"Are the Cygnal parts more susceptible to noise that other 8051 variants?"

If the question is: "Are they more susceptible than a 'standard', slow, 5V variant." the answer would be: Yes.
Being high speed, low(er) voltage devices, that should hardly be a surprise.

However, in my experience with Cygnal's controllers (that's three years now), I have not found them to be "very" susceptible. Several of my Cygnal based designs either control a GSM modem or work in close proximity to GSM 'phones and I did not have to take any 'dramatic' steps to prevent them going haywire from the RF noise.

I think that most noise related problems with Cygnal controllers can be related to sloppy PCB design. These things will not tolerate as much as a 'standard' 8051. A good solid ground plane and liberal power supply decoupling are essential.

One other thing that is important is the fact that Cygnal controllers are also able to generate plenty of noise. Their I/O pins can be configured as push/pull, which makes them fast; I have measured rise- and fall times of around 1 ns. This requires careful design and layout, especially when using an external bus. It also means that impedance control becomes neccesary with line lengths above 7cm. or so.

All in all, I think Cygnal controllers are a delight to work with and there shouldn't be any cause for concern about noise susceptability if you know what you are doing!

My € 2,-

Rob.