OPTOUSB

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Tindie-mediums.png

OptoUSB-177-720px.jpg

Features

  • USB to TTL level Serial (FTDI) Interface
    • Genuine FTDI FT230XS part
    • Standard FTDI drivers
  • Buffered lines on the TTL level side
    • Schmidt Trigger
      • Voltage level hysteresis
      • Improved slew rate
  • OptoIsolated Connections
    • Transmit, Receive, RTS, CTS
    • Configured for High Speed Serial
  • The Host USB connection is independent of the state of the FTDI side of the interface
    • Host side can communicate with the USB host whether or not the target is even powered up
    • Target power on/off doesn't cause a USB disconnect - handy for debug
  • Fast operation
  • 3.3V or 5V FTDI voltage (order option)
  • Transmit/Receive LED Connectors
    • On-board LED resistors
    • 49x49mm card
    • (4) #4 Mounting Holes

Block Diagram

OPTOUSB-BlockDiagram.png

Separate Power Domains

  • The OPTOUSB card has two distinct power domains.
    • One of the domains is the USB side where the power comes from the USB connector/cable.
    • The other power domain is on the TTL side where the power comes from the FTDI connector.
  • This is unlike "normal" FTDI interfaces where the USB connection can power the FTDI side.
    • This is the purpose of the optical isolation - to isolate the ground of the USB side from the target (FTDI) side.

Connectors

J1 - Mini USB Connector

  • Standard USB Mini connector

J2 - Transmit/Receive LED connections

  1. PUP0 - LED Pullup to USB Power
  2. TXLED - Transmit LED
  3. PUP1 - LED Pullup to USB Power
  4. RXLED - Receive LED

J3 - FTDI Connector

  • Signals are references to external system
  1. GND
  2. RTS (In to card)
  3. VCC (3.3V or 5V) - Power must be supplied to this pin
  4. Receive (Out from card)
  5. Transmit (In to card)
  6. CTS (Out from card)

Drivers

  • [ FTDI Drivers]

Layout

OPTOUSB-CAD-X1.PNG

Transfer Rate Characterization

  • LTSpice Simulation results
  • 8.6uS per bit = 115,200 baud

OPTO-USB LTSpice Simulation.png

Opto-Isolator Drive Circuits

  • The card has two independent, isolated power domains
    • The USB side is powered from the USB 5V
    • The FTDI side is powered from external power
      • Connected via header or backfeeding the FTDI connector with voltage
      • Voltage is one of either 3.3V or 5V
  • 4N25 Optoisolator used between the two sides
    • Optoisolator has Vf = 1.3V typical
    • Optoisolator has 20% CTR (min)
  • Optoisolator can be fed with up to 50 mA of drive current
    • Set nominal LED drive current at half (25 mA)

USB to FTDI Path

USB TO TTL PATH.PNG

USB Drive Side

  • USB side has MMBT3904 (2N3904) transistor for driving the optoisolator
    • Vce(Sat) at 10 mA is 0.2V
    • Vce(Sat) at 50 mA is 0.3V
  • 5V drops are across transistor (0.3V), optoisolator (1.3V) and series resistor
    • Opto voltage is 5 - 0.3 - 1.3 = 3.4V
    • Opto drive current needs to be 5x the output current (due to the 20% CTR)
    • Assume output current (5 mA)
    • Optoisolator LED drive current needs to be 25 mA
    • 3.4V / .025A = 136 Ohms (Use 150 Ohms for drive series resistor)
      • 150 Ohms will be 22.6 mA drive (good compromise drive current value)
      • 22.6 mA drive current at 20% CTR will allow collector current to be 4.5 mA
  • Transistor base resistor
    • HFE(min)=100 @Ice=10mA, HFE(min) = 60 @Ice=50mA
      • Use HFE=60
    • 4.7V across resistor, HFE=60, Ice =25mA = 11.3K Ohm resistor
    • Drive with 5V, 4.7V across 10K is 0.47 mA
    • 0.47mA * 60 = 28.2mA (>25 mA)

FTDI-Side Collector Pull-up Resistor

  • 25 mA into opto
  • CTR(min) = 20%
  • ~5mA output current

4N25 Collector resistor value cross over chart.PNG

  • Optimal resistor value is at cross-over point where the H-L and L-H prop delays trade-off
    • ~1.2K
5V FTDI side Collector Pull-up Resistor
  • For 5V, that's about 1.2K
    • 5V at 1.2K ohms is 4.1 mA
      • 4.5 mA collector current (from above)
  • Use 1.2K pull-up from above
3.3V FTDI side Collector Pull-up Resistor
  • 4N25 is not characterized for 3.3V operation in the chart but assume the same current
  • Vce(sad) = 0.3V
  • 3.3-0.3 = 3V
  • 3V / 0.0041A = 731 ohms (680 or 750 are standard 5% values that are near)
  • Use 820 ohm pull-up

FTDI to USB Path

  • Drive FTDI side from 74AC14
    • 74AC14 outputs 24 mA Iol or Ioh

TTL TO USB PATH.PNG

5V Drive from FTDI Side

  • 150 Ohm series resistor
    • 5V-1.3V = 3.7V
    • 3.7V / 150 Ohms = 24mA
  • Could drop to 180 Ohms for better margin or double up with 2 drivers/series 300 Ohm resistors in future re-design

USB Side Collector Resistor

  • Same as above 5V Collector resistor calculation, 1.2K ohms

3.3V Drive from FTDI Side

  • 82 Ohm series resistor
    • 3.3V-1.3V = 2.0V
    • 2.0V / 82 Ohms = 24mA
  • Could drop to 100 Ohms for better margin or double up with 2 drivers/series 180 Ohm resistors in future re-design

Optoisolator Speed-ups

  • Optoisolators have asymmetric propagation delays Tplh vs Tphl on their outputs

4N25 Collector resistor value cross over chart.PNG

  • At Collector Load Value of 1.2K Ohms
    • Tplh = 20 uS
    • Tphl = 1.5 uS
  • This effect limits signal fidelity
  • Ideal would be to have matching Tplh and Tphl values
  • These can be balanced by adding a resistor from the output side transistor base to ground
  • Hard to find "right" value empirically but checking with a scope it's possible to find a good value
    • 18 K Ohms for 5V operation
    • 15 K Ohms for 3.3V operation
    • Top speed is more than 57.6 kbaud with 4N25M optoisolator
  • Note: Testing in loopback mode doubles this problem

Optoisolator Improvements

  • 4N25M has a low CTR of 20%
    • 4N27 is even worse with CTR of 10% - unusable
  • CNY17-4X006 has a much better CTR of 160%-320%
  • Tested and works reliably up to 200 kbaud with 18K base resistor (above)
  • Could also reduce base current
  • Might be able to get FTDI side to run at 3.3V or 5V

Factory Acceptance Test

OPTOUSB P18131 720PXV.jpg

  • Uses dedicated test fixture
    • Built on GRID49 card
    • Card marked OPTOUSB
  • Test fixture features
    • 3.3V or 5V power to OptoUSB FTDI TTL level interface
    • Tx/Rx LEDs
    • Loopback select jumpers

Loopback Select Switches

OPTOUSB P18131 Ann 720PXV.jpg

  • TX/RX loopback Switch
    • L = Open (Rx pulled up to Vcc)
    • R = Connect TX (out) > RX (in)
  • Handshake level Switch
    • L = Active handshake
    • R = Off (Vcc)
  • Handshake Switch
    • L = Use Center handshake Switch value
    • R = Loopback CTS > RTS

Hardware Setup

  • Test "Fixture"

OPTOUSB P18128 720PX.jpg

  • Unit Under Test (UUT)
  • Power Supply
    • 5V or 3.3V Power Supply (card build option)
    • Cabled to 5V Terminal block
  • FTDI cable
    • From UUT to fixture FTDI jack
    • Left side has Black wire
  • LEDs cable
  • USB Mini cable to PC

Test card

  • Run puTTY on PC
    • Find port with Device Manager
    • Baud rate = 115,200 baud
    • Use Hardware handshake RTS/CTS
  • Switch settings - Normal operation
    • Left switch in right position (loopback data)
    • Middle switch in left position (handshake = active/ground)
    • Right switch in right position (loopback handshake)
    • Type on keyboard and verify it returns data
    • Both Green and Blue LEDs should blink
  • Switch settings - Test Tx/Rx isolation
    • Move left switch to left position
    • Type data nothing should come back
    • Verify Blue LED only blinks
    • Move left switch to right position
  • Switch settings - Test RTS/CTS
    • Move right switch to left position (handshake controlled by middle jumper)
    • Type and characters should return
    • Move middle switch to right position (hardware handshake off)
    • Type and nothing should go through
    • Move middle switch to left position (hardware handshake on)
    • Characters that were buffered should come in at once

Revision History

Various incarnations of the card

Rev 5

  • Adds test point pads
    • Useful for setting TX/RTS Optocoupler output Base resistor values
    • Tune for equal low/high pulse widths
  • Cleaned up ref des silkscreen locations, increased sizes

OPTOUSB REV5 TESTPTS.PNG

Rev 4

  • Did not build any

Rev X3

  • Fixed silkscreen sizes and relocated
  • Adds V/G pads on each side
  • Add Rx-, TX+ LEDs J2 marking
  • Bigger pads for hand soldering discretes

OPTOUSB REVX3 CAD.PNG

Rework - Rev X1 PWBs

OPTOUSB REVX1 CAD.PNG

Rework-U1-3.PNG

Assembly Sheet