freecalypso-docs: FC-handset-spec comparison

comparison FC-handset-spec @ 79:468d43c0d8cb

FC-handset-spec: document our plan of using NCR18650B

author	Mychaela Falconia <falcon@freecalypso.org>
date	Tue, 21 Sep 2021 02:29:40 +0000
parents	a05195c86d3a
children	3e32484f8832

comparison

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 The battery in our FC Libre Dumbphone handset will be single-cell Li-ion.  It
 goes without saying that this battery will be freely removable and replaceable
 by end users.  The specific size, form factor and mAh capacity of this battery
 won't be addressed until later in the project, when we get closer to building
-the actual handset.
+the actual handset.  (2021-09 update: we are now looking at using Panasonic
+NCR18650B as our canonical battery on the Venus board, but we are not yet
+committing to anything for the actual handset.)
 Our Calypso+Iota chipset dates from the era when the cellular handset industry
 was transitioning from NiMH to Li-ion batteries, and the Battery Charger
 Interface (BCI) block in the Iota chip supports both battery types, or at least
 TI's documentation claims so.  Given that we are going against the mainstream
 society's ideas in so many other ways, I have given thought to the possibility
 of using a NiMH battery instead of Li-ion.  However, the problem with using a
 NiMH battery is that we would be going into completely uncharted territory
-without any guidance.  In the case of Li-ion batteries the charging process is
+without any guidance.  Here are some of the difficulties that would arise with
-well-understood in both theory and practice, and our FCHG logic based on reverse
+a NiMH battery:
-engineering of Pirelli's firmware works well both on the same Pirelli and on
-Motorola C1xx family.  In contrast, if we went with NiMH, we would have
+1) In the case of Li-ion batteries the charging process is well-understood in
-absolutely no guidance in implementing the necessary charging control logic
+both theory and practice, and our FCHG logic based on reverse engineering of
-(TI's LCC code is useless), causing a huge risk to the project.  Furthermore,
+Pirelli's firmware works well both on the same Pirelli and on Motorola C1xx
-determining the state of charge from Vbat for the purpose of the bars icon is
+family.  In contrast, if we went with NiMH, we would have absolutely no
-already somewhat challenging even with Li-ion, given the relatively flat middle
+guidance in implementing the necessary charging control logic (TI's LCC code
-part of the discharge curve - and with NiMH we can only expect the problem to be
+is useless), causing a huge risk to the project.
-even worse, as their discharge curve is said to be even flatter.  For these
-reasons, we are going to play it safe and stick with Li-ion.
+2) When TI designed their canonical battery charging circuit to be controlled
+via Iota BCI, they assumed a charging power source that puts out somewhere
+between 6 and 7 V, not 5.0 V - those were the days before USB charging.  For
+Li-ion batteries charging at around 500 mA, a USB +5V charging power source
+is good enough, as proven by Pirelli DP-L10.  However, this lower charging
+voltage may not be enough for NiMH - consider the note in the Iota chip
+datasheet (TWL3025_SWRS021.pdf) that reads "Ni-MH/Ni-Cd 3-cell battery
+voltage can reach 5.5 V at the end of a charge cycle."
+3) Determining the state of charge from Vbat for the purpose of the bars icon
+is already somewhat challenging even with Li-ion, given the relatively flat
+middle part of the discharge curve - and with NiMH we can only expect the
+problem to be even worse, as their discharge curve is said to be even
+flatter.
+For these reasons, we are going to play it safe and stick with Li-ion.
 1.10.1. Battery ID resistors or thermistors
 In all classic dumbphones from the era which we seek to revive (the ones with
 user-removable batteries), the battery pack (flat pouch) has 3 or 4 contacts
 ID resistors or thermistors included.  Both Motorola C1xx and Pirelli DP-L10
 batteries have 3 contacts, and so does Nokia BL-6C battery form factor that was
 copied by FIC/Openmoko.  In contrast, cylindrical Li-ion battery cells like
 18650 have no such third terminal.
-Because we are deferring our handset battery design to a later project phase
+As of this writing (2021-09), we do not anticipate using any kind of battery ID
-(not included on the Venus board), we don't know at this time whether or not
+resistors or thermistors, neither on our Venus board nor on our final handset.
-our final handset battery pack will have any ID resistor and/or thermistor
+FC Venus will have a provision for bringing out Iota ADIN2 like on Leonardo
-connected to Iota ADIN1 or ADIN2.  Our current FCHG battery charging driver
+(see section 3.3.1.1), but we do not anticipate actually using it.  The battery
-does not touch ADIN1 or ADIN2 at all; if we do end up having an ID resistor or
+on FC Venus will most likely be an 18650; on the final handset we will most
-thermistor and needing to support it, that custom support will have to be added
+likely either keep the same or use a custom-made flat pouch battery pack, and
-at the time of the actual handset design.
+if we go with the latter, it will still most likely only have two terminals.
+Our current FCHG battery charging driver does not touch ADIN1 or ADIN2 at all.
+Looking at TI's old PWR reference code and the disassembly of Pirelli's
+firmware, it appears that when mainstream proprietary firmwares do check those
+ID resistors or thermistors on ADIN1 or ADIN2, they only act as artificial
+blockers: the firmware refuses to charge the battery if it sees something it
+doesn't like, as opposed to using data from these sensors to tune or adjust the
+charging process in some positive constructive manner.  In FreeCalypso we stand
+against all such artificial blocking, instead we uphold the principles of user
+empowerment and personal responsibility - thus we do not currently anticipate
+ever implementing any kind of ADIN1 or ADIN2 logic.
 1.11. Charging circuit
 Our FC Libre Dumbphone will feature a USB port (mini-B, device role only, no
 OTG) that combines two logically separate functions: battery charging and
 same connector - the Mother's intent is to use a common off-the-shelf 18650
 battery holder and wire it up to the necessary Weidmuller plug.  When a real
 battery is used, it will also be possible to charge it via Calypso+Iota BCI and
 FreeCalypso FCHG just like in a real handset - see the following section.
+Because of the need for battery-specific tuning in FCHG, it will be difficult
+to use "any" 18650 battery for the real-battery mode of operation on FC Venus -
+instead we will be on much more solid footing if we select some specific battery
+model and stick to it.  The Mother's current plan is to use Panasonic NCR18650B
+as our canonical battery.
+3.3.1.1. Iota ADIN2 connection
+Our previous development boards FCDEV3B and Caramel2 use only pins 1 and 3 on
+the 3-pin Weidmuller battery power input connector - however, TI's Leonardo
+schematics also connect ADIN2 to the otherwise unused middle pin.  Just for the
+sake of flexibility, we are going to replicate this Leonardo-style ADIN2
+connection on our Venus board, allowing the possibility of experimenting with
+battery packs that include some kind of ID resistor or thermistor and provide a
+third terminal for it.  However, as of this writing (2021-09), we do not
+anticipate actually using this functionality - the Mother's current plan is to
+use an 18650 battery that does not provide any kind of third terminal.
 3.3.2. USB subsystem
 Our Venus development board will include the same USB subsystem as intended for
 the final FC Libre Dumbphone handset, as described in sections 1.11 and 1.12,
 consisting of a USB mini-B interface connector, a charging on/off switch with

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comparison FC-handset-spec @ 79:468d43c0d8cb