I had a problem with the AT89S8253 a while back. Since Atmel has changed the specs on the clock oscillator loading (see the January data sheet), you can really only use up to about a 16MHz crystal externally and have it start up correctly every time. This is assuming that you still want to correctly load the crystal with the correct value burden caps to get within the typical +/- 30ppm rating. for a 20pF CL that translates to 30pF and for 18pF crystals that translates to about 26pF (really 27pF as 26 is not a real world value). I run mine at 12MHz X2 to get the 24MHz performance. BTW it will run on a 24 MHz external clock at X2, but I don't know how reliable that is. I agree with Jan and Kai on this. Probably safer to limit it to 24 MHz total. I definetly wouldn't design a commercial product with higher than a 12 MHz clock running X2. Atmel sure are vague with the details sometimes. I have an FAE looking into this for me but YES they are SLOW (even slower than Government workers ;-).

I haven't used the S2051 or S2052. Checkout the new AT89LP4052, it's a screamer! Single cycle core! I've been having lot's of fun with it. PWM's work great too. I only wish it had 1 more timer,as it takes a timer per PWM port. I was looking at this for microstepping stepper motors...

Trev