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Design Languages >> Verilog-AMS >> Question about PLL example given in this website
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Message started by amber555 on Mar 10^th, 2008, 8:57am

Title: Question about PLL example given in this website
Post by amber555 on Mar 10^th, 2008, 8:57am

Hi, everyone. I just download an example PLL code in Verilog-AMS from this website and it works fine. However, when I look at the code, I found that for the charge pump (cp.vams), there is a "-" before the "transition". Could anyone explain for me why we need a "-"? Thanks a lot.

Title: Re: Question about PLL example given in this websi
Post by amber555 on Mar 12^th, 2008, 4:12am

The Verilog-code that I mentioned for modeling the charge pump is as follows:

module cp (pout, nout, inc, dec);
parameter real cur = 1m; // output current (A)
input inc, dec;
electrical pout, nout;
real out;

analog begin
@(initial_step) out = 0.0;

if (dec && !inc)
out = -cur;
else if (!dec && inc)
out = cur;
else out = 0;

I(pout, nout) <+ -transition(out, 0.0, 10n, 10n);
end
endmodule

I am wondering why we need to add "-" before "transition". Thanks in advance.

Title: Re: Question about PLL example given in this websi
Post by Geoffrey_Coram on Mar 12^th, 2008, 5:04am

Surely it's just a sign convention; one could have written instead
I(nout,pout) <+ transition(out, 0.0, 10n, 10n);

Title: Re: Question about PLL example given in this websi
Post by Ken Kundert on Mar 12^th, 2008, 10:38am

Current is positive if it enters pout and exits nout (associated reference direction). Since this is a source, we think of positive current leaving the source. Thus we need a negative sign to convert our conventions internal to the model, that of a source, to the conventions used by the language, associated reference direction.

-Ken