Why use active-low in bus signal definition?

[tony_taoyh]

In my BUS specification, some control signal is defined
as active-low.

what is the advantage to do like that?

Thanks a lot.

 

[flatulent]

It makes the system faster. Putting an extra inverter in that part of the signal path would cause extra delay.

 

[tony_taoyh]

But, during the design, most signal is active high..
And we must use one inverter to do the transformation.

Thanks.

 

[kxchorus]

active low may be quick to be asserted. The signal change from high to low is through NMOS transistor, which have better driving strength than PMOS.

this technique is often used in memory array.

 

[laobian]

Maybe for speed, but dont care about it in your design.

 

[maxsnail]

Posted: 18 Feb 2005 3:07 Post subject: Why use active-low in bus signal definition?

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active low may be quick to be asserted. The signal change from high to low is through NMOS transistor, which have better driving strength than PMOS.

this technique is often used in memory array.

great right ,for memory use active low, for compatible reason, so now system use active low

 

[tony_taoyh]

HI, Maxnail,

Thank you for your good reply.

It make sense.

Thanks a lot.

Best Regards,
Tony

 

[Thomson]

another reason is that the nmos transistor is more easily fabricated than the pmos transistor and of cause the required processes during the fabrication and the money required in the doping process.

Since the substrate is often P-style, then the nmos transistor can be fabricated using only one time of doping with B(oron).