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Kai Klaas
11/06/05 07:02
  11/06/05 07:14

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#103336 - Solid ground plane solves your problems
Responding to: Prahlad J. Purohit's previous message
Prahlad said:
I am designing a 8051 based system. As shown in the attached file i need to route 21 port pins to a Common FRC type Ribbon cable connector. Now the problem is I remember to have read somewhere encircling the xtal with signals as shown in attached file couples noise into them. Is it true? if yes how should i route them to avoid problems.

As you have a mixed system in terms of using analog and digital circuitry, you need a bit more buffering and filtering, anyhow whether you route signals next to crystal signals or not.

Oscillator signals aren't so much different from other signals, only there are more togglings per time period resulting in some higher noise, when being capacitively coupled (injected) to other signals. But danger can also arise from standard signals, as they also show fast transition times of edges, like the oscillator signals.
The only remedy against injecting noise from signal to signal via stray capacitance is the rigid use of solid ground plane!! If you have a solid ground plane under the crystal, burden caps, oscillator signals and arround the crystal section on component side of PCB, and if all these grounds are connected to each other by the use of as many vias as possible, then you can route other signals in close distance to the oscillator. Use a minimum distance of 5mm and you won't have problems.
Using of solid ground plane results in an extremely efficient shielding of signals due to the elimination of stray capacitance: If there's a ground fill between signals and if the signals sit on a solid ground plane, then stray capacitance between signals is almost zero. This shielding effect is so powerful, that you would need to separate the signals by several centimeters to get the same low stray capacitance as with the use of ground planes, if at all. Yes, only the use of solid ground planes gives you a chance in succeeding when using modern fast digital chips!!! It cannot be focused on this issue too often! The solid ground plane is the most important thing with any application using modern digital chips.

Two other mechanisms are known which can cause trouble by oscillator noise, ground noise due to voltage drops along ground routing and magnetic fields due to loops formed by the signal traces. In both cases the use of solid ground plane will provide highest advantage again: There's nothing showing lower impedance (ohmic resistance and inductance) than a solid ground plane and there's nothing which results in smaller current loops and magnetic field emissions than the solid ground plane, which guarantees all the return currents to be running in as closest distance to associated signals as possible! So, you need not to worry about encircling oscillator signals, when choosing shortest connections with all the oscillator components and, yes, when using a solid ground plane and ground fill on component side of PCB!

A big disadvantage of quasibidirectional port ciruitry is that source impedance is in the 10...100kOhm range, when emitting logic high. Remember that the strong internal pull-up is only activated for two oscillator periodes when toggling from logic low to logic high. After this short period only the weak internal pull-up is turned on. This forbidds the driving of longer signal traces, especially if they leave the board. Why? The source impedance is much too high to effectievely absorb injected noise. Only a low source impedance can shunt injected noise to ground.
So, route the signals to suited 74HCMOS buffers, sitting not too far away from the micro (<10cm) and pull all the signals high by the help of 10kOhm pull-up-arrays.
Run each signal which leaves the board through a low pass filter, consisting of 470 Ohm and 47pF, or so. This filter provides a rise time of 50nsec and yields a corner frequency of about 7MHz. This helps to prevent digital noise from being coupled into the analog section.
You can additionally feed all the lines leaving the board through a soft ferrite ring core to minimize commom mode interference.

Don't forget, that ESD can be a problem with your application! So, connect the ground plane of your display- and keyboard to metal enclosure by the help of some 10nF capacitors, so that ESD hitting your keyboard will find a path directly to enclosure, not needing to flow via the cable to the micro's PCB! Using this soft ferrite additionally helps to keep the ESD out of the internal electronics.


List of 6 messages in thread
Signals encircling xtal.        Prahlad J. Purohit      11/03/05 08:38      
   the issue      Erik Malund      11/03/05 08:46      
      Trace lengths and buffering.      Prahlad J. Purohit      11/03/05 08:53      
         In that case I probably would not buffer      Erik Malund      11/03/05 09:07      
   Solid ground plane solves your problems        Kai Klaas      11/06/05 07:02      
      Thanks Kai.      Prahlad J. Purohit      11/08/05 23:39      

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