FX3: Questions on Self-Powered Design – KBA233244
Question1: How do you power USB I/O when FX3 is used as a self-powered device?
Answer: The USB I/O requires a 3.3-V regulated power supply. This supply is internally driven from either the VBUS or VBATT external supplies. If the USB port is used, one or both supplies must be present. For an FX3 self-powered design, VBATT can be connected to the system battery or a stable 3.2 V–6 V voltage rail from the PMIC; the VBUS pin of FX3 can be connected to VBUS of the USB connector to detect the upstream USB connection. If VBUS and VBATT are both present and in their specified ranges, VBUS becomes the primary supply to the USB I/O; the FX3 device draws power from the VBUS supply by default unless there is a software/firmware override. FX3’s USB block can be configured to work off VBATT power instead of VBUS using the CyU3PUsbVBattEnable API. Please refer to the FX3 API Guide from the FX3 SDK for more details about the API.
Question2: What firmware changes are needed when FX3’s VBUS pin is not connected to the USB connector’s VBUS signal?
Answer: When the FX3 is functioning as a USB device, it performs VBUS detection by default and will connect to the host only when VBUS voltage is above 4.1 V. When FX3’s VBUS is left unconnected, the user is expected to enable/disable the USB connection in firmware at appropriate times. In this case, both the enable and useVbatt parameters of the CyU3PUsbControlVBusDetect API should be set to CyFalse, and the CyU3PConnectState API should be called directly by the user application. As mentioned in Question1, use the CyU3PUsbVBattEnable API to have the device draw power from VBATT instead.
Note: If FX3’s PMODE lines are configured for USB boot (Z11) and VBUS pin is not connected to the USB connector’s VBUS signal, the device will not enumerate. In this case, it is recommended to configure the PMODE lines for boot options other than USB boot.