After reading the stm32F401 reference document, it appears jtag boundary scan is not supported unless using full 5 wire JTAG interface and SWD will prevent access to this feature. Can I get confirmation on this?

[GHale.1]

..

[Tesla DeLorean]

SWD doesn't prevent it, it is just a subset, and the ST-LINK for example can use either depending on what you wire. The CM0 parts don't support JTAG

Expect to connect TDI, TDO, TMS, TCK, NRST, optionally NJRST. The ARM 10-pin interface should suffice.

[GHale.1]

So you are confirming you must use the 5 wire JTAG interface mode not serial wire (SWD) to use boundary scan?

SWD only uses SWCLK(TCK) and TMS(SWDIO) and after the SWD sequence the chip disables boundary scan TAP. According to the documentation, stm32F401 reference manual section 23.5 "Important: Once Serial-Wire is selected using the dedicated Arm ® JTAG sequence, the

boundary scan TAP is automatically disabled (JTMS forced high)".

[Uwe Bonnes]

Do not remap NJRST when full JTAG is required The errata "Full JTAG configuration without NJTRST pin cannot be used" has not been even for STM32U5.

[GHale.1]

Yes unless I misread it,the reference manual says to use boundary scan NJTRST must be held low. ST doesn't seem to like boundary scan