Y-cable and USB-OTG: mutually exclusive / the four connectivity solutions

Andy Green andy at openmoko.com
Thu May 1 22:10:32 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| Andy Green wrote:
|> What we can't do is set current limit from adapter as we can on USB.  We
|> cannot actually treat it as a second USB interface for pcf50633 purposes
|> as it is I think.
| Yes, we may need some current limiter. This is starting to look a bit
| like the charger circuit in HXD8, which also has a third power source,
| namely car power. (HXD8 is messier, though, because it uses the
| PCF50606, which doesn't even know about USB.)
| If you look at the HXD8 schematics, you'll find that there's a lot of
| glue logic and even more analog trickery. (And as you might expect,
| making all that work properly in all possible modes of operation gave
| us some headache.) That experience is actually one of the things that
| motivated me to want an MPU - being able to replace all that messy
| glue logic (that invariably turns up) with software.

Yeah.  I think we can maybe "specify out" the need for an external
current limit here though -->

| We have to consider which current limits we care about. There are:
| 1) 100mA: we get this one for free - if there's any voltage, we can
|    draw 100mA
| 2) 500mA: we get this if we tell the FTDI in the EEPROM that we want
|    that much power, or if we make 500mA out ceiling
| 3) 1A (or such, could be "unlimited", if the charger doesn't mind):
|    we get this if we make it our ceiling
| How do we feed the JTAG/console subsystem ? From a Vsys combined from
| all three possible sources, or from the USB-debug 100mA path ?

Just USB-debug direct Vbus I think.

| I think we want the "high-current" path, i.e., 1A. I'm less certain
| about the 500mA path.

Right.  I think what we can do is treat the 100mA case as we should not
use adapter power... in the later diagrams I sent I treat it as same
situation as USB suspend there, "there is voltage but we can't take any
power" -- that boils down to falsely reporting ADAPTSNS to absent.
100mA case is atypical anyway.

"1A" is definitely OK as we can again use 2A adapter and it is a
bottomless pit enough for us.  500mA is the question, can we allow to
enable charge under these circumstances when in the worst case we have
an OTG peripheral plugged in the other side also sucking down 5V @
500mA.  And because we don't have the input-side current limit for
adapter, it would be easy to exceed 500mA just from the OTG charge pump
and the rest of the system without even considering charging the battery.

Maybe it turns out we should only enable adapter power usage at all
(above just powering the FTDI) when we have an actual adapter there and
we allow it 1A fast charge + powering the device + any OTG devices
hanging off it.

This is the kind of thing MSP430 will manage by itself including
reporting and configuring to pcf50633.

- -Andy
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