In SX Microcontrollers, SX/B Compiler and SX-Key Tool, whelm wrote:

1.  Actually the SX-Key is quite picky.  I have an application with a multiple output SMPS and the CPU power is on an output that is not directly regulated.  It is fine unless the output with the feedback has no load, in which case the CPU power drops to 4.5 to 4.7 V, which is fine for operation, but WON'T program.  The SX-Key fails unless I insure there is some load on the other output so the CPU gets closer to +5, which is a pain since a reset SX default state removes all the loads on the other supply.  Of course, since the SX-Key draws power from the CPU supply, that makes the effect even more pronounced.

2.  Powering an SX-Key separately is a bad idea, because the output voltages it provides are going to be based on that 5V.  If the SX is like most ICs, at least one of the pins in question has a diode to Vcc that is normally reversed biased, but becomes conductive if the pin goes more than .7 volt above Vcc.  This would pull the application up to > 4 V and probably burn out the diode and associated bond wire in the process because they aren't designed to sustain that much current.  Destructive latchup is another possibility!

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