| ??? 02/23/07 15:43 Modified: 02/23/07 15:56 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#133554 - Some hints... Responding to: ???'s previous message |
Hendryawan said:
Sorry I'm swear, its true. I already tell that I have been design four PCBs properly. After install into panel one of them was problem. Why only one? If all PCBs hung maybe my design is not correct. I believe you. Of course, chips are not always 100.00000%-ly identical. Nevertheless, your conlusion, that all prototypes must fail to prove that your design is wrong, is dangerously wrong. Why? Because you must always provide some headroom when designing a project. I tell you, the fact that even only one prototype failed is a prove that your design is wrong. You just must take into consideration, that chips are slightly different. Your project must work even with these eventually more sensitive chips. I have had a brief look at your homepage (link given in your user profile) and found a schematic of a different project. Let's have some disussion:  This plot shows a single layer board (!) containing some bridges. This layout looks very nice and you need only 4 bridges. But the mistake is, that this layout is only suited to handle low frequency analog signals, but not the fast digital signals of a microcontroller circuit! Your GND traces are much too narrow and long, which will result in extreme inductance. Also, your GND routing forms a closed loop arround the entire board, working like a transformer winding, sensing each magnetic field and adding noise on GND. This loop alone can be responsible for your noise sensitivity!! Open the loop somewhere! But much better is the use of a solid ground plane, which is nearly a must when working with todays micros. Have a look at this little demonstration layout (take note, vias have still to be added, which are omitted here for clarity): 
The signals and ground fill (top layer): 
The solid ground plane (bottom layer): 
Some enlarged detail of top layer:  It's only a double sided board, but noise immunity is incredibly better than with a single layer board!!! Take note, how much better the Vcc decoupling of micro is!!!!! The solid ground plane offers the least inductance ever possible!! Some more hints: Add decoupling caps directly to each chip. Add decoupling caps directly to each connector. Add decoupling caps directly to the regulator. Add an electrolytic of at least 47ยต/25V directly at the input of regulator. (No wonder, that the relay makes trouble.) Add some filter resistance or perhaps some soft ferrite choke where the supply voltage enters the board. This makes your circuit not only immune against HF noise coming from outer world, but also stops the noise leaving your board! Good luck, Kai |
