high impedance state

[bichel]

what is the use of high impedance state in digital circuits..how do they work??..i.e one of the uses given was that they are meant to isolate devices from each other..pls explain

 

[ericgibbs]

what is the use of high impedance state in digital circuits..how do they work??..i.e one of the uses given was that they are meant to isolate devices from each other..pls explain

hi,
In many logic circuits the output signal lines are OR'd together, this means more than one IC can control the data line.
If the logic circuit output driving the data line had an active pullup to to Vcc and and pull down to 0V no other devices output could be connected to the same line.
If the IC's on the OR'd data line have open circuit/high impedance output state, then when in the high impedance state, the IC is effectively not connected to the OR'r data line.

 

[bichel]

thanks for the reply..so if the ic isn't "effectively" connected to a bus then how does data transfer occurs when needed..and can data transfer take place to all the ICs(with o/p in high-z state) accessing this bus simultaneously?

 

[ericgibbs]

thanks for the reply..so if the ic isn't "effectively" connected to a bus then how does data transfer occurs when needed..and can data transfer take place to all the ICs(with o/p in high-z state) accessing this bus simultaneously?

hi,
If the circuit and program are correctly designed not more than one output is active on that line at any one time.
Say I had two IC's A, and B which had their outputs connected to the same input of another device, call that C input.
When A wants to transfer data to C, B will be 'quiet' in the high impedance state [not connected to C] and A will be in an active state and what ever the output of A is, high or low, then thats what 'C' will have as an input.
When B wants to transfer data to C, A will be 'quiet' in the high impedance state [not connected to C]

The micro controller controls the hZi states using an 'Enable' pin on 'A' or 'B'.

OK.??

 

[bichel]

thats really ok..thank you very much