GTA02 is now an expensive brick / pcf50633 defaults

Andy Green andy at
Tue Jan 6 16:17:38 CET 2009

Hash: SHA1

Somebody in the thread at some point said:
| Mike Montour wrote:
|> If you program the PCF50633 as:
|>   - charger enabled
|>   - 100mA current limit
|>   - do not wake on USB insertion (avoids the brownout issue)
|> then it will have the behaviour that you want - even with a
|> completely-dead battery, you can plug it in and it will slow-charge the
|> battery in hardware. I think it still technically violates one of the
|> USB standards by drawing current without talking to the host, but IMHO
|> it's unlikely to cause any real-world problems.
| 1. This really ought to be default behaviour, the current way is really
| *really* bad.
| 2. Why stick to 100mA? I thought the USB standard was for ports to support
| 500mA?
| 3. I'm sure the FR doesn't need to negotiate with its charger.
| 4. How easy is it to make this change?

The issue is more complicated than this.

pcf50633 has more than one level of hardcoded default actions, some
registers get reset when you tell it to enter what it calls "standby"
but we would more naturally call "off", and more get reset again when
the small backup battery that is also present on GTA02 becomes
exhausted, called NOPOWER state.

The "real default" settings for our variant that occur with NOPOWER
forces the charger disabled.  So if you stick your FR in a drawer, the
main battery is exhausted, the backup battery goes down a day or
whatever later, nothing that we did in previous session about charger
status sticks, the charger is off.  In addition, due to VB_SYS issue, we
can't get easily get to the point of running any code on A5 to enable it
without a loan from the battery to get us started.

Sometimes the battery is in cutoff and does have some power in the cell,
it's just disconnected from the terminals.  That's when any kind of
external kick will get us out of the hole.

There is no violation of USB standards if you only pull 100mA, but
another default set by going "off" even is that we pull 500mA from the
USB host until the CPU can get started, and that is in violation.  But
since we can do nothing about it, and we didn't find a host that cares
yet, it's OK.

There's two interesting ideas from Mike though, one is that disable USB
insert as ON will help by giving longer for VB_SYS to charge and the
other is leave charger enabled.  For both of these, they are defeated
(USB insert is ON action, charger disabled) by NOPOWER and we have no
control then.  If the backup battery had not decayed, it could help, but
it only has very partial impact and is not magically delivering "the
behaviour you want", GTA02 with this issue still will not start if left
long enough for backup battery to fall below the pcf50633 threshold for

- -Andy
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