Objective Development Forums

Re: Isolation?

2011-11-25T10:45:11+02:00

Micha wrote:
eslavko wrote:
Micha wrote:
What are R1/R2/D1/D2 for? You also have to consider that many/most optocouplers have an open collector output and therefore need another pullup.

That's are pullup as photocoupler are open collector. And diode asc as tristate switch when anode is low.

I will try that when I have little more spare time.

Statistics: Posted by eslavko — Fri Nov 25, 2011 10:45 am

Re: Isolation?

2011-11-25T08:18:40+02:00

eslavko wrote:
Micha wrote:You could add another (non-inverting) buffer that drives the LEDs of IC4. Or you connect R5 and R6 also to AVR_DIR and place a MOSFET which you drive with USB_D+/-.

R5 and R6 are already driven by AVR. Probably the USB bus can't sink or source enought current to drive LED directly so some buffer will be needed.

I know that they are already driven by the AVR. But you have an inverter (IC1) in there. If you connect the resistors directly to AVR_DIR you could use the inversion which is coming through driving the LEDs by a MOSFET. Something like: AVR_DIR -> R5.1/R5.2 -> LED.A/LED.C -> N-chan.D/N-chan.S -> USB-GND where N-chan.G is connected to USB_D+/- (.1/.2/.A/.C/.D/.S/.G are the corresponding pins of the components).

What are R1/R2/D1/D2 for? You also have to consider that many/most optocouplers have an open collector output and therefore need another pullup.

Statistics: Posted by Guest — Fri Nov 25, 2011 8:18 am

Re: Isolation?

2011-11-24T14:46:34+02:00

Micha wrote:You could add another (non-inverting) buffer that drives the LEDs of IC4. Or you connect R5 and R6 also to AVR_DIR and place a MOSFET which you drive with USB_D+/-.

R5 and R6 are already driven by AVR. Probably the USB bus can't sink or source enought current to drive LED directly so some buffer will be needed.

Statistics: Posted by eslavko — Thu Nov 24, 2011 2:46 pm

Re: Isolation?

2011-11-24T09:23:55+02:00

You could add another (non-inverting) buffer that drives the LEDs of IC4. Or you connect R5 and R6 also to AVR_DIR and place a MOSFET which you drive with USB_D+/-.

Statistics: Posted by Guest — Thu Nov 24, 2011 9:23 am

Re: Isolation?

2011-11-23T18:49:41+02:00

The pullup for device recongnision to D- is not drawn.
The pullups shown was enabled only when AVR is in transmitt mode. When receiving there are actualy pulldown but diodes make that like Tristated. I'm afraid that USB lines are to weak to drive optocouplers (AVR in receive mode)

Statistics: Posted by eslavko — Wed Nov 23, 2011 6:49 pm

Re: Isolation?

2011-11-23T17:33:23+02:00

Hmm, maybe this could work. Don't forget pullups on the AVR side (depending on the optos you choose). But I think enumeration will be a problem as a 1k5 pullup has to be either on D- for low speed OR on D+ for full speed devices. No idea what happens when there's a pullup on each signal.

Statistics: Posted by Guest — Wed Nov 23, 2011 5:33 pm

Re: Isolation?

2011-11-23T13:59:44+02:00

Something like this...

Power supply and Rx/Tx control isn't isolated here http://slavko.kocjancic.eu/vusb.pdf

Statistics: Posted by eslavko — Wed Nov 23, 2011 1:59 pm

Re: Isolation?

2011-11-23T13:09:58+02:00

That's true. You can invert the optocoupler outputs by adding an additional transistor and a resistor. Maybe you can create a draft of your suggestions and we can have a look on it and comment it.

Statistics: Posted by Guest — Wed Nov 23, 2011 1:09 pm

Re: Isolation?

2011-11-23T11:47:06+02:00

I think the isolating 2 wires should be easyer than 24.... just need to try.

Statistics: Posted by eslavko — Wed Nov 23, 2011 11:47 am

Re: Isolation?

2011-11-23T09:48:28+02:00

And what about isolating the AVR from CPLD/FPGA/whatsoever?

Statistics: Posted by Guest — Wed Nov 23, 2011 9:48 am