Powering part of circuit from output of logic chip

[Mat]

Hello everyone!

Just a quick question. Here is what I want to do : power the "secondary" of a digital isolator from the output of a shift register to enable or disable communications to that part of the circuit. The max current output of the shift register chip is of course enough.

Would there be any obvious reason for that not to work?

Thanks!
M.

[Mat]

Nevermind, found a better way to achieve the same thing with what's called a "bus switch".

M.

[penfold]

Assuming this shift register is on the secondary side of the isolator and has an isolated power supply, then its possible and would work but would not exactly good practice.
You may be well considering something like an AND gate with the signal in order to pass or block it, you could also use a transmission gate etc.

[Mat]

Thanks!

Yeah I figured it was not good practice...

The problem with the AND gate is that the signals to be switched are bidirectional (I2C).
I think I will be using the following chips :
- ISO1540 I2C isolator
- and then, on the isolated side, SN74CBT3306 to switch the signal on or off

I suppose I need pull-up resistors before the isolator, between the isolator and the switch, and after the switch, otherwise, it won't work..

M.

[penfold]

Just a hunch, but assuming that the bus switch is purely bi-directional, which by the looks of it, it is. Then you shouldn't need the extra pullups either before or after the switch.

Sorry for the immediate 'state the bleeding obvious' post but I'd already written it by the time you'd replied