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If your external supply is between 1.71V - 1.95V, then you need to power up the device with the Vccd / Vcca pins, but teh internal regulators will remain powered by default. After power up, you should disable the regulators using register PWR-SYS.CR0.
More details can be found in the PSoC 3 / PSoC 5 Technical Reference Manual: http://www.cypress.com/?rID=35180