[SI-LIST] Re: Drive strength to Load conversion
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sun, 17 Aug 2014 02:41:10 -0700
If we take the rash assumption that your rise and fall times are so slow
that you can treat your traces as simple ohmic interconnects, then you
can analyze your timing like it was 1985. The rational disconnect here
is that if your rise and fall times are that slow, then why would you be
concerned about skew from capacitive loading? Do you have an enormous
fan out?
If your rise and fall times are fast enough that the interconnects are
transmission lines, then if you have so much capacitance at the far end
that |Ztxline|*C >> [Trise, Tfall] then you have a reflection issues and
failing to control your interconnect impedance is going to make that
even worse. If your timing budget cannot handle capacitive charging at
your loads, then it is extremely unlikely that it will tolerate a number
of round trip delays to settle out. If any of the signals are clocks or
timing strobes then you could be walking into multiple transition
nightmares.
I think that what you need to do is understand your signaling
requirements first. That will tell you how sloppy your timing can be
and whether you need to engineer as though it is the current or past
century.
Steve.
On 8/16/2014 11:20 PM, Ajay Dhingra wrote:
> Hi Lee
> Thanks for your reply.
> I further have two things in my mind.
>
> First: I don't have a controlled impedance environment.
> Second: for any Transmission line, there is a cap load at the end of the
> transmission line. The cap load requires certain amount of charge so that
> it's voltage reaches logical threshold with a desired ramp. at the end of
> the day cap will change the ramp and hence timing. And eventual ramping of
> that voltage is determined by drive strength of the driver. though it
> happens in stages, first charging the transmission line and then
> transmission lines forwards the same ramp to cap load. So my question is
> how to approximate the drivers capability for a particular load or what max
> load a drive can drive based upon its drive strength.
>
> Thanks
> Ajay
>
>
> On Sat, Aug 16, 2014 at 11:50 PM, Lee <leeritchey@xxxxxxxxxxxxx> wrote:
>
>> We don't drive capacitive loads any more. We drive impedances on the
>> order of 50 ohms. transmission lines.
>>
>>
>> -----Original Message----- From: Ajay Dhingra
>> Sent: Saturday, August 16, 2014 11:09 AM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Re: Drive strength to Load conversion
>>
>>
>>
>> Sent from my Huawei Mobile
>>
>> -------- Original Message --------
>> Subject: Re: [SI-LIST] Drive strength to Load conversion
>> From: Ajay Dhingra <ajay.dhingra@xxxxxxxxx>
>> To: Lee <leeritchey@xxxxxxxxxxxxx>
>> CC: si-list@xxxxxxxxxxxx
>>
>> Just for first order estimation if that particular drive strength is
>> sufficient for an equivalent cap load. Or in other words what max cap load
>> can be driven by x mA drive strength.
>>
>> in a situation when only drive strength in mA information is available how
>> to assess the driver capability.
>>
>> Thanks
>> Ajay
>>
>> Sent from my Huawei Mobile------------------------
>> ------------------------------------------
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>
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