The problem would be intermittency. It may work sometimes but not others. It may work, then become unstable and latch up.
This is actually the problem with this interface, as you described.
However they typically aren't specified to operate below 4.5V so their behaviour at 3.3V will be unpredictable.
I assume, if I would disconnect the voltage selector from this vcc line and feed the ic's from a
stable 5v source that wouldn't harm the electronic, or?
I assume that is a designing mistake (it could be because this is a low cost china interface) of the pcb
manufacturer. The voltage selector voltage goes also from the interface to the BDM port and
powers the CPU with the selected voltage outside the interface.
In that meaning the internal electronic on the interface board has nothing
to do with the vcc voltage of the targeted cpu and the 74HCT ic's vcc.
It's not normal for me to use on the same ic let's talk about the 74HCT two type of vcc.
One 3.3v and than the higher 5v.
I don't think there is any equivalent ic for this two above who can operate in a good condition
if it is feed with 3.3v and with 5v.
Or am I wrong?
One more question pls.
What would the be best way to determine what type of ic should be used on
an electronic device where I know the type of the ic but don't know
should be used a variant of TTL or CMOS ic?
I also assume on this board is something mixed up with TTL and CMOS ic's.
If I would design such of board I would make a decision to use a TTL or CMOS
type of ic's, of course it depend of the behavior of other I/O ports.
But if the devices ( ic's) should communicate between each other on the same pcb
that would be a big mistake to mix them up with TTL and CMOS type and
always correcting the voltage to meet the requirement off the mixed type of ic's.
Thanks.