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I'm trying to create a device that will control a few independent RS485 lines using single USB port.

My first idea was to use GL850G usb hub controller (4 downstream ports), 4x FT232R, 4x MAX485. I had to provide 12MHz clock source for both hub controller and FT232R.

  1. Should all those devices use single clock source? Why? Why not?

I started my design with common universal clock generator (sg-51 at 12MHz), but FT232R requires 5V level on clock input while GL850G requires 3.3V

  1. So maybe there is a way to shift 5V clock source to 3.3V?

  2. If not - should I:

    • use crystal for GL850G and common clock generator for FT232R
    • use five independent crystal oscilators
    • use crystal for GL850G and internal oscilator for FT232R
  3. What are disadvantages of using internal oscilator?

If stability not cost is the key - how should I do it?

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  • \$\begingroup\$ There are plenty of off-the shelf products. Why not consider them? \$\endgroup\$ – Eugene Sh. Sep 1 '15 at 19:36
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    \$\begingroup\$ why not use the FT230X instead of the FT232. The FT230X has a builting oscillator and can be used for the RS-485 applications as shown in the datasheet \$\endgroup\$ – Kvegaoro Sep 1 '15 at 19:52
  • \$\begingroup\$ @EugeneSh. this is more complex device, usb to rs485 will be only a part of it \$\endgroup\$ – peku33 Sep 1 '15 at 20:00
  • \$\begingroup\$ @Kvegaoro +1 for FT230X. Can I safely use usb hub with 4 client devices without common clock source? \$\endgroup\$ – peku33 Sep 1 '15 at 20:47
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    \$\begingroup\$ I think so because each USB packet has a sync field to synchronize clocks, see beyondlogic.org/usbnutshell/usb3.shtml#USBProtocols Also, each device on the bus gets assigned an address so they would they can share the bus. I also in the past from the usb hub I have ran 4 independent usb-rs485 converters to do load testing to a device meant to be a rs485 network switch. \$\endgroup\$ – Kvegaoro Sep 1 '15 at 21:27
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A simpler option than four FT232Rs and a hub controller would be a single FT4232H.

The FT232H family supports a superset of the FT232R's features; it is frequently used as a JTAG controller. In this application, though, what matters is that the FT4232H can be configured to provide four UARTs. This allows you to do away with the hub controller entirely, along with all of its consequent added complexity.

And to combine some things further, you might be better off using a pair of MAX3030E and MAX3096 RS485 quad transmitters and receivers than using four MAX485s.

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FT232R requires 5V level on clock input

This is not true. It requires VCC rail > 4.0V to use its internal oscillator --- see sections 5.2, "DC Characteristics" and 7.6, "Using the External Oscillator" in the datasheet. To use an external oscillator/generator, you need VCC < 4.0V. VCC = 3.3V ois good. Therefore you have no problem here to feed up all the ICs by a single 3.3V clock.

Should all those devices use single clock source? Why? Why not?

If stability not cost is the key - how should I do it?

You should use so called preferred conditions. The preferred conditions for FT232R is to use its internal oscillator. If you want an external generator to be used, try to use another version of FT232-, not -X or -R.

What are disadvantages of using internal oscilator?

In your application, imo, no one exists.

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