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I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

FT232RNL Datasheet

https://ftdichip.com/wp-content/uploads/2024/12/DS_FT232RN.pdf

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!

EDIT

Changes to the schematic derived from the comments of Stefan Wyss and Justme

I added level shifters (SN74LVC1T45) on the RX and TX signals. They are doubling and should have no problem since in the datasheet I read:

This device is designed for glitch free power sequencing without any supply sequencing requirements such as ramp order or ramp rate.

In addition to this, I also added an LED for the presence of VCC_TTL. I used a transistor and VBUS.

Now VCCIO is derived from the 3V3 generated by FT232RNL itself.

Is this more in line with the datasheet?

I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

FT232RNL Datasheet

https://ftdichip.com/wp-content/uploads/2024/12/DS_FT232RN.pdf

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!

UPDATE: I’ve revised the schematic based on the feedback. New version posted here.

I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

FT232RNL Datasheet

https://ftdichip.com/wp-content/uploads/2024/12/DS_FT232RN.pdf

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!

I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

FT232RNL Datasheet

https://ftdichip.com/wp-content/uploads/2024/12/DS_FT232RN.pdf

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!

EDIT

Changes to the schematic derived from the comments of Stefan Wyss and Justme

I added level shifters (SN74LVC1T45) on the RX and TX signals. They are doubling and should have no problem since in the datasheet I read:

This device is designed for glitch free power sequencing without any supply sequencing requirements such as ramp order or ramp rate.

In addition to this, I also added an LED for the presence of VCC_TTL. I used a transistor and VBUS.

Now VCCIO is derived from the 3V3 generated by FT232RNL itself.

Is this more in line with the datasheet?

I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!

I’m seeking a full design review of my USB/UART interface project, which is based on the FT232RN chip. This project is intended as a didactic exercise, and while I’d appreciate feedback on all aspects of the design, I’d like to focus particularly on the following points.

Variable External Supply Voltage (VCCIO)

• The chip’s VCCIO supply voltage is provided externally and may vary between 1.8V and 5V.
• I’m not entirely sure if the datasheet’s examples, which typically assume VCCIO values of 5V or 3.3V, adequately address cases where VCCIO may be as low as 1.8V.
• Given the lack of a specific reference design for variable VCCIO scenarios in the datasheet—even though the electrical characteristics include VCCIO = 1.8V

TX/RX LED Reference on VBUS

• The datasheet example shows TX/RX LEDs being driven by a digital one-shot that switches the CBUS pins from a tri-state to a low state, with the LEDs referenced to VBUS (typically 5V). This allows me to size the resistor in series with the led without worrying about reference variation.
• I question whether referencing the LEDs to VBUS is the best choice when VCCIO might be lower (e.g., 1.8V or 3.3V) from a functional standpoint.
• I wonder if this design decision might introduce any drawbacks in terms of consistent LED functionality, and whether additional precautions, such as precise resistor sizing, are needed to ensure reliable operation.

Schematics

FT232RNL Datasheet

https://ftdichip.com/wp-content/uploads/2024/12/DS_FT232RN.pdf

I appreciate any insights, suggestions, or potential drawbacks you might identify regarding the overall design. Thank you in advance for your time and expertise!