hmm, sorry, data cable question

Sylvain Munaut 246tnt at gmail.com
Tue May 24 13:35:04 UTC 2011


Hi,

> you are not frying your mobile only because you are lucky: your com port
> fails to deliver the 12~15v standard to the rs232 and delivers 5v instead
>
> so the 7805 regulator fails to deliver 5v because it's a linear regulator
> with a high drop out (internal voltage loss- it is designed to work at at
> least 6 or 7v ) plus the diodes voltage drop and it's delivering 3.4v

Yup exactly.


> if your connect that cable to a good com port, it will drive the calypso at
> 5v and it will fry it. right now is delivering 2.5v on tx, and it seems
> that's just enough to drive the calypso serial

Actually his cable is delivery the 3.4 V.

The 2.5V comes from the calypso because its serial port _is_ 2.5V and
not 3.3V. We use 3.3V converters because they are more common and work
fine, but it's still overdriving the calypso.

> also, did anyone tried attaching a phone to a 5v cable to see if it really
> dies?

It might not die right away, the ESD diodes will take some of it at
first ... but for how long is the question.
It also depends on the impedance of the driver, if you have a 1k
resistor on the path, it would help I guess.
But there is also the issue of the 2.5V from the calypso needs to be
detected as 1 by a 5V cable.


Cheers,

    Sylvain




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