next-generation MPU discussion

joerg at joerg at
Thu Apr 24 17:40:23 CEST 2008

Am Do  24. April 2008 schrieb Werner Almesberger:
> Andy Green wrote:
> > We probably can share the battery but I didn't really think of what
> > better features it gets us.
> It could provide power until a sufficiently low-voltage regulator
> is active. I don't remember the part number of the battery, so I'm
> not sure if the voltage is rights.
> In any case, if we share the backup battery to power the MPU when the
> system is in PMU.Standby, the MPU won't be able to do much, or it'll
> drain the backup battery.
> > Everyone seems to like that method of shared IO, it sounds like a good
> > policy.
> It's nice, simple, and fool-proof. But it also has the distinctive
> disadvantage of needing lots of GPIOs on the MPU :-(

I always felt the lots of GPIO on MPU will be used anyway when we hook up 
G-Meter etc to it, what it needs is additional IO from mainCPU what we have 
plenty I think. 

> By the way, wakeup sources (i.e., CPU.SLEEP -> CPU.NORMAL) can also
> go to the CPU, which then signals the MPU that it wishes to be woken
> up. (With XPWRRGTON on the 6400 (WHMKSUPSCHCRZNMS?), PWREN on 2442.)
> So buttons or similar that can only wake us from CPU.SLEEP, but don't
> have to wake us from PMU.Standby don't need to go to the MPU. (Of
> course, if we want to keep the option open to use such sources also
> to wake from PMU.Standby, we can't make this simplification.)

Simply spoken, the MPU can press the button, and can instantly detect user 
pressing the button - whether or not we will do any further actions on MPU.
Imagine the MPU overriding the wakeup of CPU by instantly asserting a sleep 
sequence (higher prio irq or sth) as soon as MPU detects the buttonpress. If 
MPU is broken (frozen..), the wakeup will succeed.
Maybe weird example, whatever it's worth.
The idea is *every* GPIO is I *and* O, so you have maximum flexibility.

Imagine we got 8 LED. In 'emergency' case, CPU and MPU might share these to 
build a bidir 8bit-databus ;-) (maybe looks funny too ;)

BTW: which number of LED is GTA04 


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