Serial object stops updating after a few seconds (native USB CDC board - Seeed XIAO ESP32S3)

Hi everyone,

(Note: this post was drafted with the help of an AI assistant, but reflects my own project and the actual issue I'm experiencing.)

I'm receiving simple button-state data from a Seeed XIAO ESP32S3 board over USB (native USB CDC, not an external FTDI/CH340 chip) into Max via the [serial] object, and after a few seconds it simply stops updating entirely - no errors immediately obvious in the patch, values just freeze and nothing further comes through.

Setup:

- Board: Seeed XIAO ESP32S3, using native USB CDC ("USB CDC On Boot: Enabled" in Arduino board settings)

- Two boards actually - a sender (reads two buttons, transmits over ESP-NOW) and a receiver (gets the ESP-NOW packet, prints it over USB serial to the computer running Max)

- The receiver sends simple lines like "A 1", "B 0" over Serial.println() at 115200 baud, only when a button state changes (so it's not a high data rate - maybe a few messages per second at most when pressing buttons)

- Confirmed via Arduino IDE Serial Monitor (reading the receiver board directly) that data comes through continuously and correctly - no gaps, no dropped messages - so this looks isolated to Max/the serial object rather than the firmware or hardware.

Sender code:

#include <esp_now.h>
#include <WiFi.h>
#include "esp_wifi.h"

// ==================== Configuration ====================
const uint8_t RECEIVER_MAC[] = {0x68, 0xEE, 0x8F, 0x46, 0x4E, 0xFC};
const int WIFI_CHANNEL = 1;

const int numButtons = 2;
const int buttonPins[numButtons] = {D1, D3};
const char buttonIDs[numButtons] = {'A', 'B'};

const unsigned long debounceDelay = 20; // ms

int buttonStates[numButtons];      // confirmed, stable state
int lastButtonStates[numButtons];  // raw last reading (may bounce)
unsigned long lastDebounceTime[numButtons] = {0};

// ==================== Function declarations ====================
void initESPNow();
void registerPeer();
void onDataSent(const wifi_tx_info_t *tx_info, esp_now_send_status_t status);
void readButtons();
void sendButtonState(char id, int pressed);

// ==================== Setup ====================
void setup() {
  Serial.begin(115200);
  delay(1000);

  for (int i = 0; i < numButtons; i++) {
    pinMode(buttonPins[i], INPUT_PULLUP);
    buttonStates[i] = HIGH;
    lastButtonStates[i] = HIGH;
  }

  initESPNow();
}

// ==================== Main loop ====================
void loop() {
  readButtons();
}

// ==================== ESP-NOW setup ====================
void initESPNow() {
  WiFi.mode(WIFI_STA);
  esp_wifi_start();
  esp_wifi_set_channel(WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE);

  if (esp_now_init() != ESP_OK) {
    Serial.println("ESP-NOW init failed");
    while (true);
  }

  esp_now_register_send_cb(onDataSent);
  registerPeer();
}

void registerPeer() {
  esp_now_peer_info_t peerInfo = {};
  memcpy(peerInfo.peer_addr, RECEIVER_MAC, 6);
  peerInfo.channel = WIFI_CHANNEL;
  peerInfo.encrypt = false;

  if (esp_now_add_peer(&peerInfo) != ESP_OK) {
    Serial.println("Failed to add peer");
    while (true);
  }
}

// ==================== Send callback ====================
void onDataSent(const wifi_tx_info_t *tx_info, esp_now_send_status_t status) {
  // Kept silent to avoid cluttering Serial - uncomment for debugging
  // Serial.println(status == ESP_NOW_SEND_SUCCESS ? "SUCCESS" : "FAIL");
}

// ==================== Button handling (array-based, proven debounce logic) ====================
void readButtons() {
  unsigned long currentMillis = millis();

  for (int i = 0; i < numButtons; i++) {
    int reading = digitalRead(buttonPins[i]);

    if (reading != lastButtonStates[i]) {
      lastDebounceTime[i] = currentMillis;
    }

    if ((currentMillis - lastDebounceTime[i]) > debounceDelay) {
      if (buttonStates[i] != reading) {
        buttonStates[i] = reading;
        int pressed = !reading; // 1 = pressed, 0 = released
        sendButtonState(buttonIDs[i], pressed);
      }
    }

    lastButtonStates[i] = reading;
  }
}

void sendButtonState(char id, int pressed) {
  // Simple 2-byte message: button ID + value
  uint8_t payload[2] = {(uint8_t)id, (uint8_t)pressed};
  esp_now_send(RECEIVER_MAC, payload, sizeof(payload));

  Serial.print((char)id);
  Serial.print(" ");
  Serial.println(pressed);
}

Receiver firmware (this is the board Max actually reads from over USB):

#include <esp_now.h>
#include <WiFi.h>
#include "esp_wifi.h"

// ==================== Configuration ====================
const int WIFI_CHANNEL = 1;

// ==================== Function declarations ====================
void initESPNow();
void onDataRecv(const esp_now_recv_info_t *info, const uint8_t *incomingData, int len);

// ==================== Setup ====================
void setup() {
  Serial.begin(115200);
  delay(1000);

  initESPNow();
}

void loop() {
  // Nothing to do here — everything happens in onDataRecv
}

// ==================== ESP-NOW setup ====================
void initESPNow() {
  WiFi.mode(WIFI_STA);
  esp_wifi_start();
  esp_wifi_set_channel(WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE);

  if (esp_now_init() != ESP_OK) {
    Serial.println("ESP-NOW init failed");
    while (true);
  }

  esp_now_register_recv_cb(onDataRecv);
}

// ==================== Receive callback ====================
void onDataRecv(const esp_now_recv_info_t *info, const uint8_t *incomingData, int len) {
  if (len < 2) return; // safety check

  char id = (char)incomingData[0];
  int value = incomingData[1];

  Serial.print(id);
  Serial.print(" ");
  Serial.println(value);
}

Questions:

1. Is there a known issue specifically with native USB CDC boards (as opposed to FTDI/CH340-based boards) and the [serial] object freezing after establishing a connection?

2. Aside from dtr 1, are there other serial object attributes worth trying (baud handshake settings, flow control, buffer size)?

3. Would switching to a different serial implementation (e.g. the jasch comport external, or a Node for Max serial bridge) be a more reliable path for USB CDC boards specifically, based on others' experience?

Any help appreciated - this is for a project with real-time button/sensor feedback so the freezing is a blocker.

Max patch:

Thanks!