[SI-LIST] Re: drive ability
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 26 Jan 2005 09:44:34 -0500
> It is said that the maximum = output current of drive pin should
> be more than the sum of all input pi= n driven.
You should consider the above to be the absolute minimum drive capability,
below which your circuit has no chance of ever working. If the drive
strength is less than this, then your signals won't reach valid logic levels
at your inputs even if you wait forever for all transient effects to die
down.
As you already noted, for CMOS inputs, the desired output current to support
reasonable switching (transient response) needs to be more than given by the
above statement.
There was a time, and there were logic families, where you had to pay close
attention to the static (stable phase) input currents and simply make sure
the output had enough current capability to drive them.
This was sometimes the situation with old TTL (almost never used in new
designs anymore), where one output could drive as many as (say) eight
standard input loads, and you could almost ignore the transient effects
because traces were short and gate delays were in the tens of nanoseconds,
so it wasn't a big deal to wait several more nanoseconds for a non-optimum
transient response.
Today, when every nanosecond matters, you can't ignore the transient
response, so you need to choose your output drive strengths to give you
clean switching, which has nothing to do with CMOS input currents.
Regards,
Andy
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