Does anyone know of an ARM Cortex-M3 microcontroller with both the 3.3V peripheral and 1.8V core voltages supplied by external sources? Everything I have found so far has an internal regulator for the 1.8V CPU core. My problem is that I'm working in a high temperature environment and the internal regulators all have a thermal shutdown circuit to protect the chip which shuts the 1.8V off at 125C. I would like to supply the 1.8V with my own extended thermal protection that will run up to 200C but have not been able to find a microcontroller that will accept one.
Microchip has MCUs that are qualified for 150C operation.
The maximum operating temperature of the cortex M3 chips I've looked at is 125C - which is right about where you found the thermal shutdown protection.
You'll have to call the manufacturer for use beyond those conditions, but I expect that you won't get the answer you need. You may need to locate the computing elements away from the heat, or use some active cooling solution because there are very few microcontrollers that operate above 125C.
If you want to play with pushing them beyond your limits, though, you're right, you need one that allows an external regulator. I believe both the PIC32 and AVR32 have internal regulators, but they are on accessible external pins and may be bypassed with your own power supply. They aren't ARMs, though, so if that's a core requirement then you may simply need to contact a manufacturer.