How to isolate I2C

[Popolvár]

Hi,

I am working on new preamp. It consists of more separate modules (MCU + RC5 decoder, volume, input switch, output buffer). I would like to separate all other modules from MCU module by optocouplers. For volume control I plan to try CS3310/PGA2310 or serial chain of binary Pi or Tee attenuators. CS3310/PGA2310 are driven through serial interface that can be isolated simply by optocouplers. I would like to drive attenuator relays through PCF8574 (I2C bus expander) and driver transistors. But I2C is bi-directional - slave device send one acknowledge bit. Bi-directional opto-isolation is not so simple and it could save me some space on the board if I could just ignore this acknowledge bit and connect I2C data wire simply through optocoupler from MCU to PCF8574. Is it correct - may I do it so?

Thanks,

Daniel

 

[guido]

Yes, you can ignore the ack bit. If you are the only busmaster with the micro it is no problem (get some opto's with open collector out, otherwise the devices get into problems bringing the line down (or was the ack up, cant remember...).

I guess you don't have a i2c sender in your preamp, only receivers... One better option might be to isolate the display and put the micro into sleepmode. That's what i did, check www below.

Did not use a display, since it generates noise...

Greetings,
Guido

 

[Popolvár]

Hi Guido,

Thanks for your reply.

Yes, you can ignore the ack bit. If you are the only busmaster with the micro it is no problem (get some opto's with open collector out, otherwise the devices get into problems bringing the line down (or was the ack up, cant remember...).

Yes, you are right - slave device acknowledges data by pulling the SDA line low. BTW, I plan to use "speedy" 4N35 optocouplers.

I guess you don't have a i2c sender in your preamp, only receivers... One better option might be to isolate the display and put the micro into sleepmode. That's what i did, check www below.

There has to be at least one - MCU 😉 I do not have other senders. Putting the MCU into sleep mode is cool idea - I have to try it. I checked your page, nice work. My ideas and plans are similar. I like in circuit programming - I want to do the same with ATmega8 avr chip but I will have ISP connector on MCU board rather than RS232 connector at the back plate of the preamp.

Did not use a display, since it generates noise...

😕 I plan to use it (LCD or VFD). How critical it is? I have found all digital circuits to be noisy - I want to separate them (MCU, display unit) mechanically by steel or copper screen from other modules. Display could be also switched off when the MCU is not receiving any RC5 signal.

Thanks

Daniel

 

[guido]

In my setup there is no noise with the micro: in sleep mode all lines are 0 or 1: the whole digital circuit is 'dead', all lines are in one state. Think even including clocklines (check the pic's datasheet, the startup part of the oscillator). There are AVR's that can do the same, i think. But i did not look into that.

Can't comment on noise using a display: no experience there.
Remember those cd players where switching off the display should/does (?) bring an improvement ? Does it, well i don't know.

But i had no need for a display (had one in an earlier version of this preamp), it is cool and a nice programming exercise. But during day to day live, it is not needed. Functions on the preamp are too simple.

Gr,
GuidoB