Peter,

I think the reason no one has designed your better onchip watchdog timer is because its considered only to be insurance for some lockup conditions. Most people are happy with having some insurance and few seem to worry about the details and quality of their insurance. So the quick answer to your question is probably that the chip manufacturers think what they have is good enough for most buyers.

I decided to avoid the external watchdog when using FLASH. There are ways to disable it but generally not worth the results. The MAX812 buys us some more insurance and I'll use th internal Watchdog in my 8051. I'm not a fan of software watchdogs.

Our ISP hookup is designed an a minimal cable connection. I only want a technician to power up the board and plug in the cable (or press-fit PCB probe connections in a fixture) from a PC station to change the FLASH program. Moving and removing header jumpers increase the probabilities for mistakes in production.

We change our ISP vector before we ship so that no one can reprogram the unit without our protocol locks. And we blow all the security fuses to protect the code.

-Jay C. Box