>> I don't believe the resistor is necessary. Perhaps someone who does 
>> feel
>> it is necessary can provide an explanation why.

> In a production unit where you KNOW you will not need to be changing 
> a control input then tying directly to logic low or logic high is 
> OK.

>> If, however, a line is bidirectional (could be an output, 
>> especially on
>> power up prior to initialization) a resistor is appropriate to 
>> limit
>> current during the time the line is being driven by the chip. If 
>> it's just
>> an input, though, I don't see a reason for the resistor. I'm open 
>> to
>> comment from others, though.

In an ideal world what you set stays set. Inputs stay that way and 
outputs do too. Circuits don't change with time, the code never needs 
rewriting because it was perfect first time and the user spec was so 
good it anticipated every need and no updates were needed . Ever.

In a real world things change. Murphy brings electrostatic discharge, 
power supply glitches, power supply sags, battery spikes, .... .
IF a pin tied high or low changes to an output the processor will 
sink/source appropriately. This may not matter at all. And it may 
cripple the product. It MAY cause physical damage to the processor. It 
MAY cause a battery to flatten far far sooner than it ought. And it 
may not. Code or hardware changes may short the open or open the 
short - the pin's unused so nobody may notice.

If an unused pin is set to output and left unconnected and Murphy 
flips it to an input, all may be well. Current drain may not be 
grossly affected and product operation may be just spiffing. But, the 
product may fail, current drain may cause problems, processor MAY be 
damaged (unlikely but possible), A2Ds may be noise up, oscillations 
may occur and cause RFI. And more.

Whether such dangers are worth protecting against is a considered 
engineering tradeoff. In many cases adding a high value resistor (1 
megohm is usually OK) is a reasonable tradeoff.


        Russell McMahon



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