From: sophana <jobarjo78_at_yahoo.fr>
Date: Tue, 22 Jun 2004 18:57:55 +0200

Matthias.Klumpp_at_gmx.de wrote:

>>Open collector transistor must be a NPN! This cannot work.
>>don't forget then the open collector scheme does invert the
>>logic, which is not what you might want to.
>>If your atmel output is an uart, it won't work.
>>
>>
>
>Well, maybe you will not believe it, but it works perfectly in my
>application.
>If you do e.g. a PSPICE simulation, you will see - no problem.
>
>
>
It is normal. A NPN transistor is a piece of silicium with 3 connected
parts: N,P,N
The P is the base (always in the middle). the 2 other N are the
collector and emiter. their size are not the same, that's why one is
called collector and the other emiter.
If you invert the collector and emiter, the transistor effect will still
work, but it is not designed to work like that.

So your scheme is not open collector but it is open emitter.

Note that a "real" open collector scheme has a resistor between the UC
and the base, to limit the current. without that, your UC could be damaged.

>>I had almost the same behaviour. except that the archos does boot if there
>>is a steady high level during boot.
>>
>>
>
>Sorry to say, but in fact you did NOT have almost the same behaviour,
>because your JB DID boot ;-))
>Would you explain the similarity of your problem with mine to me ?
>
>
>
Are you sure that your high level on remote pin is applied before the
archos dc power?
and there is no transistion during the boot?
Note that I use 5v instead of 3.3v. I don't know if that does count.

>>I use 19200 bit/s and I must not send anything before the rockbox has
>>
>>
>booted.
>
>I'm running with 9600 Baud (JB Studio).
>How is it possible to transmit with different Baud rates ?????
>
>
simply change the serial bitrate in the rockbox serial init.

>Ok, but we don't have TTL inside the JB.
>
>
>
All modern chips (above 3.3v) are ttl compatible.
Vil=0,8v
Vih=2v
maybe 2.5v chips io are also ttl compatible.

>>If you don't sink high current a simple diode would do the same.
>>
>>
>
>How ?
>If I imagine a Z-diode as level shifter I also do have 0,7V drop.
>
>
>
any low signal diode like the 1n4148 will do the same.
If the UC pin drives a 0, the voltage will be sinked to 0.7v
if it drives 1, the diode will block and it won't drive anything.
As there is no charge in your application, the transistor is useless.

>THX Sophana,
>all of that is very interesting and was increasing my low knowledge of HW,
>but it does not really answer my question, that is still:
>
>Why do I have to apply 0,7V on the JB's remote pin to make it boot correctly
>?
>
>
>
can you try 5v pullup instead of 3.3v?
I can assure you it does work in my car.

>Greetings
>
>Matthias
>
>
>
>

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Received on 2004-06-22