In 2022-10 Vadim Yanitskiy discovered the existence of a previously unknown
family of Calypso phones: Sony Ericsson K200/K220, usually with 'i' suffix.
The only diff between K200 and K220 appears to be the FM radio receiver
(present on K220, unpopulated PCB footprints on K200), whereas the suffix letter
indicates GSM frequency bands: 'i' for 900+1800 MHz, 'a' for 850+1900 MHz.

As of this writing (early 2022-11) Vadim and steve-m appear to be working on
adding support for this phone model to their beloved pet OsmocomBB.  In
contrast, I (Mother Mychaela) currently have no plans to add support for this
phone model to FreeCalypso fw: it is not an interesting target for the purpose
of turning it into a liberated end user phone by way of FC fw (for one thing,
K200a or K220a phones with North American frequency bands appear to be
unobtainium), and we don't need yet another alien phone running voice
pseudo-modem firmware.

However, I am gathering some notes about K200/220 hw and original fw in this
directory, for two reasons:

1) To provide better guidance to Vadim in his quest to support this weird phone
   model in his beloved pet project;

2) To gather knowledge in case the situation changes and a use case does arise
   for porting our FreeCalypso fw to this target.

If anyone does fancy the idea of running FreeCalypso fw on these SE K200/220
phones, even if only as an experiment, the primary pain points will be mostly
the same as with any other alien phone:

* Like with almost every other alien Calypso device (phone or modem), the FFS
  maintained by the original fw is not directly suitable as-is for FreeCalypso,
  i.e., sharing the same FFS between original fw and FC, freely going back and
  forth in fw with the same FFS, would be a bad idea.  Therefore, we would have
  to decide whether to put our own FFS (with different content) in the same
  sectors as the original, perhaps even produced from the original with our FFS
  editor tool, or to put our aftermarket fw FFS in some other sector location.

* In the case of Pirelli DP-L10 we were lucky to find an area of flash which
  the original fw leaves unused and untouched in the vast majority of use cases
  (staging area for OTA fw updates, unused at all other times), thus we could
  put our separate-from-original FFS instance there.  This separate FFS
  arrangement is what allows our FC fw on the Pirelli to be run via fc-xram,
  without flashing.  But we don't have the same luck on K200/220: looking at
  flash dumps, there does not appear to be any area that is totally unused and
  could be repurposed for aftermarket fw FFS without disturbing the original fw.
  Therefore, we won't have the option of non-invasive run-from-RAM operation,
  and we would have to flash our fw instead, like on Mot C1xx.

* The requirement of having to flash our fw, as opposed to running from RAM,
  combines badly with the lack of phone UI functionality in voice pseudo-modem
  fw: a phone that appears completely dead to the user but speaks some special
  binary protocol on the UART when you press the PWON button feels extremely
  weird and unnatural.

* Having to flash our fw also means that we would need to either implement
  battery charging in our fw (requiring thorough reverse eng of charging
  specifics on this model) or use another phone (running original fw) to charge
  batteries, moving batteries back and forth between phones.

All of the above factors, which apply to *any* alien (meaning not FC-made) phone
model, should make it clear why running our fw on alien Calypso phones is no
longer the direction of interest in FreeCalypso - instead we need to focus on
our own FreeCalypso hardware with phone handset functionality, starting with
FC Venus.