Checked with current oficial version - result is same
resource SERIAL_TX 3 B10
resource SERIAL_RX 3 B11
resource SERIAL_TX 4 A00
resource SERIAL_RX 4 A01
resource PULLUP 1 NONE
resource PULLUP 2 NONE
resource PULLUP 3 NONE
resource PULLUP 4 NONE
resource PULLDOWN 1 NONE
resource PULLDOWN 2 NONE
resource PULLDOWN 3 NONE
resource PULLDOWN 4 NONE
Its worth trying a pull-up resistor on the pin to see if that fixes it. Might be something completely different I suppose.
Good news! This version worked for me.
Serial3, ELRS is OK
Serial4, GPS + Compass is OK
Serial7, Mavlink1 set baudrate to 57600 is OK
Will have to check if i can make a full flight in a few days. 
Thank you very much @andyp1per for the help!
Thank you again andyp1per
Should have my replacement Blitz flight controller soon
So see how it goes
Just a quick question about orientation should the arrow be underneath or facing upwards pointing forward ect…
Sorry for hijacking this post
Thanks again
Steve w
Doesn’t matter - just make sure you set AHRS_ORIENTATION correctly for whatever you choose
Thank 
you for all your support
Can you please point me to source code where you was able to made changes with pullup?
I’ll try to play around locally.
Thanks!
No DMA on UART 4, but might work for you.
It would be helpful if you could verify that its only the RX pin that needs this.
I’m trying to compile like this:
Keep UART RX pins high to prevent ELRS going into bootloader mode
PA2 USART2_TX USART2 OUTPUT HIGH
PA3 USART2_RX USART2 OUTPUT HIGH
PB10 USART3_TX USART3 OUTPUT HIGH
PB11 USART3_RX USART3 OUTPUT HIGH
PA0 USART4_TX USART4 OUTPUT HIGH
PA1 USART4_RX USART4 OUTPUT HIGH
default to all pins low to avoid ESD issues
DEFAULTGPIO OUTPUT LOW PULLDOWN
I’m not shure about changing order is it make sence or not
Work OK with those changes (based on yours).
hwdef.zip (2.5 KB)
I commented # DEFAULTGPIO OUTPUT LOW PULLDOWN in the hwdef.dat - we do have it in BL.
Not sure why we do have this in two places. If you have an idea - please let me know
In the BL I’ve changed like this:
# Keep UART RX pins high to prevent ELRS going into bootloader mode
PA0 UART4_TX UART4 OUTPUT HIGH
PA1 UART4_RX UART4 OUTPUT HIGH
PA2 USART2_TX USART2 OUTPUT HIGH
PA3 USART2_RX USART2 OUTPUT HIGH
PB10 USART3_TX USART3 OUTPUT HIGH
PB11 USART3_RX USART3 OUTPUT HIGH
# default to all pins low to avoid ESD issues
DEFAULTGPIO OUTPUT LOW PULLDOWN
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What happens if you comment out the UARTx_TX lines? Does it still work?
I haven’t tried this.
I tried to compile for stable version 4.4.4. At the last step it crashed with an error for some reason
But comparing with the description of ELRS, it should work.
TX2 is not on the board at all - it can be removed. RХ2 is present
I’ll try to try this in day or two.
Tha main problem for me compilation takes in about one hour (I’m using wsl under windows)
Make sure you are using ccache. Need a faster computer!
Tested with AP 4.4.4 and new bootloader with removed TX - works OK
# Chip select pins
PA15 FLASH1_CS CS
PE4 OSD1_CS CS
PA4 GYRO1_CS CS
# Keep UART RX pins high to prevent ELRS going into bootloader mode
# PA0 UART4_TX UART4 OUTPUT HIGH
PA1 UART4_RX UART4 OUTPUT HIGH
PA3 USART2_RX USART2 OUTPUT HIGH
# PB10 USART3_TX USART3 OUTPUT HIGH
PB11 USART3_RX USART3 OUTPUT HIGH
# default to all pins low to avoid ESD issues
DEFAULTGPIO OUTPUT LOW PULLDOWN
PD15 LED0 OUTPUT LOW GPIO(90) # Blue LED
define HAL_LED_ON 0
I was able to create build for the stable AP v4.4.4
Can be downloaded from here: Telegram: Contact @DyggytyFPV
Special thanks to @andyp1per
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Amazing effort everyone! I’ll download and check. Thank you very much!
Please note that you need to use version with bootloader one even if you have AP installed
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