Latching

[danwinslow]

What does it mean, exactly, to 'latch' a pin or a port? I assume it means to preserve the value?

[+Stephen Moss]

What does it mean, exactly, to 'latch' a pin or a port? I assume it means to preserve the value?

853356[/snapback]

 

You are correct, there are several logic devices that can be used for latching, the most common of which is the D-Type latch.

A D-Type latch has bacically two inputs pins, Data (D) and a Clock. When the signal on the clock pin (provided manually or via other logic) reaches the appropriate state the signal on the Data pin (logic 1 or 0) is sent to the output. The output will remain in this state regardless of any change to the signal on the data pin and is therefore said to be latched until the next clock signal occurs at which point the signal currently on the Data pin will again be latched to the output.

 

Having said that a pin or port is said to be "latched" when in no longer responds as expected, always inputing or outputing the same information even though different information is being presented to it. If it helps think of this an being like a piece of software that stops responding.

AFAIK there is no definitive reason as to when this happens, as incorrect input combinations, static build-up in a control pin or intermittent joints could all cause such a problem.