Hi there,
So using the code tags above " </> " just paste it in there, will yield better results and higher quality help .
So looking at this you are close but conflated some stuff…
Your not explicitly setting up com port 1 correctly for hardware serial.
only software serial for the assignable ports can be used,
// testRD03E radar sensor
// JCL, 4June2025
// Rd-03D mmWave Radar: Multi-Human Tracking with Distance, Speed & Positioning
#define RX_PIN D2
#define TX_PIN D3
#define BAUD_RATE 256000
#define BUFFER_BYTES 64
// Variables
uint8_t RX_BUF[BUFFER_BYTES] = {0};
uint8_t RX_count = 0;
uint8_t RX_temp = 0;
// Single-Target Detection Commands
uint8_t Single_Target_Detection_CMD[12] = {0xFD, 0xFC, 0xFB, 0xFA, 0x02, 0x00, 0x80, 0x00, 0x04, 0x03, 0x02, 0x01};
// Multi-Target Detection Command
uint8_t Multi_Target_Detection_CMD[12] = {0xFD, 0xFC, 0xFB, 0xFA, 0x02, 0x00, 0x90, 0x00, 0x04, 0x03, 0x02, 0x01};
void setup() {
Serial.begin(115200); // Debugging info to Serial Monitor
delay(500);
Serial1.begin(BAUD_RATE, SERIAL_8N1, RX_PIN, TX_PIN);
Serial1.setRxBufferSize(BUFFER_BYTES);
Serial.println("RD-03D Radar Module Initialized");
// Send single-target detection command
Serial1.write(Single_Target_Detection_CMD, sizeof(Single_Target_Detection_CMD));
Serial.println("Single-target detection mode activated.");
delay(500);
// Send multi-target detection command
// Serial1.write(Multi_Target_Detection_CMD, sizeof(Single_Target_Detection_CMD));
// Serial.println("Multi-target detection mode activated.");
// delay(1000);
RX_count = 0;
Serial1.flush();
}
void loop() {
// Read data from Serial1
while (Serial1.available()) {
//if (Serial1.available() > 0) {
RX_temp = Serial1.read();
RX_BUF[RX_count++] = RX_temp;
// Prevent buffer overflow
if (RX_count >= sizeof(RX_BUF)) {
RX_count = sizeof(RX_BUF) - 1;
}
// Check for end of frame (0xCC, 0x55)
if ((RX_count > 1) && (RX_BUF[RX_count - 1] == 0xCC) && (RX_BUF[RX_count - 2] == 0x55)) {
processRadarData();
}
}
}
void processRadarData() {
printBuffer();
memset(RX_BUF, 0x00, sizeof(RX_BUF)); // Reset buffer and counter
RX_count = 0;
}
// Function to print buffer contents
void printBuffer() {
Serial.print("RX_BUF: “);
for (int i = 0; i < RX_count; i++) {
Serial.print(“0x”);
if (RX_BUF[i] < 0x10) Serial.print(“0”); // Add leading zero for single-digit hex values
Serial.print(RX_BUF[i], HEX);
Serial.print(” ");
}
Serial.println();
}
You may want to take a look, I had something similar. and did it this way… YMMV
https://forum.seeedstudio.com/t/reading-distance-using-esp32-and-seeed-24ghz-human-static-lite-sensor/275885/6?u=pj_glasso
for a general rule though, I follow this understanding…
they are not all the same , USE a hardware UART ports whenver High Speed or timing is more critical, i.e. Serial for USB, Serial1 for D7 & D6
Those are assigned UARTS, Software_Serial Allows you to use any GPIO pins as a comport pair,via BIT_Banging but some limits are obvious doing it this way.
Hardware + Software Serial on ESP32 (C3/S3/C6)
#include <HardwareSerial.h>
#include <SoftwareSerial.h>
// Hardware Serial on UART1 (TX = GPIO10, RX = GPIO9 for example)
HardwareSerial Serial1(1); // Use UART1
// Software Serial on arbitrary GPIOs (TX = GPIO6, RX = GPIO7 here)
SoftwareSerial mySoftSerial(6, 7); // TX, RX
void setup() {
// USB Serial for debug
Serial.begin(115200);
delay(1000);
Serial.println("=== Serial Ports Example ===");
// Start Hardware Serial1
Serial1.begin(9600, SERIAL_8N1, 9, 10); // RX = 9, TX = 10
Serial.println("Hardware Serial1 started on GPIO9/10");
// Start Software Serial
mySoftSerial.begin(9600);
Serial.println("Software Serial started on GPIO6/7");
}
void loop() {
// Send message out both ports
Serial1.println("Hello from HardwareSerial1");
mySoftSerial.println("Hello from SoftwareSerial");
delay(1000);
}
Notes:
- You can remap UART pins using
Serial1.begin(baud, config, rxPin, txPin) on ESP32.
- Avoid using GPIOs 0, 1, 3, or others with boot functions unless you know they’re safe on your board.
- For the Xiao ESP32C3/S3, UART1 and UART2 are available depending on your board config.
- SoftwareSerial can be flaky on higher baud rates—best kept at 9600–19200 for stable use.
Try yours like this, and get back …
#include <HardwareSerial.h>
#define RX_PIN 2 // Adjust for your board C6 is GPIO02 &GPIO21 for D2 & D3 if the pin macros are not supported.
#define TX_PIN 21
#define BAUD_RATE 256000
#define BUFFER_BYTES 64
uint8_t RX_BUF[BUFFER_BYTES] = {0};
uint8_t RX_count = 0;
// Command frame
uint8_t Single_Target_Detection_CMD[12] = {
0xFD, 0xFC, 0xFB, 0xFA,
0x02, 0x00, 0x80, 0x00,
0x04, 0x03, 0x02, 0x01
};
void setup() {
Serial.begin(115200);
delay(500);
Serial1.begin(BAUD_RATE, SERIAL_8N1, RX_PIN, TX_PIN);
Serial1.setRxBufferSize(BUFFER_BYTES);
Serial.println("RD-03D Radar Module Initialized");
Serial1.write(Single_Target_Detection_CMD, sizeof(Single_Target_Detection_CMD));
Serial.println("Single-target detection mode activated.");
RX_count = 0;
Serial1.flush();
}
void loop() {
while (Serial1.available()) {
uint8_t RX_temp = Serial1.read();
RX_BUF[RX_count++] = RX_temp;
if (RX_count >= sizeof(RX_BUF)) RX_count = sizeof(RX_BUF) - 1;
if (RX_count > 1 && RX_BUF[RX_count - 2] == 0x55 && RX_BUF[RX_count - 1] == 0xCC) {
processRadarData();
}
}
}
void processRadarData() {
Serial.print("RX_BUF: ");
for (int i = 0; i < RX_count; i++) {
if (RX_BUF[i] < 0x10) Serial.print("0");
Serial.print(RX_BUF[i], HEX);
Serial.print(" ");
}
Serial.println();
RX_count = 0;
memset(RX_BUF, 0, sizeof(RX_BUF));
}
HTH
GL 
PJ 