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[SOLVED] 74hc573 8-bit IO Expander

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Hello Dear All,

I would like to expand the IO with using 74HC573. I have some doubt regarding the input part (U1). What will happen on Q0-Q7 if the U1-pin1 (/OE) is goes Hi and the BUS data will affected after the /OE = H? AD0-AD7 BUS might control for other device.

In the datasheet said if /OE=H, LE=L then the output is Z (High Impedance Off-state). What does it mean High Impedance Off-state? and it doesn't mention /OE=H, LE=H condition. I wonder will Q0-Q7 is open circuit while /OE=H.

Hopefully can received your advice.

Thank you,..
 

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You should read up on "tri-state".

The High Impedance state basically means the output driver is disconnected completely inside the IC, in other words it does not drive the pin either high or low. This is useful when more than one IC is in parallel on a bus structure because you wouldn't want conflicting signals to meet head on. It also means that if the IC is driving out to a peripheral circuit, going Hi-Z could let the connection 'float' and assume unwanted or unstable logic levels so you have to be careful that the remainder of the circuit is safe under those conditions.

Brian.
 
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Hello Brian,

Thanks for your reply.

What do you meant read up on tri-state?

High Impedance, which means once the /OE pin is go Hi the output Q0-Q7 will be float/unstable logic? If this the case, can we use resistor to pull high method?

Thank you,.
 

I mean try a Google search on the term "tri-state", it should explain how and why the output drivers are turned off.

Yes, you can use pull-up or pull-down resistors. The only time you would see a problem is if the following circuit relies on the output of the '573 being in a particular state. When /OE is high it stops driving so unless something else is establishing a fixed logic level, the pins will 'float' to any voltage and might be interpreted as high, low or even oscillating between the two.

Brian.
 
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Hello Brian,

Ya, i did search the "tri-state" term. A lot of good information on net.

Good explanation. Now then i understand. Thanks a lot.
 

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