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Question: How should you connect the USB-Serial Bridge Controller (CY7C65211/CY7C65213/CY7C65215) in systems with multiple power supplies?
This article describes the hardware design guidelines for systems where the USB-Serial Bridge Controller has a different power supply than the rest of the system. It is recommended to use the system’s power supply to drive the VCCIO/VDDD* of the USB-Serial Bridge Controller. The VCCIO/VDDD input voltage powers the GPIO and determines the serial interface voltage levels. If the system power supply and VCCIO/VDDD are on two different power domains, the following design guidelines should be followed to avoid backpowering of the USB-Serial Bridge Controller.
Backpowering will happen when the power supply to the USB-Serial Bridge Controller is switched OFF and power to the rest of the system is still ON. In this scenario, the USB-Serial Bridge Controller will be backpowered from the input signals coming from the rest of the system. To avoid this scenario, a diode with a pull-up resistor should be connected at all input pins of the USB-Serial Bridge Controller.
Figure 1 shows a system with the Cypress USB-Serial Bridge Controller powered from a separate power supply.
V_Supply1 is the power supply to the USB-Serial Bridge Controller
V_Supply2 is the power supply to the rest of the system
To prevent any backpowering from the RX pin of the USB-Serial Bridge Controller, a diode with a pull-up resistor is connected at the RX pin of the USB-Serial Bridge Controller as shown in Figure 1.
* VCCIO for part number CY7C65213, and VDDD for part numbers CY7C65211 and CY7C65215
Figure 1. System Setup with a Diode Connected at the RX Pin of the USB-Serial Bridge Controller
This implementation will apply to all input pins of the USB-Serial Bridge Controller that are connected to the rest of the system.
The following are the recommended values of the resistor and diode: