M5StickV で UART 文字列受信サンプル。uart.any() で受信バッファの状態を確認して readline() で読むのが簡単。https://twitter.com/ksasao/status/1239058196334923776

boot.py

import lcd
import image
import time
import uos

lcd.init()
lcd.rotation(2) #Rotate the lcd 180deg

from Maix import I2S, GPIO
from Maix import GPIO
from fpioa_manager import *
from machine import UART

# serial port setup
# grove connector
fm.register(35, fm.fpioa.UART2_TX, force=True)
fm.register(34, fm.fpioa.UART2_RX, force=True)
uart = UART(UART.UART2, 115200,8,0,0, timeout=1000, read_buf_len= 4096)

# initialize camera
import sensor
while 1:
    try:
        sensor.reset() #Reset sensor may failed, let's try some times
        break
    except:
        err_counter = err_counter + 1
        if err_counter == 20:
            lcd.draw_string(lcd.width()//2-100,lcd.height()//2-4, "Error: Sensor Init Failed", lcd.WHITE, lcd.RED)
        time.sleep(0.1)
        continue

sensor.set_pixformat(sensor.RGB565)
sensor.set_framesize(sensor.QVGA) #QVGA=320x240
sensor.run(1)

# main
text = ""
try:
    while(True):
        if uart.any():
            data = uart.readline()
            text = data.decode('utf-8')
            print(text)
        img = sensor.snapshot() # Take an image from sensor
        img.draw_string(40, 50, text, scale=3)
        lcd.display(img)

except KeyboardInterrupt:
    a = kpu.deinit(task)
    sys.exit()

m5stickcsample.ino

#include <M5StickC.h>

const int MAX_RX_BUFFER_SIZE = 1024;
char rx_buffer[MAX_RX_BUFFER_SIZE];

void setup() {
  M5.begin();
  M5.Lcd.setRotation(1);
  Serial2.begin(115200, SERIAL_8N1, 32, 33);
}

void loop() {
  M5.update();
  if ( M5.BtnA.wasPressed() ) {
    println_ext("Button A");
  }  
  if ( M5.BtnB.wasPressed() ) {
    println_ext("Button B");
  }
  int axpButton = M5.Axp.GetBtnPress();
  if ( axpButton == 1 ) {
    println_ext("Axp Button      (long)");
  }
  if ( axpButton == 2 ) {
    println_ext("Axp Button      (short)");
  }
}

void println_ext(char* str){
  Serial2.write(str);
  M5.Lcd.fillScreen(TFT_BLACK);
  M5.Lcd.setCursor(0, 5, 4);
  M5.Lcd.println(str);
}