All historical Calypso phone or modem designs which we (FreeCalypso) consider
interesting enough to copy use a combined MCP (multi-chip package) for their
flash and XRAM, the latter being our term for board-level RAM, as opposed to the
relatively small IRAM inside the Calypso chip itself.  The following Spansion
MCPs (all of them no longer made and available only from surplus, sadly) are
most interesting to us:

* S71PL032J (4 MiB flash, various RAM options) would be fine for an AT-command-
  controlled modem like Openmoko - however, we are not building any such product
  at the present, and this flash capacity is too small for the functionality
  that must be supported on FC Venus.

* S71PL064J is 8 MiB flash with various RAM options; S71PL064JA0 is the version
  with 2 MiB RAM.  The latter part is used inside mass-produced iWOW TR-800
  modules - there is a rumor that they may have used S71PL064JB0 initially,
  with 4 MiB RAM, but then went down to 2 MiB - but S71PL064JA0 is the chip
  inside those TR-800 modules which are presently available as a large surplus.
  This chip seems ideal: 8 MiB flash + 2 MiB XRAM is sufficient memory capacity
  for all TCS211 firmware configurations, and the physical footprint of 7x9 mm
  is the smallest we've ever seen for such MCPs.

* S71PL129J and S71PL129N are 16 MiB flash families, presented as two flash chip
  select banks of 8 MiB each, specifically made for use with processors like
  Calypso that have a limit of 8 MiB per chip select.  RAM options are
  correspondingly large; the chip used on FCDEV3B (copied from Pirelli DP-L10)
  is S71PL129NC0HFW4B, and the XRAM capacity is 8 MiB.  Compared to S71PL-J
  chips, S71PL129N has more stringent reset timing requirements; the impact on
  Calypso-based designs is that Calypso FDP output cannot be used as the flash
  reset signal, and a different circuit based on ON_nOFF signal is needed - and
  the latter circuit requires adding one more little IC, Nexperia 74AXP1T34, a
  dual supply translating buffer.  The latter design with 74AXP1T34 has been
  proven on FCDEV3B V2.

As far as FC Venus PCB design goes, meaning flash+RAM MCP footprint and the
flash reset circuit, the following options are up for consideration:

Option 1: copy the 7x9 mm MCP footprint from iWOW TR-800.  This footprint
accommodates S71PL064J but not S71PL129J or N; because populating S71PL129N is
not possible on this footprint, flash reset can be sourced from Calypso FDP
output as was done in TI's original design.

Option 2: enlarge the MCP footprint to 8x11.6 mm with 8 extra mechanical-only
balls, but keep the old and simple reset circuit.  This option will allow
populating either S71PL064J or S71PL129J, but not S71PL129N - the latter would
cause sleep mode problems and sometimes even boot problems as seen on FCDEV3B V1
where we made this mistake.

Option 3: use the larger MCP footprint for 16 MiB flash and also incorporate the
74AXP1T34 flash reset circuit from FCDEV3B V2.  This option will allow any of
S71PL064J, S71PL129J or S71PL129N to be populated and work correctly.

The Mother's original plan for FC Venus was to do Option 1, but this plan is now
being changed to Option 3.  The reasons for this change are:

* S71PL129NC0HFW4B parts are already on hand at FreeCalypso HQ, and have been
  used successfully in FCDEV3B board builds.  With Option 1 we would need to go
  back to our Chinese grey market supplier and procure S71PL064J chips, and then
  take the risk of possibly bad parts.

* For psychological reasons it is important for FC Venus to be a no-worse
  successor to FCDEV3B.  Even though 8 MiB flash + 2 MiB XRAM is perfectly
  sufficient memory capacity for all of our fw configurations (and according to
  TI's docs, it would be sufficient even for their pdt_2272 config with MMS
  functionality, which we don't have), a reduction from FCDEV3B's 16 MiB flash +
  8 MiB XRAM will undoubtedly be seen by some community members as a downgrade.
  There is also potential value in being able to load and run non-flashed
  fc-xram firmware builds, which is only possible with gigantic 8 MiB XRAM.

* Because *all* of our suitable Spansion MCP options are no-longer-made
  surplus-only parts (much like the core Calypso chipset itself), it makes good
  sense to design our PCB in such a way as to allow as many options as possible,
  not excluding any otherwise suitable (and known) option through our PCB
  design choices.

The downside of this chosen approach (compared to our original approach of
Option 1) is the increase in MCP BGA footprint, plus the little bit of extra
room needed for the 74AXP1T34 IC.  The Leonardo core layout inside TR-800 is
very tight with no room for any extras, thus if we were seeking to clone or
semi-clone a Tango module, this increase in PCB real estate for the flash+RAM
MCP would not be acceptable.  However, in the case of FC Venus, our core
shieldcan section already includes many additions beyond Leonardo/TR-800:
consider U401 through U404, all of which need to go into this expanded core
section.  Thus if we are already expanding the core for other reasons, we should
be able to throw in this flash+RAM MCP expansion as well.

Reset circuit population option
===============================

If U303 is populated and R372 is unpopulated (necessary for S71PL129N MCP, the
current plan for initial board build), the flash reset signal will be fed with
ON_nOFF, translated to 2.8V via U303.  However, if the populated MCP is S71PL-J,
then there are two options:

* Flash reset may be generated "in the new way" like for S71PL129N, with U303
  populated and R372 unpopulated.

* If U303 is omitted and a 0R jumper is populated at R372 instead, the circuit
  reverts to TI's old way of using Calypso FDP for flash reset.

The unpopulated R372 footprint will also serve as a pair of test points for
oscilloscope probing, allowing both Calypso FDP output and the actually-used
output of U303 to be observed.