From: [IDC]Dragon <idc-dragon_at_gmx.de>
Date: Tue, 13 Jul 2004 23:11:06 +0200 (MEST)

> # 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.
>
> So I'll do it, just to be sure:
>
> +5V
> |
> +-------------------------------+
> | |
> +-+ +-+
> | |22k | |4.7k
> | | | |
> +-+ +-+
> +---------------------+ |
> +-+ | |
> | |12k | |
> | | | |
> +-+ | |
> | | |
> | | |
> --- | |
> |G |
> ------ |
> D| |S |
> uC Out port--------+ +--------+-- AJB remote
>
> Like this ?

Almost. You need to swap the values of the divider. And I forgot to mention
you should have a pullup on the uC side, too.
Hmm, the circuit is bidirectional, for open collector on both sides, which
is not exactly what you want. Your uC serial output is a plain driver.
Forget what I said and simplify to this:

                                   +5V
                                    |
                                    |
                                   +-+
                                   | |4.7k
                                   | |
                                   +-+
                                    |
                                    |
                      Schottky |
     uC Out port--------|<|---------+-- AJB remote

The pullup won't hurt even if a box already has one. The diode will allow
your uC to pull the pin low, but will prevent it from being driven with high
output current to 5V. Instead it "gently" floats up via the pullup. Use a
Schottky because those have low voltage drop across, allowing for a better
"low" level.

Jörg

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Received on 2004-07-13