On one board I have a microcontroller with one power supply and on another I have a Flip Flop with a separate power supply. I want to connect a pin of the microcontroller to the reset pin of the flip flop. Can I just put a wire accross or does it need more than that? I was going to do that but now I'm not sure that would work because it wouldn't be a complete circuit and the two boards may not have quite the same ground levels. The two boards have to have separate power supplies because the board with the DSP is premade and the power comes from the mains. Thanks
Electronic – Communication between microcontroller and separately powered PCB
Consider this a theoretically biased answer - I've not dealt with multiple ADCs and a separate ground plane. This will (hopefully) not be your star answer but may raise some issues worth noting. Also - if any of this sounds like hogwash or ill advised (variations on the same theme :-) ) please say so (preferably gently) - leaving uncommented advice which you consider misleading reduces the worth of the material as a resource for others. .
What you have done sounds close to ideal. A second ground plane is a luxury not always available in "lesser" systems.
One may be tempted to partition the ground plane into N segments radially expanding from the single common ground point, but that has good and bad points.
Considering where and how you return the grounds of the signal sources can be an interesting exercise.
If possible you return the sources' grounds to the analog ground plane, but that then raises issues re sources which are powered but which do not themselves have separate power and analog grounds. How do you return the source power ground to the power ground plane and the source analog ground to the analog ground plane?
In the case of eg instrumentation amplifiers this may be easy as the analog ground is conceptually separate from the power ground.
In the case of single ended sources you may need to look closely at what happens to ground currents between power and analog. If the local power ground has a potential dc offset relative to analog ground you may wish to isolate this component from analog ground. To do this you may even go as far as providing an AC filtered DC feed to power ground for the sources analog portion and an AC ground path to the analog ground plane. This effectively creates a local analog ground for the source's circuitry - eg perhaps an inductor from power ground plane to local analog ground with a capacitor from local analog ground to analog ground plane.This sort of magic is liable to be needed only in extreme cases - it is to be hoped that in cases where DC components are large enough to matter that the device designers have accommodated it (as they have done with your dual gnd ADC's.
An example where this may not be the case is eg a microcontroller with internal DAC being used as a signal source for an ADC. For this arrangement to make sense (DAC-ADC) there will probably be some other analog function or convolved signal as well as the DAC output. In this case, how do you treat the microcontroller ground and what differences do the choices make.
Both ground planes will probably be interrupted by vias interconnecting other planes. In extremely demanding cases, which yours sounds like, care needs to be taken re unbalancing of go and return signal paths for critical analog signals. An analog signal track which crosses a break in it's analog ground plane creates a slot antenna which may be both a radiator and a receiver. In many cases the effect may be small enough to be neglected but you need to know that this is so by design and not by good (or bad) luck. Ground plane breaks also provide increased loop area which can be important in critical cases. (Loop area between go and return can occur in fully balanced cases when tracks are used for both paths - usually eliminated by proper groundplane use.)
If you choose CAN then the "best" micro is one with CAN built in (many have this, Coldfire MCF5225x is one example).
I2C or RS485/RS422 are good choices too, depending on the electrical characteristics of the situation (distance, noise, power). The rest comes down to the protocol you use for communicating - how to address messages, control flow, avoid collisions.
I'll wish you good luck finding a micro which will work at 180c, automotive spec tops out at 125c and I don't think even military spec improves drastically on that.
You need to start with layer 1 (electrical) to work out what the physical interface has to do, after that it's mostly a software issue on how you talk. These days, ethernet might even be a viable option if the micros have the space to run a modest OS.
A single wire may or may not work - check to see if your boards have a common-enough ground. You may get lucky and it will be just fine (since both boards will be connected to the mains ground, at least in some way). At the very least, you can probably run two wires - one to tie the two grounds together and another for the data. If for some reason you can't have a common ground, you can look into optoisolators.