4-wire JTAG and EXTI4 on the F405RG

[Jack1221]

Hi,

I am designing a custom board with a STM32F405RGTv being flashed by a FT2232 over the 4-wire JTAG protocol, where the FT2232 chip triggers the STM RESET line as needed.  This circuit has been verified in my previous designs.  However this new board contains a EXTI configuration for pin PA4.  In creating the CubeMX configuration for the board, I added this EXTI configuration and that disabled my 4-wire JTAG configuration.  I attempted to add it back and found it greyed out.  Is there a conflict between running JTAG without the nJRST line and the EXTI4 interupt controller?  Is this related to errata 2.2.7, which although different mentions how one cannot use PB4 as a GPIO when the other 4 wires of JTAG are active?  I want to ensure I can move foward with this EXTI channel before I finalize board design, any help would be much appreciated.

TLDR: Do 4-wire JTAG and EXTI4 channel conflict on the STM32F405RG?  If so, can I be pointed to a resource as to why?

[STOne-32]

Dear @Jack1221 

NJTRST pin cannot be used for another purpose than NJTRST . Also alternate function is forbidden. Only NJTRST function is allowed for correct JTAG operation.

Regarding The Errata  2.2.1 - "Full JTAG configuration without NJTRST pin cannot be used"

Description
When using the JTAG debug port in Debug mode, the connection with the debugger is lost if the NJTRST pin (PB4) is used as a GPIO. Only the 4-wire JTAG port configuration is impacted.

Workaround
Use the SWD debug port instead of the full 4-wire JTAG port.

=> It says essentially that when using the JTAG debug port one must have it configured in 5 pin JTAG mode, even if NJTRST is not connected to the JTAG header. if the wordings is not clear or creates confusion, we can enhance it in future.

Now regarding the question: Do 4-wire JTAG and EXTI4 channel conflict on the STM32F405RG?  If so, can I be pointed to a resource as to why?  - No direct link ( only PB4 can not used) , you can use EXTI4 - with another port like Port A PA4.   

Another Alternative to use SWD Mode and not JTAG. not sure if FT2232 firmware is supporting it yet. 

Hope it helps you

STOne-32.