab Exercise 4 (Stretch): Hex-to-ASCII	15 min
2:20	Wrap-up and Day 12 preview	10 min

In-Class Mini-Lecture (30 min)¶

UART as an FSM + Datapath Design Exercise (10 min)¶

Everything from Weeks 1–2 comes together: FSMs, counters, shift registers, testbenches
Block diagram: baud generator (counter), bit counter, shift register, FSM controller, TX output
The FSM states: IDLE (TX high), START (TX low for 1 bit period), DATA (shift out 8 bits LSB first), STOP (TX high for 1 bit period)

Baud Rate Math (10 min)¶

25 MHz / 115200 = 217.01... ≈ 217 clocks per bit
Rounding considerations: 217 gives actual baud = 115,207 Hz, error = 0.006% — well within tolerance
Other baud rates: 9600 → 2604 clocks; 19200 → 1302 clocks
Design decision: Parameterize CLKS_PER_BIT so the module works at any baud rate

Live Build (10 min)¶

Walk through the UART TX code structure together
Highlight the critical details: LSB first, start bit = 0, stop bit = 1, idle = 1
Show the provided architecture diagram (distributed as a handout or on screen)
Terminal setup: 115200 baud, 8N1, no flow control

Lab Exercises¶

Exercise 1: UART TX Implementation (40 min)¶

Objective (SLO 11.1, 11.2, 11.3, 11.5): Build a working UART transmitter from the architecture diagram.

Tasks: 1. Implement the UART TX module with: - parameter CLKS_PER_BIT = 217 (115200 baud at 25 MHz) - Inputs: clk, rst, tx_start, tx_data[7:0] - Outputs: tx_out, tx_busy - FSM: IDLE → START → DATA (8 bits) → STOP → IDLE - Baud rate counter for bit-period timing - Bit counter (0–7) for tracking which data bit to send 2. Simulate first: Write a testbench that: - Sends the byte 8'h41 (ASCII 'A') - Verifies the TX line goes: IDLE(1) → START(0) → D0 D1 D2 D3 D4 D5 D6 D7 → STOP(1) - Verify tx_busy is asserted during transmission - Use a short CLKS_PER_BIT value (e.g., 4) for fast simulation 3. View waveforms in GTKWave. Verify the bit timing.

Checkpoint: Simulation shows correct UART TX waveform for 'A' (0x41).

Exercise 2: Single Character on Button Press (20 min)¶

Objective (SLO 11.4): Get the first character from the Go Board to the PC.

Tasks: 1. Create a top module that: - Debounces a button (reuse Day 5 module) - On button press, loads ASCII 'A' (or any character) and asserts tx_start - Connects tx_out to the UART TX pin via .pcf 2. Synthesize and program. 3. Open a terminal emulator (e.g., screen, minicom, picocom, or PuTTY) at 115200 8N1. 4. Press the button — see 'A' appear on your PC terminal.

Checkpoint: Character appears on the PC terminal on button press.

Exercise 3: Transmit "HELLO" (25 min)¶

Objective (SLO 11.3, 11.4): Build a multi-byte transmitter using a ROM + sequencer.

Tasks: 1. Store the string "HELLO\r\n" in a ROM (7 bytes). 2. Build a sequencer FSM that: - On trigger (button press or reset), starts at ROM address 0 - Sends each byte via UART TX, waiting for tx_busy to deassert before advancing - Stops after the last byte 3. Synthesize and program. Verify "HELLO" appears on the terminal, followed by a newline.

Checkpoint: "HELLO" prints on the PC terminal from the Go Board.

Exercise 4 (Stretch): Hex-to-ASCII Converter (15 min)¶

Objective (SLO 11.4): Transmit human-readable data.

Tasks: 1. Implement a module that converts a 4-bit value to its ASCII hex character ('0'–'9', 'A'–'F'). 2. Use it to transmit the counter value as readable hex text: e.g., "0A\r\n", "0B\r\n", ... 3. Transmit a new value every ~1 second.

Deliverable¶

"HELLO" appearing on the PC terminal from the Go Board, with a simulation waveform showing correct UART TX framing.

Assessment Mapping¶

Exercise	SLOs Assessed	Weight
1 — UART TX implementation	11.1, 11.2, 11.3, 11.5	Core
2 — Single character TX	11.4	Core
3 — "HELLO" string TX	11.3, 11.4	Core
4 — Hex-to-ASCII	11.4	Stretch (bonus)

⚠️ Common Pitfalls & FAQ¶

Day 11 is your first communication interface. UART is unforgiving about timing — one wrong clock count and every byte is garbled.

Characters show as garbage in the terminal? The most common cause is a baud rate mismatch. Double-check your CLKS_PER_BIT calculation: for 115200 baud at 25 MHz, it should be 25000000 / 115200 = 217. Also verify your terminal emulator is set to 115200 baud, 8N1.
UART sends LSB first. This catches everyone at least once. Bit 0 of your data byte goes out on the wire first, then bit 1, etc. If you're seeing characters that are "almost right but wrong," check your bit ordering.
Sending multiple bytes and getting corruption? You must wait for o_busy to go low (or o_done to pulse) before sending the next byte. The "HELLO" sequencer in Exercise 3 forces you to handle this — don't shortcut it.
Terminal emulator won't connect? Common causes: wrong COM port (try them all), hardware flow control enabled (disable it), or another program has the port open. On Linux, check ls /dev/ttyUSB*; on macOS, ls /dev/cu.*.
Which pin is TX on the Go Board? Check go_board.pcf for the UART TX pin. It must match the pin connected to the USB-to-serial chip on the board. If you get this wrong, the terminal will show nothing at all.

Preview: Day 12¶

UART RX completes the communication loop — type on your PC, see it on the Go Board. You'll also build an SPI master and get hands-on with AI-generated protocol testbenches, learning to prompt for timing-aware verification.