Ground and Ground plane on 2 layer board

Hi everybody,
I am doing a design of 2 different boards and I am struggling about the
use (or abuse?) of the Ground Plane.
Both the boards have a SMPS (LM2575) that generate 3.3VDC from an input
of 12/24 V AC/DC. The CPU on the 2 boards are 8 bit microcontrollers
(PIC18) , one running at 20 Mhz and the other at 4Mhz. No signal on
both the boards is running faster than these frequencies.
Both the boards have a SPI EEPROM, an RTC and a RS485 transceiver. The
20Mhz board has 4 relay outputs and 4 optoisolated inputs, whilst the 4
Mhz board has only 1 relay output.
The boards are both routed 95% on the top layer, I use the bottom only
for VCC and GND and for few lines that I wasn't able tofit on the top
layer.

My question is: how to use the ground plane? I've read plenty of
threads about it, but I didn't fully understand which is the best way
to do. My doubts are mainly about:

1) GND of the SMPS: the SMPS is located in a definite area of the PCB,
so I made a GND plane "all for it", and connected to the rest of GND
plane in a single point witha thick trace. Is it correct or is it
better to make a big plane for all the pcb?
2) GND of the relays: I made a GND line only for relays , and connected
it with the GND plane only in one point. Correct or not? (in the relay
area there is no GND plane at all, top and bottom layers)
3) grounding of the RTC (DS1302): should I take particular precautions
about the GND plane surrounding the RTC and its crystal? (also about
the 20Mhz crystal)
4) FUll or grid gnd plane? For my pcb manifacturer it's the same, so I
have to choose...

more generally speaking, not having RF stages on my boards, once
finished the routing I filled in almost all the unused PCB area with a
GND plane, both top and bottom layer. Of course I was checking that I
had as few interruptions as possible, specially on the bottom layer,
even if I need to have some (for Vcc and for the 5% of routing descrbed
above)
But is this a correct approach or I am making everything wrong? Should
I use only the bottom layer for the GND plane? or is ok to fill
everything fillable? :-)

I know that using a 4 layer board would make everything easier, but I
have to keep my design in a 2 layer... no way to change this!

I think that's all, sorry for the lenght of this thread but I wanted to
put out all my doubts, so that maybe it can be useful also for other
people.

Thanks since now to everybody who can help, all the best,

Liuc

.

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    ... I am doing a design of 2 different boards and I am struggling about the ... The boards are both routed 95% on the top layer, ... how to use the ground plane? ... so I made a GND plane "all for it", and connected to the rest of GND ...
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