IIR Filter could be causal or always non causal

[bhq]

I want to know that Is IIR filter is always non causal. if not than please send me an example of IIR Filter in difference equation that is causal.

Please think about

y = y(n-1) + x(n-1) + 2*x

Is the above system is causal or non causal?
Is the system is IIR or not ?

 

[nalino]

this is a causal system as y only relies on it's past values y(n-1) and x's present and past values...transfer function is

y = y(n-1) +2*x + x(n-1)
Y(z) = Y(z)*z^-1 + 2*X(z) + X(z)*z^-1
H(z) = Y(z)/X(z) = (2+z^-1)/(1-z^-1)

=> 1st order low-pass filter..

 

[xulfee]

this system is causal the example u given

 

[premkiran]

The filter equation , written by you, is a causal iir filter.

Only the required specifications in designing a filter , can drive you either to causal/non causal filer equations.