On 6/1/06, 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). 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 > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist