Jan,

You are "dead on" as the quote from the web site repeats:

"Worlds Fastest Drop-In 8051 with In-Application Programmable Flash"

I like the DS'4x0 series a lot and have used quite a few of them in products.

Atmel has recently joined the single machine cycle fray with thier "LP" products (a 20MHz '2051 with mostly single cycle execution rate is a killer part).

Noticably, some of the design features of the DS'4x0 demonstrate a better ability as a "drop-in" upgrade as the clocks/timers retaining 12x divider unless you program them to be faster. This means SIO works "out of the box" and if you've kept time critical routines tied to these timers they perform the same (unless a faster code path has implication).

The ISP for the Atmel "LP" parts is not as forgiving as using the DS'4x0 3 wire serial connection (with bootloader) but is a fine alternative to poping out the part and re-flashing it.

The serial programming capability of the DS'420 makes things easy but the MTK program does have it's limits. If you use standard '51 crystal frequencies you don't have to "diddle" the PC uart timing (in the MTK app).

The MSC121x while not being as fast as the DS'4x0's (4x machine cycle) has a very good and frequency tolerant SIO programming capability and is a well concieved, feature rich analog platform. It is not a pin compatible processor, however.

regards,
p