There was questions up on forum threads:

https://forum.pjrc.com/threads/66076-Teensy-4-x-s-1062-MCU-has-16-bytes-of-NVRAM-on-RTC-unit?p=268385#post268385

Originally in the thread: https://forum.pjrc.com/threads/64592-Teensyduino-1-54-Beta-5?p=268336&viewfull=1#post268336

That said that the 16 byte memory that was battery backup was not defined nor worked.

So added the defines for the 4 registers.  Only one early on alias was defined before this for the first one...

Also added in the SNVS_LPCR_GPR_Z define which the register bit was talked about in a few other register definitions, this bit was not actually defined in the reference manual, it said it was reserved.  But reads or writes without this bit set in the register will zero out the 4 registers.

Tried example sketch to make sure it worked

There is a small typo in teensy3/usb_desc.c, teensy4/usb_desc.c.

Should read `capabilities` rather than `capbilities`.

...prevent overheating when overclocking and exiting loop()

configure usb_mtp also from usb.c

modified and adapted from rawhid

mtp file derived from rawhid and modified slightly

If called second time - fix the counts to reflect just the size of the new buffer plus the static buffer.

Also as a precaution, resets the TX or RX head tail pointers back to 0 to make sure it is not indexing into memory that may not exist.

To allow future MTP_SERIAL implementations and facilitate use of MTP_t4, that provides possibility of MTP_SERIAL

An experiement to see how much work again it would take to add the T4 methods to allow user sketches to increase the size of Serial ports RX and TX buffers, without having to edit the Source code which does it then for all sketches.

T3.x SerialN Add extra memory capabilities for Serial1-6

Also

Fixed serialEventX yield handling when you call begin multiple times

There is an issue that if you are not using the serialEvent type code and you call SerialX.begin multiple times, it will add that item to the process list multiple times, which can overrun the memory and cause crash or the like.

Fix was to check the list first and see if we are already in the list.

Wondered  if this had already been done earlier.

Was not so am trying it now.

One side effect was had to change interrupt.c as there were
#defines for things like DR, GDIR which then caused compile issues with the structure.  So renamed the defines.

Built about 3 programs with it so far.  Which appear to still be working.

Missed a few, pllus one register fields name had wrong register name

https://forum.pjrc.com/threads/63842