OPTOUSB

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

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

Transfer Rate Characterization

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

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 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

• 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

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

• 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

• 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

• 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"

• 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

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

Rework - Rev X1 PWBs

Assembly Sheet

• OPTOUSB Assembly Sheet