Still sounds like the problem is entering ISP rather than the ISP itself. So you either go with Atmel and have an easy time initiating ISP but have a hard time performing it because you need special, sensitive cables, or you can go with UART-based options where there is the potential for problems initiating it, but once you've initiated ISP you're pretty much good to go since you're using a standard serial cable.

I personally don't like the idea of using the UART itself to initiate an ISP procedure. I don't think a break should initiate ISP nor do I think some special string of characters should initiate ISP. That just seems too dangerous to me in terms of potentially being able to accidentally enter ISP mode.

I still think the best approach is essentially how TI has done it: Hold PSEN low while resetting and the device powers up in ISP mode and you can perform ISP via the serial port. That way stray garbage on the serial line doesn't impact you but you have the reliability of the RS-232 line; plus if you already have an RS232 connection you can re-use that same circuitry for the purpose of ISP.

Let me ask this question: Does anyone know of any UART-based ISP problems with chips that use something besides the UART to enter ISP? I.e., TI uses PSEN, Dallas uses PSEN and EA. Has anyone heard of any ISP problems with these kinds of chips?

Regards,
Craig Steiner