SIMtrace v0.6 schema

[Harald Welte]

Hi Kevin, 

thanks for posting your most recent schematics.

On Sat, May 07, 2011 at 10:12:23PM +0200, Kevin Redon wrote:
> here the schema.
> all the components are on it, but I don't know if the electrical part is
> right.
> Any comments/advices are welcome.

1) Regarding dexters mail: I defintely would want JTAG, it helps a lot
   for debugging.  We don't have to solder the connector for production
   units after R&D has finished.

2) It might be an idea to use a 2.5mm jack (like Compal phones) for the
   DBGU, instead of the FTDI-6pin-header.  I guess more people in and
   around the Osmocom projects have a T191 cable than a FT232 cable with
   the 6-pin header.  What do the others think?   It may be worth
   putting both footprints (for the 6-pin cable and the 2.5mm jack)
   on the PCB.  This way we can later still decide which one to place on
   the board.

3) It might be nice to have an ADC input connected to the VCC_OUT line,
   to be able to measure the SIM card supply voltage as it is output
   by the phone.  ADCREF of course would have to be connected, too.

4) It is generally good 'style' to connect the nWP to some I/O line of
   the CPU (together with a pull-down).  This way the memory is
   write-protected by default.  Only if the CPU sets the GPIO to high,
   it can write to the flash.

5) I think the IN/OUT naming is a bit misleading.  E.g. "VCC_OUT" is the
   source of the VCC power, i.e. the phone.  It may be better to call
   the signals leading to the phone {VPP,I/O,CLK,RST,VCC}_PHONE and the
   signals leading to the sim {VPP,I/O,CLK,RST,VCC}_SIM

6) The USB pull-up mechanism is missing.  This is required for properly
   signalling a USB bus reset to the host controller.  Please refer to
   the OpenPCD schematics for an example circuit.  It just requires one
   transistor and some resistors.  It has also proven useful to connect
   the nRESET to that pull-up circuit to make sure a cpu-reset leads to
   a bus reset as well.

7) A 5.6V Zener diode makes sense as over-voltage protection at the USB
   5V power supply line

8) The two 47uF capacitors at the 3.3V line and the 22uF capacitor at
   the 5V line might be above the USB specifications for capacitive
   load.  See Section 7.2.4.1 of the USB spec, where I believe it says
   you can only put a cumulative capacitance of 10uF at the downstream
   end of a Vbus.  We might ignore that, or use something like a 
   LM3525 (or even a TI TPS2151 which is a LDO with built-in in-rush
   current limiter).  OpenPCD1 v0.4 is not being sold anymore exactly
   due to this problem.

9) I would only use a jumper for the TEST singal, as we probably only
   need it very rarely during sam7dfu testing/porting.  After that, we
   will use USB DFU for updates

10) However, it is useful to have a push-button for DFU-mode activation,
    this is a simple switch between Vcc and a GPIO.  Normally, DFU is
    activated over USB itself.  However, if the firmware is dead, this
    BOOTLOADER switch can help with recovery (once again, see OpenPCD)

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