freeze dspic brushless drive

[mahdi70]

tnx alot my friend ..temtronic..newguy..Ttelmah

Today I did an experiment. To be able to understand EMI and the effect it has on circuit, I use aluminum foil between 2 pcbs and see interesting result. I start the motor with the first pic and the other pic does not freeze or reset and work properly...aluminum foil connected to GND.





But there is a problem. The first pic never resets or freezes with or without alum foil. When i start the motor with the second pic and alum foil, at up to 80% of power, the second, third and the fourth pic are reseting continuously or freeze, but the first pic works very well. Why the first micro not involved with EMI noise?

tnx alot

[Ttelmah]

Well, you have demonstrated that the problem definitely is EMI. However, there are lots of other things that come into play. Each of the chips has a different actual layout, so 'one' behaving differently to the others, could just simply be a difference in the layout to this one.
This is why an earlier poster asked what happened if you switched 'which' chip you used as the 'first'?.

Other things also come into play:

First, Temtronic has already mentioned capacitance. Generally on anything like this there wants to be a capacitor with good high frequency performance immediately adjacent to every chip that is switching at any speed (includes the PIC's themselves), and particularly if power is involved. Take a look at a PC motherboard. You will find that on a typical board, there are probably perhaps 200+ capacitors!. Perhaps 50 in the PSU, and then ones adjacent to perhaps every second chip across the board.

Then there is the physical layout of the tracks. I like to think of the electricity, like currents in the sea, or a river. Go and look at a river sometime, and notice how behind the legs of a bridge, you get eddies. Similarly how on a tight bend, the current flows fast on the inside, but there is hardly any flow on the outside (These are your sharp corners). Then look at a place with an island. Notice how objects floating can even sometimes go 'upstream' at the downstream end of an island (currents flowing backwards....), and how the rates on the two sides of the island can be different, even if the channels are very similar. Then look at the turbulence, where the two streams come back together. These are all the current effects that will cause problems. Also note how an object sent down one side of the island, may well get further in a given time than an object on the other. Now think what would happen if there was a channel 'across' the island. Water would start to flow from one side of the island to the other. This is what happens if you have multiple circuits, and their ground connections go back to different points in the PCB (often called a 'ground loop'). If your chips all have control signals coming in to set the speed, they need to have their grounds all referenced back to the same point, or there can be unwanted current flows....
So, if your 'first' chip is by it's connection defining the point in the circuit where the signal ground actually connects, then it will behave differently to the others....

[mahdi70]

[Ttelmah]

Good.

One other thing often not realised, is just how high the instantaneous currents can be on things that are switching. Even the little PIC may only draw a few mA 'average', but at the actual moment when a line changes, can easily have a few nSec, drawing dozens of times the average current figures. Obviously things like PWM drives get even worse here.
I think one of the earlier posters mentioned using an AM radio, to do some basic listening for radiated EMI, and this is well worth doing. You can learn quite a lot from what this gives!...

[mahdi70]