Fwd: Adding a system management MPU

Wolfgang Spraul wolfgang at openmoko.com
Thu Mar 20 07:57:15 CET 2008


fyi

Begin forwarded message:

> From: Andy Green <andy at openmoko.com>
> Date: March 19, 2008 7:43:02 AM GMT+08:00
> Subject: Re: Adding a system management MPU
>
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> Somebody in the thread at some point said:
>> Andy Green wrote:
>>> There is a plan to basically put the debug board FTDI chip on the
>>> mainboard for JTAG / debrick purposes, along with a micro USB conn  
>>> that
>>> isn't accessible without opening the case.  So we discard the debug
>>> board as a "product" and all phones expose a "USB UART" console  
>>> and JTAG
>>> if you want it.
>>
>> I would fear the added cost of the FTDI chip - it's also ugly to  
>> force
>> the user to open the case.
>
> Well, IIRC it was $3.80 / 1K, no doubt it is cheaper for FIC.  We also
> don't have to deal with the debug board, connector, or cables any more
> to balance it a bit.  Issues around the level of paranoia and  
> protection
> otherwise needed for built-in debricking (eg, write-once at factory
> memory now found in GTA02) are solved by this too -- and hackability
> "built-in" is a feature for this product AIUI.  But I take your  
> points.
>
>> In terms if de-bricking I could imagine to expose the RFCOMM- 
>> interface
>> of the Bluetooth chip
>
>> I decided in my EBook design to stick to a discrete CSR Bluecore4  
>> chip
>> with UART + USB interface. To debrick it you would for example hold a
>> given set of keys that are anyway connected to the MSP and use MSP to
>> set the ARM-core into UART boot mode (from ARM core mask ROM)after
>> resetting it. The MSP will then upload the stepstone over UART into  
>> the
>> internal SRAM of your main ARM CPU.
>
> I can see this will work for your design (must be highly impressive to
> witness), but we would need a new BT module with private UART IF like
> yours.  Also in our case compared to the FTDI solution it wouldn't in
> itself provide actual JTAG which can be useful for desperate  
> debugging,
> although we could somehow do it via the MSP GPIO driven by the same BT
> UART I guess.
>
> I think it's okay in the end if someone with a brick has to crack the
> case for a guaranteed fix.  $3.80 here and $3 there and pretty soon
> you're talking real money is true though.
>
> - -Andy
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