Air32F103/MH32F103

There is another STM clone on the market.  Namely, one can buy
board with chip marked Air32F103.  Internet says that this is
rebranded MH32F103.  There is datasheet in Chinese which
says that there are 5 variants of MH32F103, smallest one with
32kB RAM and 128kB flash (that is what is sold as Air32F103)
but there are bigger with RAM going up to 96kB and flash up
to 1MB.  I got Blue Pills which vendor maketed as "chinese STMF103".
The chip on them reports 64kB RAM and 256kB flash.  I think
that this is next member of MH32F103 family.  Namely, set of
available devices matches MH32F103 and various tests (except for
RAM and flash size) give the same results on Air32F103 and
this chip.

There is limited info on the net about those chips and in particular
very little info in English, so I decided to share some information
that I found out.  First, there are incompatibilities with STM.
For me biggest one is that compare output on channel 3 of timer 2
(and probably also timers 3, 4, 5) does not work, it behaves like
compare output was stuck at 0.  Flags is interrupt register seem
to change in right way, so actual compare probably works.  And one
can toggle output line by reconfiguring channel for negated output.
But simple LED blinker which uses PWM mode of the timer does not
work.   Chinese info probably says about this, but I first discovered
the problem and only after that I noticed that chinese text says
something about channel 3 of timers 2-5 (I do not understand
chinese so I do not know what exactly they say).

There is other incompatiblity: NVIC has 71 positions and interrupt
priorities have only 3 bits.

OK, that are bad things that I found out.  But there are also good
things.  MH32F103 can run at 216 MHz.  Chinese datasheet says
that above 2.3V flash has 0 wait states up to 108MHz and 1 wait
state above 108MHz to 216MHz (and more when supply voltage is
lower).  Of course, bigger RAM and flash are good.  MH32F103 has
more devices than STM32F103CB: timers up to timer 7, second DMA
controller, SPI3, DAC.

I admit that some parameters look strange: real flash that fast
looks unusual.  OTOH datasheet says that smallest chip needs 300uS
to wake up from deep sleep mode and bigger chips need 1200uS.
This is much larger than corresponding time for STM32F103CB, but
look too short for copy from flash to RAM used by GD32 processors.
Also, voltage dependent wait states are normal for flash, but
would be strange otherwise (as core and RAM is supplied at fixed
voltage from internal regulator, so does not see changes in
voltage).

Let me also mention thing that other folks discovered: the chips
actually have 97kB RAM.  There is special unlock sequence
(probably intended to be secret) that activates extra RAM.
Again, it is not clear what it really means.  One possibilty
is that manufacturer uses the same die for different models.
Since largest has 96kB RAM that would explain 96kB of extra
RAM.  Apparently manufacturer wanted extra private RAM for procedures
in "system memory", that explains extra 1kB.  But bigger models
have more devices.  One can see GPIO ports up to port G, so
probably there is enough GPIO machinery for biggest model (which
is 100 pin one).  It is not clear about extra devices (apparently
one can activate QSPI on smallest model...).  Another question
is flash: biggest model has 1MB flash.  Do they put that much
flash in all chips?  Or maybe flash is on separate die,
sized as needed?

Anyway, this looks like interesting chip.  Some info on the
net:

https://www.cnx-software.com/2022/11/21/air32f103-is-a-clone-of-stm32f103-clocked-at-up-to-216-mhz/

chinese info about differences
https://wiki.luatos.com/chips/air32f103/switchFromSxx.html

report about extra memory
http://www.hrrzi.com/2022/12/the-air32f103.html

-- 
                              Waldek Hebisch