Help with measure statements for calculating prop. delays

[phanidee]

hspice find propagation delay of a gate

hi,

Sorry, if this is the wrong place to ask, pls redirect me to the right place. i am very much new to this forum.

The problem is that, i am simulating a simple full adder in hspice for 1000 random inputs, and want to measure the propagation delay of each gate in the full adder for every input.

Iam not able to write the measure statements with particular rise and fall numbers to measure the delay.

Are there any post-processing tools which will help me in doing that ?

I am very much new to this stream, pardon me if the questions sound too naive

Thanks in advance,
Phani

 

[erikl]

how to calculate propagation delays of a circuit

... i am simulating a simple full adder in hspice for 1000 random inputs, and want to measure the propagation delay of each gate in the full adder for every input.

Iam not able to write the measure statements with particular rise and fall numbers to measure the delay.

Are there any post-processing tools which will help me in doing that ?

Do you want to find out the longest propagation delay path of your full adder, and the single delays through all the gates involved? If so, simulation is not the right method, much less analog simulation (for a digital circuit!). Takes much too long time.

What you should use in such case, is STA (Static Timing Analysis). For a starter on this method, see here.

 

[phanidee]

calculate propagation delay of a gate

yes, i want to calculate the single gate delays of all the gates involved.

i thought this could just be done with some commands in hspice.

 

[erikl]

xor gate schematic

yes, i want to calculate the single gate delays of all the gates involved.
i thought this could just be done with some commands in hspice.

Of course it can be done in hspice, if it's a 2bit-, a 4bit-, or even an 8bit-full-adder. In this case you could find out the longest propagation delay path by thoroughly studying the schematic, then run a single simulation and extract all the single gate delays. Or you could run simulations for all possible combinations. Would take a rather long time for an 8bit-full-adder.

With a 16bit- or a 32bit-full-adder, however, there are so many (thousands) of possible paths, that it won't be easy to find out the longest path just by checking the schematic, i.e. you'd have to run hundreds or even thousands of simulations (and you'll never be sure if you really had found the longest path).

The STA method finds the longest propagation path very quickly, telling you also the single delays of the involved gates.

 

[phanidee]

hspice measure propagation delay

i just want to calculate the propagation of 1 xor gate in 2nd full adder in a 16bit ripple carry adder. I would like to take the average of the propagation delays for 1000 inputs 10 16 bit ripple carry adder. so to calculate just this prop. delay of the xor gate i am not able to write the exact rise and fall numbers for 1000 inputs.

is there any way to get out of this problem ?

 

[erikl]

how to measure delay propogation with hspice

i just want to calculate the propagation of 1 xor gate in 2nd full adder in a 16bit ripple carry adder. I would like to take the average of the propagation delays for 1000 inputs 10 16 bit ripple carry adder. so to calculate just this prop. delay of the xor gate i am not able to write the exact rise and fall numbers for 1000 inputs.

is there any way to get out of this problem ?

If you just want the propagation delay of 1 xor gate, you could extract this very part of your schematic, i.e. isolate this gate together with all gates controling its inputs as well as all gates loading its outputs. Then - with only a few simulations - you can get all propagation delays of all possible paths.