On 5/31/06, Vasile Surducan <piclist9@gmail.com> wrote:
> On 6/1/06, brusque@hotpop.com <brusque@hotpop.com> wrote:
> > Hello,
> >
> >     I'm working on a small board with a PIC18F4525-I/P and a RTL8019AS.
> > Right now I'm only trying to make the communication work. The connection
> > between the RTL and the PIC is identical to another board I have that is
> > working ok.
> >
> >     The fact is that it doesn't work. I'm noticed a very strange
> > behaviour that I would like someone to told me if this is right or
> > problematic. When the PIC sends data (PORTD) to RTL (SD0 to SD7) the
> > waveform is the spected "square" waves. But, when the RTL is sending
> > data to the PIC (PORTD pins are changed to input) there's slow risetimes
> > of 1.2us and very fast falltimes (7.0ns, measured with a digital scope).

One more thought, the rising edge could be a tri-state (high Z) behaviour.
Does it change if you're making changes in the firmware ?

>
> 1. how long is the bus between PIC and RTL ?
> 2. the bus is made on dual layer board with routes on one layer and a
> continuous groud plane on the other layer ?
>
> This behaviour is identical with a parasitic capacitance on the bus lines.
> It could be solved (if the boards are already manufactured) by trying
> an adapting resistor network near the RTL (possible near the PIC too).
> Usually a 3k3 to 10k network conected from the bus to VCC may help,
> but if the bus is very long or the RTL is supplied wrong will not help
> to much.
>
>
> >
> >     Is this behaviour normal? I noticed that the other board (the
> > working one) shows similar behaviour regarding the risetimes and falltimes.
> >
> >     If you would like to see the schematic of the communication part of
> > this board please take a look at
> > http://brusque.dyndns.org:8080/temp/net_board.png.
>
> You've used some magnetic perls on both VDD and VCC ? In my experience
> these helps on VDD but are making problems on GND.
> Try to change the ground point for your scope probe (ie near the GND
> of the RTL )and measure again. Use a comparative measurement with 1:1
> and 10:1 probe and see if any difference.
>
> greetings,
> Vasile
>
>
> >
> >     In fact there's not much thing besides communication, only some
> > 74HC595 latches and connectores.
> >
> >     Thank you very much,
> >
> >     Brusque
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>
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