SIMtrace prototype bug report

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Sylvain Munaut 246tnt at gmail.com
Mon Jun 20 09:19:19 UTC 2011


Hi,

> well, if you're sure the ~20 ohms is not a problem, then I think we can
> indeed stop searching for a proper switch.  The big question is though:
> Why does it not work yet?

Well, my 20 ohm statement is based on a fixed resistor. In this case I
suspect what you measured (probably with a multimeter I guess ?) might
not be valid due to high non-linearity effects (see below and in the
datasheet).

> The nRST, CLK and I/O are still routed via the switch.   The switch is
> in the correct position, as I can apply 3.3V with a 10kOhm resistor to
> any of the three lines on the 'phone' side, and I can see the
> corresponding signal changing on the SIM card side.  However, the
> high-voltage is only about 2.5V on the SIM card side.

And what's the voltage on the 'phone side', (after the pullup) ?
If there 0.8v voltage drop across 10k + 20 ohm, most of it must should
due to the 10k ...

But now that I look at the QS3244 datasheet, the Ron is highly non-linear.
At Vcc=5V and Vin=3.3V, it's even outside of the graph ... so at
Vcc=3.3v you can only imagine.

Vp (Voltage pass) is speced at 4V with Vin=Vcc=5V ... so 1V drop min
in that case.
Here we're in a much less ideal case of Vin=Vcc=3.3V ...


I still think a bus switch is appropriate ... just not that one. (or
try powering it from 5V).
The SN74CB3Q3244 mentioned by Kevin seems to have much better specs at
Vcc=3.3v and low voltage inputs.


Cheers,

    Sylvain




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