Serial output is blocked by M_UDREx not resetting

[vintagepc]

Stock firmware does not boot, it waitspins in MarlinSerial::Write() because it appears M_UDREx is not getting reset. I'm not sure whether this is a quirk of Marlin serial or the emulator since the bootloader can talk over PTY and flash firmware.

[vintagepc]

I wonder if something is up with the handoff from the bootloader. I did some experimenting and if I force to write to the second serial port I can get output and no bootloop.

echo: 3.9.0-RC1-3272. echo: Last Updated: Apr 8 2020 19:15:59 | Author: (none, default config).. Compiled: Apr 8 2020.. echo: Free Memory: 1652 PlannerBufferBytes: 1792.. echo:Hardcoded Default Settings Loaded.. adc_init. W25X20CL not responding.. CrashDetect ENABLED!. echo:enqueing "M83".. echo:enqueing "G1 E-1.000 F2700".. echo:enqueing "G1 Z10.000 F800.000".. FSensor ENABLED (sensor board revision: 03 or older). echo:SD init fail.. Wizard state: 0. Error:Temperature heated bed switched off. MAXTEMP triggered !..

[leptun]

@vintagepc Did you push the required changes for the emulated serial port (primary)?

[vintagepc]

I pushed up an updated source but not an updated firmware as my local build has some other crap in it for debugging.

(114 in MarlinSerial.h)

		if (selectedSerialPort == 0)
		{
			while (!((UCSR1A) & (1 << UDRE1)));
			UDR1 = c;
		}

You should be able to just rebuilt it now with make MK3S.afx as I added a build rule and lifted a script from the main firmware repo

[vintagepc]

Note -the changes make it write to the first serial port.

By default it outputs to the console you run the program in. To make it a real "port" you can talk to with pronterface, uncomment the uart_pty_connect(&hw.UART1,'1'); line in setupSerial()
and watch the console output for a /tmp/.... device node you can connect to.

[vintagepc]

I made some progress on this. MarlinSerial::Begin clobbers the value of UDRE0. Presumably, on the hardware, this is reset to true automatically, but in simAVR it is left in this state.

As a result, it will spinlock waiting for it to be true in the first MarlinSerial::write() call.