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Subject: Re: AW: AW: Still Problems with AJB Remote

Re: AW: AW: Still Problems with AJB Remote

From: [IDC]Dragon <idc-dragon_at_gmx.de>
Date: Tue, 13 Jul 2004 08:59:13 +0200 (MEST)

> # It's pin 2 of the board-to-board connector. No pullup drawn.
>
> Aha, so the remote pin of the Recorder's needs to be driven actively,
> correct ?

Only if no internal pullup is present. To keep it bidirectional, I'd
recommend an external pullup. If you drive it, stay below 3.3V and don't
drive it if the Archos is powered off. Else you would try to power it via
the internal protection diode of the CPU, stressing that.

>
> # Rockbox site search got me there:
> # http://rockbox.haxx.se/mail/archive/rockbox-archive-2004-03/0049.shtml
>
> There is a application not mentioned:
> http://www.semiconductors.philips.com/acrobat/applicationnotes/AN97055.pdf

Yes, that's the one I meant.

> Could you maybe be so kind and describe the circuit that would be needed
> in
> my case (starting from the uC's out to the remote pin) ?
> What values do the R's have to have ?
> Which type of resistors will be suitable ?

OK. You need a small N-channel MOSFET. I think I used a BS170, which is very
cheap and easy to get:
http://www.fairchildsemi.com/ds/BS/BS170.pdf
But keep in mind these are very sensitive to ESD. Get some more of it. I
blew one or two while tinkering with this. If you can measure any resistance
lower than infinite (insulation) between gate and another pin, it's gone.

The "circuit" is very simple: connect the source to the Archos, drain to
your TTL input/output, gate to a 3.3V reference. The transistor draws no
power from there, it's just a reference. However you do this, maybe a
voltage divider from 5V. (eg, 22kOhm towards ground and 12 kOhm towards 5V
will give 3.2V at the midpoint)

If your Archos needs an external pullup, you can connect it via maybe
4.7kOhm to 5V. This is safe, the pin is capable to limit the voltage if the
current is limited by such a resistor.

I'm no good in ASCII drawing, but I hope it's understandable without.

Jörg


Jörg

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Received on 2004-07-13

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