Tutorial — Practice 8: Serial communication

Learn by programming: follow each step, write your own code, then validate with the lab questions and exam. Adapt pins and routines to your board when needed.

What you will build

Send and receive UART data at 9600 baud between the PIC and a PC.

Practice 8 schematic with MAX232 and serial communication — PIC UART (RC6 TX, RC7 RX) through MAX232 to PC serial connector. — Practica 8 — Comunicacion serial · UNEXPO

1
Configure RC6 (TX) and RC7 (RX)

RC7 input (RX), RC6 output (TX). With MAX232: PIC ↔ MAX232 ↔ DB9/USB-serial. `ADCON1 = 0x0F` for digital pins.
```
								        BSF     TRISC, 7    ; RX
        BCF     TRISC, 6    ; TX
							
```
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2
Calculate SPBRG for 9600 baud

At 20 MHz with BRGH = 1: `SPBRG = (Fosc / (16 × baud)) - 1` → ~129. Document the calculation in your report. Configure TXSTA (async, BRGH, TXEN) and RCSTA (SPEN, CREN).
```
								uart_init:
        BCF     TXSTA, SYNC
        BSF     TXSTA, BRGH
        MOVLW   D'129'
        MOVWF   SPBRG
        BSF     RCSTA, SPEN
        BSF     TXSTA, TXEN
        BSF     RCSTA, CREN
        RETURN
							
```
Tip: Formulas: /en/comunicacion/uart/
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3
uart_tx transmit routine

Wait for TXIF = 1, write the byte to TXREG. Send a greeting (`PIC18 UART\r\n`) at startup to confirm the PC receives data.
```
								uart_tx:
        BTFSS   PIR1, TXIF
        GOTO    uart_tx
        MOVWF   TXREG
        RETURN
							
```
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4
Echo loop in main

If RCIF = 1, read RCREG (clears flag), send the same byte with `uart_tx`. Open terminal at 9600 8N1. Each key should echo on screen.
```
								bucle:
        BTFSS   PIR1, RCIF
        GOTO    bucle
        MOVF    RCREG, W
        CALL    uart_tx
        GOTO    bucle
							
```
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5
Test with PC terminal

PuTTY or Tera Term: correct COM, 9600 baud. Garbage means check BRGH/SPBRG or crystal. No data means check MAX232 and crossed TX/RX. Save screenshot for the report.
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Before you code — pre-lab

Pre-laboratory — reference research

Research for Practice 8 (Serial communication, UNEXPO). Links to course EUSART.

Q 1 PIC18F4550 EUSART module

The EUSART sends and receives asynchronous serial bytes:

- TXSTA — transmit control (TXEN, BRGH, etc.).
- RCSTA — receive control (SPEN, CREN, RX9).
- SPBRG — baud rate generator.
- TXREG / RCREG — TX / RX buffer.

Pins: RC6 = TX, RC7 = RX. Set `TRISC` for alternate functions and enable SPEN in RCSTA.

Q 2 SPBRG calculation for target baud rate

With BRGH = 1 (high speed, common at 20 MHz):

`SPBRG = (Fosc / (4 × baud)) - 1`

Example: Fosc = 20 MHz, baud = 9600:

`SPBRG = (20_000_000 / (4 × 9600)) - 1 ≈ 520`

For 115200 the value is lower. Show your calculation and percent error in the report.

Q 3 What is the MAX232 for in the schematic?

The PIC uses TTL levels (0–5 V). A PC serial port (RS-232) uses roughly ±12 V. The MAX232 converts TTL ↔ RS-232 using external capacitors. Without it the PC cannot decode the PIC's bits correctly.

RC6 = TX and RC7 = RX are the UART pins in the typical schematic.

Design your solution

Write your pseudocode

Include: SPBRG calculation, TXSTA/RCSTA setup, TXIF wait, RCIF/RCREG read and optional simple protocol.

0 characters (minimum 80)

My pseudocode draft

My flowchart draft

Reference pseudocode

START
    Configure EUSART (SPBRG, TXSTA, RCSTA)
    TRISC.6 ← output, TRISC.7 ← input
REPEAT FOREVER
    IF RCIF THEN
        rx_data ← RCREG
        process_command(rx_data)
    END IF
    IF need_tx THEN
        WHILE TXIF = 0 DO
        TXREG ← tx_data
    END IF
END REPEAT

Reference flowchart

Practice 8 serial communication flowchart — Init UART → loop polling RCIF/TXIF → send/receive bytes.

Build and adapt the code

Practica · UNEXPO

Lab Practice 8

Establish serial communication between PIC18F4550 and a PC using EUSART. Send and receive data (serial terminal).

Requirements

Configure EUSART at 9600 or 115200 baud
Pins RC6 (TX) and RC7 (RX)
PC terminal (PuTTY, Tera Term, Arduino Serial Monitor)
Optional: simple command protocol

Reference source code

After building — post-lab

Post-laboratory — reference research

Questions after testing UART with a PC terminal or Proteus.

Q 1 How do you know a byte was transmitted correctly?

Before writing TXREG check TXIF is 1 (transmit buffer empty). After writing, wait until TXIF is 1 again before the next byte. On receive, RCIF means a byte is in RCREG; read RCREG to clear the flag.

Q 2 Typical errors when the terminal shows garbage

Common causes: wrong baud rate (bad SPBRG), BRGH mismatch, TX/RX wiring error, bad MAX232 wiring, or terminal at 9600 while PIC uses 115200. Verify project Fosc (20 MHz HS).

Attach a terminal screenshot showing sent and received data.

Check what you learned

Tutorial hub · Exercises

Tutor IA

Tutorial — Practice 8: Serial communication

Configure RC6 (TX) and RC7 (RX)

Calculate SPBRG for 9600 baud

uart_tx transmit routine

Echo loop in main

Test with PC terminal