receiving Serial data crashes the ESP

[s0170071]

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Basic Infos

I have a MT681 power meter with an IR output connected to the UART. The meter sends bursts of 500 bytes every 1200ms @9600bps. So one burst lasts about 500ms.

Hardware

Hardware: WemosD1 mini
Core Version: 2.4.0rc2

Description

When the RX bursts are present upon boot, the ESP fails to connect to wifi. It does connect eventually but it takes it a unpredictable amount of time, mostly about 10min.
Obviously the watchdog kicked in. There is some output:

ets Jan  8 2013,rst cause:4, boot mode:(3,6)

wdt reset
load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v0c897c37
~ld

Workaround:
Set UART mode to tx only, RXbuffersize =0. Wait for connect , then adjust the buffer size and the uart mode according to your needs.

Settings in IDE

Module: Wemos D1mini
Flash Size: 4MB
CPU Frequency: 80Mhz
Flash Mode: ?qio?
Flash Frequency: ?40Mhz?
Upload Using: SERIAL (tried OTA too)
Reset Method: power cycle

Sketch

The sketch makes the ESP send itself some bytes over the serial. RX1 sends to swapped RX0.
You need a wire between D4 (TX1) and D7 (swapped RX0) for this.
If the wire is present the watchdog will kick in.
If you remove the wire, the ESP will start.
Observation 1: yield(); while receiving helps a lot. But how can you yield() if you do not know when bytes are coming ?
Observation 2: If the size of the RX buffer is left at the default value, things don't work either.

#include <ESP8266WiFi.h>
//#include <WiFiClient.h>
//#include <ESP8266WebServer.h>

const char* ssid = "...";
const char* password = "...";
unsigned long startuptime;
 

void setup(void){
  Serial.setRxBufferSize(400);
  Serial.begin(115200);
  Serial.println("\n\nstart...");
  pinMode(D4,OUTPUT); // check the wire bridge
  digitalWrite(D4,0); 
  if (digitalRead(D7)==0) 
      Serial.println("D4 D7 wire bridge detected."); 
  else 
      Serial.println("NO wire bridge detected. Put it between D4 and D7");

  Serial.flush(); // force ouput
  delay(100); // wait for output
  
  
  
  Serial.swap();          // route the RX pin to D7
  Serial1.begin(115200); 
  for (int i = 0; i<500; i++) 
  {
    Serial1.write(i);
    //yield();  // this fixes the issue (and breaks the example)
    // but this is not the point. Trouble occurs when recieving, not sending.
    // sending here is only done for the sake of simplicity.
    // besides: sending 500 bytes takes 39ms
    // now: pull the wire and watch the ESP run ;-)
  }
  
  
  startuptime=millis();

  WiFi.begin(ssid, password); //  now try to connect
  while (WiFi.status() != WL_CONNECTED) {
    delay(1);
    Serial1.write(0x55);
  }
  Serial.swap();
}

void loop(void){
   
  Serial.println("Wifi connected. Startup time: " + String(millis()-startuptime)  +"ms");
  Serial.println("Bytes in buffer: " + String(Serial.available()));
  for (;;) delay(1000);
}

Output:

start...
D4 D7 wire bridge detected.

 ets Jan  8 2013,rst cause:4, boot mode:(3,6)

wdt reset
load 0x4010f000, len 1384, room 16 
tail 8
chksum 0x2d
csum 0x2d
v0c897c37
~ld

edit 2017-12-22:

• added wdt output
• added example

[s0170071]

may be related to #2576 and #2239

[s0170071]

I don't mean to rush anyone but could anyone please have a look ? At least try the sketch and confirm this is a bug.
#4328 does not fix this.

[d-a-v]

You may try #4536.

[earlephilhower]

Closing because #4328 is merged, and no feedback for many months.