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Subject: Re: AW: JB not starting with remote pin connected ?
From: sophana (jobarjo78_at_yahoo.fr)
Date: 2004-06-23


Matthias.Klumpp_at_gmx.de wrote:

>>and there is no transistion during the boot?
>>
>>
>
>Well, that's the point.
>I had a more close look to the signal. In fact there is a short send command
>on the UART (I already discovered the buggy line of code).
>So resuming. When I was holding the UART pin low, during initialisation of
>my SW, I was electrically supressing the serial communication.
>Effect was, that it worked, but I was not changing the behaviour by its
>root, but by its symptom ;-))
>
>
>
that's probably your problem

>>Note that I use 5v instead of 3.3v. I don't know if that does count.
>>
>>
>
>I was once told (as I remember by Steve Russell, who did a AJB remote), that
>applying 5V to the JB's remote pin could damage it.
>That was the reason I was thinking about a OC level shifter.
>
>If you apply 5V direct to the remote input of the JB, there were no damages
>?
>
>
I use a 10k pull up resistor. applying 5v through a resistor will never
damage your device, since it is protected by clamping transistors for
electrostatic protection.
Very often, 3.3v devices are made direct compatible with 5v devices by
using special transistors on the positive side.

>>>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.
>>
>>
>
>Serial init means, the Settings or some init inside the Rockbox ?
>As I know, the default is 9600Bd, that is also used with the original Archos
>SW.
>Why did you change the default ?
>Your remote control is not anymore compatible with JB's with original SW,
>isn't it ?
>
>
>
>>>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.
>>
>>
>
>Ah, so my low level of 0,7V is maybe too close to Vil, that might cause
>communication problems in some models.......
>I think, I should modify that OC stage to soemthing that has a real 0V low
>state....
>
>
>
the more you will be close to the voltage limit, the less your design
will be tolerant to noise induced by several environemental factors.
But it can work that way.

>>>>If you don't sink high current a simple diode would do the same.
>>>>
>>>>
>>>How ?
>>>
>>>
>>>
>>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.
>>
>>
>
>I see, 100% correct.
>When I was designing that PNP stage, I was not knowing about that 0,7V low
>level I now do have.
>
>
>
>>>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.
>>
>>
>
>As I mentioned above, it was not really that I needed to apply low level to
>the remote input, I just was overwriting the faulty serial communication of
>my SW.
>
>Finilazing:
>I think I have to get a low level that is lower than 0,7V to assure a
>correct low level recognition in the JB.
>
>What do you think would be better:
>- Simply connecting the UART 5V directly to the remote pin (without OC stage
>at all) ?
>
>
This can work and I even think it is the best and simplest solution.
Simply place a 2k to 10k resistor for protection between both pins.
Do you use bidirectionnal communication?
If not there will be no problem at all since there is only 1 transmitter
so no electrical conflict.
If yes, collisions (if any) will be protected by the resistor you will
place between both outputs.

>- Designing a double npn transistor stage with last trans. as "real" OC ?
>
>
>
If you want a "real" open collector this is the solution indeed. But a
lot of components: 3 resistors + 2 transistors.
This is only needed if you absolutly want to have a real low level
during collisions of 2 transmissions.
Some protocols need that low level.
You don't need it here...

>Please remember:
>If I use npn transistors, I will need 2 of them to get the correct logic on
>the JB remote inp.
>Atmel uC's are able to have outputs configured as OC, but not the pin that
>is configured as UART. The pins do have 2 possible configurations. If you
>use the one as UART TX, you don't have access to the out port configuration
>register to make it a OC.
>As i know, there is also no way to invert the UART logic.
>
>Many thanks again,
>
>that was clarifying some stuff I guess I once heard at the
>University.....long time ago ;-))
>
>
>
>

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