next-generation (now) MPU discussion

[joerg at openmoko.org]

Am Do  24. April 2008 schrieb Werner Almesberger:
> joerg at openmoko.org wrote:
> > Keeping the "save"-state and not messing around with these state changes 
of 
> > PMU. (though I don't see any big difference between save and off state, 
> > except for RTC and some lowpower-supply chain).
> 
> The PCF50633 resets registers to mask-programmed defaults depending on
> state. Some registers only reset in NoPower while others reset in
> Standby.

Yep, that's what I meant when saying "not messing around...", already 
suspected things like these. For properly handling power up button etc, we 
will need some backup-power for PMU anyway I guess. 
So it's "share the battery" then, rather than "move it". Better idea anyway.


> > Right. Just wondering how many devices we have to sell to break even on 
> > 
bargain_on_chip/unit(hw-supply)./.additional_effort_for_small_chip(sw-manpower).
> 
> A small chip may actually save time by not tempting us to do too
> much with it ;-)

WAAAH, don't ever say this when anybody may hear! ;-) They will give you bad 
hardware, expect you to do a great job on sw for it, and probably pay less 
for it because it's less work as you stated. :-/ ;-)
Now for real: we got a lot of additional sw-fiddling with low level things on 
the small chip when comparing to the 12,95$ one, and we even aren't 100% sure 
where we will hit the limits the small RAM and missing hw-serial-I/O is 
imposing (are we?).

I'd like to push the decision about which chip to use for MPU, so we can get 
down to real development work for the circuit diagrams. (though we are also 
waiting for the LCM specs, so probably there's plenty of time to just sit and 
talk and ponder whether it's worth the 7$/unit... OTOH we can easily start 
with existing GTA02 screen and 0 effort, but for MSP430 we have to do some 
work anyway - a pity when it's done for the wrong chip)

Andy: can you give to us a short memo summarizing the benefits of a MPU we 
wouldn't get any other way, briefly explaining background and way it shall 
work for each of those points? So everybody involved in the decision on MPU 
is clear on what we are talking about, enabling us to make a more educated 
decision.

cheers
/jOERG