ATmega328P and ATmega328PB 8Mhz Optiboot bootloader doesn't work at 115200 baudrate

[arduino12]

Hi,

According to the Optiboot precompiled-binaries table it should support 115200 baudrate at 8MHz clock frequency.

I tested all the supported baudrates on my ATmega328P and ATmega328PB with their internal 8Mhz clock-
and found out that the 115200 is not working!

platformio.ini:

[env]
platform = atmelavr
framework = arduino
board = ATmega328P
upload_speed = 115200 ; 9600 19200 38400 57600 115200 230400 250000 460800 500000 1000000
board_build.f_cpu = 8000000L
board_hardware.oscillator = internal

[env:fuses_bootloader]
upload_protocol = usbasp
board_bootloader.type = optiboot
board_bootloader.speed = ${env.upload_speed}

[env:test_upload]

After uploading the bootloader and setting the fuses with pio run -e fuses_bootloader -t bootloader (when upload_speed = 115200),
running pio run -e test_upload -t upload resulting in the following timeout errors:

Configuring upload protocol...
AVAILABLE: arduino
CURRENT: upload_protocol = arduino
Looking for upload port...
Auto-detected: COM30
Uploading .pio\build\test_upload\firmware.hex
avrdude error: programmer is not responding
avrdude warning: attempt 1 of 10: not in sync: resp=0x00
avrdude error: programmer is not responding
avrdude warning: attempt 2 of 10: not in sync: resp=0x00
...

Maybe we got the wrong 115200 precompiled-binary?
or the 8MHz internal clock gets too much error for 115200 baudrate?
(On external 16MHz clock the 115200 baudrate works).

[MCUdude]

Have a look at the AVR baud rate calculator and more specifically, the U2Xn = 1 part. 115200 baud at 8 MHz results in 3.5% error if the 8 MHz is 100% accurate. The internal oscillator is usually not spot on. I'd recommend you use a different baud rate that has less error, for instance, 250000 baud.

[arduino12]

Thanks @MCUdude !
I also think that the 3.5% error + internal oscillator error are to blame.

I have added the error (if non zero) to the Optiboot precompiled-binaries table-
Can you please use it so others will know about it and add a bullet that the 115200 doesn't work with the internal oscillator?

	1000000	500000	460800	250000	230400	115200	57600	38400	19200	9600
24 MHz	X	X		X	X 0.2%	X 0.2%	X 0.2%	X 0.2%	X 0.2%
22.1184 MHz			X		X	X	X	X	X
20 MHz		X		X		X 1.4%			X 0.2%
18.4320 MHz			X		X	X	X	X	X	X
16 MHz	X	X		X		X 2.1%		X 0.2%	X 0.2%	X 0.2%
14.7456 MHz			X		X	X	X	X	X	X
12 MHz		X		X		X 0.2%	X 0.2%		X 0.2%	X 0.2%
11.0592 MHz			X		X	X	X	X	X	X
8 MHz	X	X		X		X 3.5%	X 2.1%	X 0.2%	X 0.2%	X 0.2%
7.3728 MHz			X		X	X	X	X	X	X
4 MHz		X		X					X 0.2%	X 0.2%
3.6864 MHz			X		X	X	X	X	X	X
2 MHz				X					X 0.2%	X 0.2%
1.8432 MHz					X	X	X	X	X	X
1 MHz										X 0.2%

I am surprised that the default upload_speed is 115200 because it seems to also have a 2.5% error when running of the common external 16MHz oscillator..