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Subject: Re: JB not starting with remote pin connected ?
From: sophana (jobarjo78_at_yahoo.fr)
Date: 2004-06-20


----- Original Message -----
From: <Matthias.Klumpp_at_gmx.de>
To: <rockbox_at_cool.haxx.se>
Sent: Sunday, June 20, 2004 11:41 AM
Subject: JB not starting with remote pin connected ?

> Hi,
>
> I was just observing a strange behaviour of my JB Studio 20.
> Let me explain, what I know and what happened:
>
> As I know the AJB's are operating with 3,3V.
> As somebody from this list was telling me once, there is a pull up
resistor
> (R) inside the JB's remote pin.
>
> With my remote control I'm connected with a open collector transistor (T).
> Let me try to make a schematic in ASCII art of that:
>
> 3.3V
> |
> |-|
> | |R
> |-|
> |
> +-----+
> T |
> |/C
> --|\E
> |
> - GND
>
> T is a PNP BC557 transistor controlled by a Atmel uC.

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.
If it is controlled by software, you don't need a transistor, you can just
play with the tri state command. This is what I do with the PIC.

> JB is not connected to power.
> When the uC board is powered up, the JB also receives power by a relay
> (power source is the Archos power adapter).
>
> What I was observing is the following:
> - I'm powering my uC board and so also the JB.
> - uC is setting the base of the transistor to high, so collector of PNP
> transistor is floating.
> - I'm measuring 3.3V at the JB's remote pin, so far so good.
>
> Normally the JB shows now "Charging".
>
> In that above described scenario, it is switching on the green LED, but
not
> displaying any text. It then accepts no key presses any more. I do have to
> disconnect the power from the JB, press STOP and re-connect the power to
it.
> Then ist also displays "Charging" and I can witch it on by pressing the ON
> key.
>
> If I do the same, but holding down the base of the transistor to low level
> by the uC (so remote pin is held down to 0V), the JB switches on the green
> lamp AND displays "Charging".
>
> Is anybody able to tell me the reason for that ?
>
I had almost the same behaviour. except that the archos does boot if there
is a steady high level during boot.
I use 19200 bit/s and I must not send anything before the rockbox has
booted.
I have a special routine which stop transmission until the archos boots.
I think the archos won't boot if there is any framing error during boot.

> Another question is more HW basics related:
>
> Is there any difference in the following transistor circuits ?
> What is the difference ?
>
> Open Collector (with PNP transistor):
>
> +-----to JB remote
> T |
> |/C
> --|\E
> |
> - GND
>

this must be NPN, it does invert the logic and only drives the low level.

>
> Open Emitter (with PNP transistor):
>
> +-----to JB remote
> T |
> |/E
> --|\C
> |
> - GND
>
this does use a PNP. It does not invert the logic, and is only needed if you
want to sink high current.
Note that you won't drive 0 totally because of the Vbe voltage around 0.7v.
It won't be TTL compatible anymore because the TTL low voltage must be under
0.8v
If you don't sink high current a simple diode would do the same.

>
> Many thanks to you
>
> Matthias
>
> --
> ----------------------------------------------------------------------
> Sent by: Matthias Klumpp
> E-Mail : Matthias.Klumpp_at_gmx.de
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