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I'm in a process of designing a 2 layer PCB for my device. The device is a master control board, and there will be 10 daughter boards. Various peripherals and things will be connected there: I need 3V3 for all my STM32 micros, 12V for the MOSFET drivers, 5V for the relays, and also there will be another voltage from a programmable bench PSU. All of these voltages will have common grounds, except for the 5V for the relays, which are optocoupled. What is the best strategy when routing my PCB? Should I just place a big ground plane on the bottom and connect everything? Or perhaps a star topology?

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    \$\begingroup\$ Will your design contain any sensitive, small value voltages and / or high frequencies ( > 100 MHz) "? I guess not, the components you list are all "large signal", digital and low frequency. Then I would just make one ground plane to keep it simple. \$\endgroup\$ – Bimpelrekkie May 14 at 13:17
  • \$\begingroup\$ There's a large micro clocked @ 80MHz, other than that some UART streams, but these will be quite loq speed (<100kHz) \$\endgroup\$ – Marek Ant May 14 at 13:31
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    \$\begingroup\$ Even a micro at 80 MHz I consider "low frequency", but yeah, I now work on RF circuits at 26 GHz ;-). So the "one ground plane" advise still holds. Don't forget to add plenty of supply decoupling capacitors! Add a few extra here and there that will remain unpopulated (no capacitor on these yet). But if you need some extra caps later, you will be happy if the pads on the PCB are already there. \$\endgroup\$ – Bimpelrekkie May 14 at 13:50
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    \$\begingroup\$ We can assume that the product is something that has to pass EMC tests. Having 3 different regulators and a fine pitch MCU on 2 layers ain't gonna happen, it will have "antennas" all over the place. There's no reason to use 2 layers for this anyway, the cost argument is some 20 years obsolete. \$\endgroup\$ – Lundin May 14 at 13:55
  • \$\begingroup\$ Thank You for the answers! I never laid a 4 layer PCB out, so I guess it's the time. I'm was gonna use external power supply modules for everything, I have a couple of these nearby on the DIN rail nearby. Or would it make more sense to use just one supply, then add some local regulation on board? \$\endgroup\$ – Marek Ant May 14 at 20:24
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Should I just place a big ground plane on the bottom and connect everything? Or perhaps a star topology?

Big ground planes are best in most designs. They reduce inductance and resistance of return currents because there is more copper for the current to flow on. They also increase the amount of capacitance to other traces/planes which is good to knock down high frequencies by shunting high frequencies to ground.

A point where you would not want to use a solid ground plane is if you have large currents that could run on the ground plane through a sensitive area (like low level analog or if the voltage needs to be maintained to less than 100uV) because the resistance of the plane can create a voltage offset. In this case you could either move the components away from the return current on the ground plane, create a slot or split plane. Slots and split planes come with their own problems (can become radiators) so use them with caution.

If the design is purely digital, a solid ground plane would be recommended.

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