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After making this PCB, I learnt the importance of ground plane. Besides ground plane, there is considerable amount of theory behind PCB designing which can provide immunity against ESD, surge, EMI, RFI etc.

After reading some articles and following the suggestions on my previous post, I am trying to make another PCB.

I haven't routed whole board yet because I am not very sure about the robustness of this layout against EMI, RFI, ESD etc. Since this takes experience, I need suggestions from you.

Here is what I am planning to do next:

1) Route most of the PCB on top layer so that I can provide a huge GND pour on the bottom layer in DC zone. (No GND pour in ac zone)

2) Route the remaining board, making sure that decoupling capacitors are as close to micro-controller as possible.

3) Use smd caps and resistors so that I can provide a bigger and continuous ground plane.

4) Keep current loops smaller so as to reduce internal EMI development.

5) Provide ferrite bead at incoming ac power lines to suppress sparks.

Besides this, what other improvements do you suggest for increased stability of this circuit? Any constructive suggestions from your side will be very appreciable.

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  • \$\begingroup\$ "Currently there is a 32.768kHz crystal. Is it a bad idea?" I would believe so, in both directions. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 20 '15 at 23:35
  • \$\begingroup\$ Thanks @Ignacio. I will try to move it away from beneath the RF antenna. BTW crystal is in metal casing and it's body has been grounded. \$\endgroup\$ – Whiskeyjack Apr 20 '15 at 23:38
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    \$\begingroup\$ With that slot, you're going to have PCB fragility issues. Maybe stick a few tabs across in places where creepage isn't a concern. \$\endgroup\$ – Connor Wolf Apr 20 '15 at 23:52
  • \$\begingroup\$ can you please provide a close up of the ATMEGA328P circuits, and the reset line? \$\endgroup\$ – KyranF Apr 20 '15 at 23:57
  • \$\begingroup\$ @KyranF - Here is one. :) \$\endgroup\$ – Whiskeyjack Apr 21 '15 at 0:03
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In the AVR Appnote on hardware design considerations on page 5:

To protect the RESET line further from noise, it is an advantage to connect a capacitor from the RESET pin to ground. This is not directly required since the AVR internally have a low-pass filter to eliminate spikes and noise that could cause reset. Applying an extra capacitor is thus an additional protection. However, note that this capacitor cannot be present if DebugWIRE or PDI is used. If not using High Voltage Programming it is recommended to add an ESD protecting diode from RESET to Vcc, since this is not internally provided due to High Voltage Programming. Alternatively, or in addition, a Zener diode can be used to limit the RESET voltage relative to GND. The Zener diode is highly recommended in noisy environments. The components should be located physically close to the RESET pin of the AVR. Figure 2-1 shows the recommended circuit on the RESET line.

I suggest you try an external capacitor of 0.1uF to ground, and a schottky diode to from RESET to VCC to act as an ESD clamp. Add a 5.1V Zener from RESET to GND, which comes highly recommended in the manufacturer's text.

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