SIMtrace prototype bug report

[Harald Welte]

Hi all,

I've done a lot of work on the SIMtrace prototype over the last couple
of days.  A number of bugs were easy to re-work, or are at least known
and can be improved in the second revision.

However, I think there's one pretty serious problem in the hardware
design:  The bus switch has something like at least 5 (rather 16Ohms) of
internal 'on' resistance between the sim card and the phone.  While this 
_might_ still work for I/O, nRST and CLK lines, it definitely doesn't
fly for the supply voltage (VCC).

I think there is little that can be done, except:

a) finding a better alternative analog / bus switch component
   this might be an option for the second prototype, but is not
   easy to fix in the existing board.  I'd love to see something
   that has milli-ohms internal resistance, not ohms.
   Advantage: Also removes the couple-of-ohms from the IO,RST,CLK

b) permanently connecting VCC_PHONE with VCC_SIM
   this sort-of violates our idea of splitting the SIM card and the
   phone side for MITM.

c) always supplying VCC_SIM by the SIMTRACE board, ignoring VCC_PHONE
   This would make a lot of sense, if we'd also provide VCC_PHONE to
   an IRQ-capable GPIO of the SAM7.  We could then detect VCC_PHONE
   changes in software and switch the VCC_SIM from the IRQ handler.
   This introduces some delay, but I doubt that it is more than what
   would happen in case there was some uF-range capacitor for VCC
   stabilization in the hardware.
 
I will try solution 'c' as a work-around, it simply means cutting one
trace and adding one wire.

-- 
- Harald Welte <laforge at gnumonks.org>           http://laforge.gnumonks.org/
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