On 11/12/08, Marcel Birthelmer wrote: > Hi all, > for a project, I'm designing a small LED display that will end up on a > separate board due to physical restraints. It will be a multiplexed 4-digit > 7-segment clock display. The way I have it planned currently is to switch > the anode voltage to each (common-anode) digit with a p-channel mosfet, and > to connect each of the segment cathodes directly to a PIC pin. The segment > currents will be 20mA each, and less than 10 will be on at a given time, so > the electrical specs of the PIC will not be violeted there. Usually you should consider the malfunction situation too, when only one digit is light. > > However, my question is this: if I connect the relevant signals to the > display board via a small ribbon cable - let's say 3" long - where should I > optimally connect the VDD to this board? As I understand it, the current > will flow from VDD through the segments into the PIC and out the PIC's VSS, > so how do I keep that loop as small as possible? Also, where would I put > capacitors in this case to keep the switching currents from affecting other > things? In a logic think the VDD will come from the PIC board because is the point where probably you have the stabiliser. > > On the other hand, would there be an advantage to swallowing the cost of the > extra handful of MOSFETs I would need to switch each segment to a single > common ground? This is a hobby project, but I'm trying to keep an eye on > "professional" considerations here. At such small distance and so low frequency, will be OK in any situation you may connect it. However if you have analogic stuff, avoid crossing that area with display ribbon cable. A ribbon with a succesion of VDD-signal-GND-signal-GND etc will have a better noise imunity than signal-signal-signal-VDD-GND-GND-etc > > Thanks for your insights. > Regards, > - Marcel > -- > 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