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

The current SX-Key is designed to get a 5 Volt power supply from the target system. With less than 5 Volts, it can happen that the on-board DC/DC converter for the programming voltage Vpp does not deliver the 12 Volts that are needed to program the chip. Also, when debugging, the SX-Key monitors the OSC2 pin of the target device for specific patterns. When the target runs at less than 5 Volts, the signal level on OSC2 may be too low to be read correctly which causes the Debugger to stop working.

"Officially", the SX-Key is specified for supply voltages of 5V only. Due to some component tolerances, it may work on lower voltages, if you are lucky. Parallax sold the SX-Ring which allows you to feed the SX-Key from an external 5V supply but I'm not sure if this part is still available.

You can build your own adapter on a little proto board with a four-pin header, a four-pin socket, and a barrel-type DC jack. Connect the OSC1, OSC2,  and Vss pins straight through from the header to the socket but leave the Vdd pins open. On the header that goes into the SX-Key socket, connect the Vdd pin with center pin on the DC jack, and the outer ring of the DC jack with the Vss pin, and you are ready to go.

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