The ESP32 processor hangs on a second call to Serial.begin(115200) when compiled for ESP32 Dev Module.

[ZinggJM]

// SerialBeginTwice test by Jean-Marc Zingg
//
// Purpose: test issue with ESP32 compiled for ESP32 Dev Module

void setup()
{
  Serial.begin(115200);
  delay(100);
  Serial.println("going to call Serial.begin(115200); a second time. Should tell success ...");
  delay(100);
  Serial.begin(115200);
  delay(100);
  Serial.println("... success, returned from second call to Serial.begin(115200); Bye and out.");
  delay(100);
}

void loop() {}

I verified this on a ESP32 Dev Module clone, on a Wemos LOLIN32 Lite, and on the Waveshare e-Paper ESP32 Driver Board.

The GxEPD2 examples call Serial.begin(115200) in setup() to Serial.print("setup"), and again in display.init(115200).

Related post in Arduino Forum Displays: https://forum.arduino.cc/index.php?topic=487007.msg4952538#msg4952538

[lbernstone]

Please follow the ISSUE_TEMPLATE.

[ZinggJM]

Never mind if you are not interested. All relevant information is there, can be verified easily.

OS: Windows 10 - not relevant
Arduino IDE: 1.8.13
package: actual
target: 3 mentioned - not relevant
Error message: none
expected behavior: see both lines in Serial Monitor
actual behavior: only first line, processor hangs.

[lbernstone]

Can you please indicate what version of the code you are running?

[ZinggJM]

1.0.6 - is this the version you mean?
C:\Users\ZinggJ\AppData\Local\Arduino15\packages\esp32\hardware\esp32\1.0.6\variants\esp32

[lbernstone]

Well, there's an easy workaround. I'll put in a PR and see if someone with JTAG can figure out what is actually causing the hang.

[ZinggJM]

I just repeated the test with ESP32-Wrover on LOLIN D32 PRO:
works when compiled for LOLIN D32 PRO
fails when compiled for ESP32 Wrover Module

[lbernstone]

If you set core debugging to verbose it will work. This is a timing issue.

[ZinggJM]

Ok, thank you, I will try. But how to explain this to users of my library?

[ZinggJM]

Verbose didn't help, on LOLIN D32 PRO.

[ZinggJM]

Thank you for the fast response. Glad the issue is verified and a solution is found.

[stale]

[STALE_SET] This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[stale]

[STALE_DEL] This stale issue has been automatically closed. Thank you for your contributions.

[ZinggJM]

This issue seems still to be present. See e.g. https://forum.arduino.cc/t/esp32-e-paper-display-upload-but-no-output/927375/3

[DriekdeGadgetfreak]

Here is the fix:.
In uartBegin the output buffer is flushed:
uartFlush(uart);
Change this to
uartFlushTxOnly(uart,false); // also flush the input buffer:

[ZinggJM]

Maybe someone should make the old download link invalid, remove the version pointed to!
I still get issues reported by users of this outdated version.

See https://forum.arduino.cc/t/waveshare-e-paper-displays-with-spi/467865/3114

Jean-Marc