Binary garbage on the UART when powering the RPi

Question

I have a strange issue I cannot figure out. I have a thermal printer (https://www.adafruit.com/products/597) plugged to the UART (/dev/ttyAMAO).

I configured the RPi to remove the console on /dev/ttyAMA0 with the following:

echo "dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait" > /boot/cmdline.txt
systemctl disable [email protected]

My project is running fine, except for one little thing.

Immediately after powering the RPi, the printer sometimes prints 5 random characters, as if something was sent to the UART.

Here is what it looks like (each line is a new powering of the RPi)

Any idea from where it's coming from?

Additional info: - the RPi is a model B - it runs Raspbian Jessie Lite

Answer 1

The Pi's UART has a fault (in the firmware). Whenever the port is opened there is a 30 µs low glitch on TXD.

See http://elinux.org/RPi_Serial_Connection#Unwanted_serial_garbage_input and search for glitch.

You will need to find a workaround as the fault will not be fixed.

Answer 2

If these are wrong-baud-rate garblings of the the messages produced by the Kernel startup code, you could remove them by commenting out or disabling the lines in arch/arm/boot/compressed/misc.c which produce those, typically something like:

putstr("Uncompressing Linux...");

and

putstr(" done, booting the kernel.\n");

plus some atypical error cases, which you might want to leave intact.

The actual justification for this answer though, is that if you don't feel like setting up a build environment for the kernel, you could also leverage the fact that these are null-terminated strings, by using a hex editor on the raw kernel file to overwrite the initial characters of each argument ("U" and " ") with null (ie, 0x00). Make sure if you do this that you overwrite rather than insert, as the latter would render the remainder of the kernel meaningless.

An electrical gating of the transmit data line with a GPIO should also be fairly simple.