[SI-LIST] Re: Problems with system bus design
- From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
- To: <pomgud@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sun, 25 Jan 2004 11:08:57 -0500
The formula you quote to calculate the output
resistance will give the the guaranteed value
at 2.4V output voltage in the high state. With
only this value, your concern appears to be
substantiated, because this does not provide
the rise time you need. Most outputs, however,
are nonlinear: if you pull them further away from
the steady state, the current goes up more than
what you could predict the the steady-state
calculation. The highest available current is
usually at least two to three times higher than
the steady state guaranteed current.
If you need proof/disproof whether your output
can deliver the required transition time with
the heaviest load, the surest way would be
to get trusted simulation models for the output
(IBIS or SPICE) and simulate the waveform.
Regards,
Istvan Novak
SUN Microsystems
----- Original Message -----
From: "pom gud" <pomgud@xxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Sunday, January 25, 2004 4:33 AM
Subject: [SI-LIST] Problems with system bus design
> Hello,
> I am using a 32-bitprocessor in current design.
> I have one technical doubt about system bus design.
> In processor data sheet, in DC Electrical
> Specification section, it is mentioned that drive
> capability of data bus is 6 mA and output load is of
> 30 pF. I am referring a book "High Speed Digital
> Design; A hand book of Black Magic" by Howard Johnson
> for peripheral bus analysis. As per formulas given in
> this book i found that it is not possible for
> processor peripheral data bus to drive 30 pF load with
> 6 mA drive capability and meet rise time requirement
> of 1.5 nS. The design calculations are as follows,
>
> Output driver resistance = (VCC- VOH)/IOH =
> (3.3 - 2.4)/ 6 mA = 150 ohm
> So rise time = 2.2*R*C = 2.2*150*30pF
> = 9.9 nS
>
> I have a lot of confusion about these calculations.
>
> Hoping for a quick reply
>
> Thanks and Regards
> pomgud
>
>
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