Knowledge Base Article
EnCoRe III has its own internal 3.3V regulator to support the USB PHY when running off of supply voltages from 4.35 to 5.25. When used in this mode the RegEnable bit in the USBIO_CR1 register should be set to 1. When the chip is in sleep mode, such as for USB suspend, the 3.3V regulated output is shut down to conserve power. The normal D+ pull-up to 3.3V is therefore switched out and a suspend pull-up resistor of approximately 7K ohms to Vcc is switched in. It is also possible to conduct USB operations when running off of a regulated 3.3V supply that meets the USB specification requirements. In this case the internal 3.3V regulator is not used, and so the RegEnable bit in the USBIO_CR1 register should be set to 0. When the chip enters a low power state in this case the normal D+ pull-up resistor will still be used.