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PCB design

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sardineta

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The PIC16F870 has two Vss pins one of the them at the left and the other at the right.

The Vss pin near the OSC1 and OSC2 (where the Xtal goes; left) is wired with the Vss pin that is near the Vdd (right).

in other words all Vss pins from the uC are wired at a common GND


In a PCB design is it correct to wire down a component, like a resistor (thru hole)
 

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Yes all the Vss pins should be wired to a common ground. They put more then one Vss pin on some chips so the chip designer can have a good bypassed ground at multiple places in the chip.

In a PCB design is it correct to wire down a component, like a resistor (thru hole)

I think you are talking about putting wires under a resistor. Yes it shouldn't cause any problems in most cases.
 
And be sure that a PCB trace connects them. The worst mistake of all would by hooking up the battery's ground to one Vss, and then say an LED's ground to the other Vss without a trace connecting them. That's worse than leaving the second one unconnected.
 
Thank you very much!
I found it very interesting.


Another question:
In a PCB the analog ground is connected to the digital ground, isn't it?
and both of them are wired to the CENTRAL TAP of the power transformer, and finally at the earth ground and the chassis or case?

am I right?

thanks
 

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sardineta said:
Thank you very much!
I found it very interesting.


Another question:
In a PCB the analog ground is connected to the digital ground, isn't it?
and both of them are wired to the CENTRAL TAP of the power transformer, and finally at the earth ground and the chassis or case?

am I right?

thanks
It's generally the practice to separate analog and digital ground, and tie them up as close as possible to the power supply. Switching-induced noises in the digital ground may propagate to the analog side.
If your circuit frequencies are not too high and noise tolerances are acceptable in your circuit, then this requirement may not be that stringent.
 
checkmate thanks for your comments,
In fact the only high frequency the circuit has is the cristal oscillator at 4MHz.

The other digital signals are on/off at low frequencies from the uC to the base of transistor tu torn on/off relays and leds.

I have analog signals but The noise comes from the power line due to inductive loads (flourescent lamps)

A RFI filter may stop it when it is done by components inductors and capacitos isn't it? but more perfomance is get by using a RFI filter integrated (power filter)? or is the same
 
checkmate said:
sardineta said:
It's generally the practice to separate analog and digital ground, and tie them up as close as possible to the power supply. Switching-induced noises in the digital ground may propagate to the analog side.
If your circuit frequencies are not too high and noise tolerances are acceptable in your circuit, then this requirement may not be that stringent.
Well, designing a ground plane prevent from splitting the DGND and AGND.
Take a look at my last PCB (a DAC)
 

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It is interesting, but I have some troubles to understand it
I think the red color is the gnd plane, but what are the black lines,
and why they form a rectangle around the IC's
any comment
thanks
 
I've replied to your Email :wink:

Yes the red color is the ground plane located to the top layer. But there are as well some routes (for example linking C29 and C37 in this layer too.
Thus explaining why there are some "black" lines, this is a part of the layer where there is no solder, to avoid the be in contact with the ground (otherwise BOOOOM ! :lol: :shock: :wink: )


Regards
 
I have a question

Does the heat dissipator that the regulator 78xx and 79xx has
(package TO-220) is wired to the ground or it just float in the air

I do not know if this metallic part goes to ground

I have seen some PCB that has this component in the board

thanks
 

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I believe it can be wired either way. (floating or grounded). Most of the datasheets label it as a ground, so it's probably good to connect it to ground.

It is usually connected to a large "pour" on the circuit board to dissipate heat.
 
The metal back for the regulators are always shorted to the center pin as far as I know. For the 78xx, the center pin is ground, but I vaguely remember this is not the case for the 79xx. Anyway, it should be indicated on the datasheet. And yes, the hole on top of the packaging is used for mounting on a heatsink. If you have enough space on your PCB, you can bend the chip 90 degrees, such that both the back of the chip and the heatsink lies on the PCB.
 
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