How to Interface PIC18F45Q10 with PC using PL2303 USB to TTL

• Raja Gupta
• April 27, 2025

In this post, I will explain how to connect a PIC18F45Q10 microcontroller to a PC terminal using a PL2303 USB to TTL converter.
The idea is simple: blink an LED to confirm the microcontroller is running, and print “Hello World” continuously over UART to the terminal.

Along the way, I also faced a few practical issues, which I have covered here so that anyone following this guide can avoid them.

Materials Required

• PIC18F45Q10 Microcontroller
• PL2303 USB to TTL Module
• 20 MHz Crystal Oscillator
• Two 22pF Capacitors
• Breadboard and Jumper Wires
• LED and 330Ω Resistor
• 5V Power Supply
• MPLAB X IDE with XC8 Compiler
• PuTTY or TeraTerm (PC Terminal Software)

Step 1: Hardware Setup

The hardware connection is straightforward but needs attention to detail:

Important points to check:

• Cross-connect TX and RX properly.
• Ensure GND connection between PIC and PL2303.
• Power the circuit with a stable 5V supply.

Step 2: Handling the USB to TTL Driver Issue

Initially, my PL2303 module was detected by Windows but did not create a proper COM port.
The root cause was the use of an older PL2303HXA chip which is not supported by newer Windows drivers.

To fix it:

1. Uninstall the existing Prolific driver from Device Manager.
2. Install an older driver manually (version 3.3.2.102).
3. Reconnect the USB module.
4. The device should now appear with a valid COM port, ready for use.

It is also recommended to prevent Windows from automatically updating this driver later.

Step 3: Writing and Flashing the Code

Here is the complete working code for the setup:

#include <xc.h>
#include <stdint.h>

// Configuration Bits
#pragma config FEXTOSC = HS
#pragma config RSTOSC = EXTOSC
#pragma config CLKOUTEN = OFF
#pragma config CSWEN = ON
#pragma config FCMEN = ON
#pragma config MCLRE = EXTMCLR
#pragma config WDTE = OFF
#pragma config LVP = OFF

#define _XTAL_FREQ 20000000UL

void UART_Init(void)
{
    uint16_t baud_rate_value = (_XTAL_FREQ / (64UL * 9600)) - 1;

    SP1BRGL = (uint8_t)baud_rate_value;
    SP1BRGH = (uint8_t)(baud_rate_value >> 8);

    TRISC6 = 1;
    TRISC7 = 1;

    TX1STAbits.SYNC = 0;
    TX1STAbits.BRGH = 0;
    RC1STAbits.SPEN = 1;
    TX1STAbits.TXEN = 1;
    RC1STAbits.CREN = 1;
}

void UART_Write(char data)
{
    while(!TX1STAbits.TRMT);
    TX1REG = data;
}

void UART_Write_Text(const char* text)
{
    while(*text != '\0')
    {
        UART_Write(*text);
        text++;
    }
}

void main(void)
{
    UART_Init();

    RC6PPS = 0x09; // Map TX to RC6
    RX1PPS = 0x17; // Map RX to RC7

    TRISA0 = 0;    // Set RA0 as output
    LATA0 = 0;     // Initially OFF

    while(1)
    {
        UART_Write_Text("Hello World\r\n");

        LATA0 = 1;
        __delay_ms(500);

        LATA0 = 0;
        __delay_ms(500);
    }
}

Flash this code into the PIC18F45Q10 using MPLAB X IDE and your PIC programmer (PICkit, Snap, or any compatible programmer).

Step 4: Testing the Setup

Once the code is flashed:

• Open PuTTY or TeraTerm.
• Set the Serial line to the correct COM port.
• Configure:
  • Baud rate: 9600
  • Data bits: 8
  • Stop bits: 1
  • Parity: None
  • Flow control: None

If everything is connected properly:

• The LED on RA0 will blink ON and OFF every 500 milliseconds.
• “Hello World” messages will continuously appear on the PC terminal.

This confirms that both your GPIO and UART setups are working perfectly.

Understanding Peripheral Pin Select (PPS)

Unlike older PICs where UART pins were fixed, PIC18F45Q10 allows routing internal peripherals to different I/O pins through a system called Peripheral Pin Select (PPS).

In this project:

• RC6PPS = 0x09; maps UART Transmit (TX1) to RC6.
• RX1PPS = 0x17; maps UART Receive (RX1) to RC7.

This flexibility is highly useful during PCB design and can make hardware routing much easier.
It is worth noting that microcontrollers like STM32 also offer flexible pin functions but are more restricted compared to the complete freedom given by PIC18 PPS.

Common Issues to Watch For

• Wrong TX/RX wiring (they must be crossed).
• Forgetting to configure PPS mappings.
• Baud rate mismatch due to incorrect oscillator settings.
• USB to TTL driver issues (especially with old PL2303 modules).
• Incorrect serial settings in PuTTY or TeraTerm.

Always double-check each of these areas if you face any unexpected behavior.

Conclusion

This project might seem simple, but it brings together several important embedded system concepts:

• UART peripheral initialization
• Peripheral Pin Select configuration
• Handling real-world hardware driver issues
• Understanding clock and timing effects on communication

Completing this interface and seeing both the LED blink and UART communication working reliably is a solid confidence boost when working with new microcontrollers.

If you are starting with the PIC18 family, this project gives you a strong foundation to build further applications.