EfiMini Opto-Coupled 3.3 V Serial

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Optically Isolated 3.3 V Serial to USB Adapter

Since the digital supply voltage for the CPU is 3.3 V we designed a generic optically isolated Serial to USB dongle for interfacing to the 3.3 V com interfaces. Reason for it being that it is very easy to damage the CPU and / or the serial dongle without electrical isolation between the on board digital signals and the outside communication signals. Normally there are basically no protective layers umong these 3.3 V connections. After we blew a standard commercially available 3.3 V to USB dongle and didn't really know why, we decided to design our own with opto-coupled signal paths to provide the necessary protection.

The choice of the Opto-Coupler being used limits the top transmission speed to 57.6 kBd.

There is also a page available dedicated to the Design Verification of the opto circuit.

efiMini Connections :

Besides the USB connection, the efiMini uses 2 serial ports for additional communication.
One channel is used for the firmware program flash load, while the other one is used for diagnostic purpose.
The pgm flash download is the one which is necessary for flash based firmware upgrades. The diagnostic interface is handy for the tech support engineer, but of less importance for the normal user.

While there are lots of serial to USB adapters available, they can't be used for the efiMini, cause the standard RS232 interface specs are up to +12 V to -12V. But the efiMini uses on board digital supply and signal voltages of 3.3 V.

The heart of the USB to serial conversion is the FT232RL from FTDI. One advantage is that this chip is well established and all necessary drivers are either already implemented in your Windows installation or easily available directly from FTDI.

As earlier mentioned, this is a generic design and provides the additional functionality not used by the efiMini.

  • 3.3 V supply voltage.
  • 5.0 V supply voltage.
  • operation in non isolated mode.

Opto Isolated Serial Adapter Block Diagram

EfiMini Usb Serial Dwg.png


There are several config options for setting up the 3.3 V / 5.0 V Opto-Coupled Serial to USB adapter.

Jumper J4 and J5 always need to be set / removed in pairs and provide the selection for a 3.3 V / 5.0 V serial interface config.

Jumper J6 connects the USB interface ground to serial interface ground.

Jumper J3a selects 5.0 V to be connected to the Serial interfaces Vref input, while J3b selects the 3.3 V supply option. This selection mandates the ground jumper J6 to be set.

USB to Serial Config J4, J5 J3a J3b J6
efiMini on off off off
isolated 3.3 V on off off off
isolated 5.0 V off off off off
ground only 3.3 V on off off on
ground only 5.0 V off off off on
ground, supply 3.3 V on off on on
ground, supply 5.0 V off on off on

J2 also carries a 5.0 V and 3.3 V supply pin. The total power available can not exceed 450 mA.

Opto Isolated Serial Adapter Schematic

EfiMini Usb Serial.png


The overall opto board has been used in a slightly simplified version for years. The current design has been tested and R1 - R6 are re-evaluated since the opto from the previous design was changed.

Note : either R7 (default) or R8 should be stuffed, but not both.

Opto Isolated Serial Adapter Board

EfiMini Usb Board.gif


The pict above shows the board and it's configuration and connectivity

Opto Isolated Serial Adapter Design Verification

EfiMini Usb Design.gif


Click on Design Verification to get the details about the board design.

Opto Isolated Serial Adapter Layout


EfiMini Usb Serial Layout.png


For the one interested about how a dual layer pcb layout looks like, we provide a pict of it above.

The total size of the 3.3 V Optically Isolated USB to Serial is 43mm x 22mm (1.7" x 0.85").


Note : All published design documentation is Copyright efiLabs.com and its use is without exception FOR NON COMMERCIAL PURPOSE ONLY. For commercial licenses contact efilabs.com ... try us, we're reasonable :)

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