CS355 Sylabus

Intro to Sequential Circuits

• So far, you have only seen combinatorial digital circuits. We are about to embark into studying the second category of digital circuits: sequential digital circuits

If you have forgotten about the different types of digital circuits, read this again: click here

• A very important sequential digital circuit is a one-bit memory

A one-bit memory is a device that is capable of remembering 1 bit value. So you can store a 0 or a 1 value in this device, and recall the value at a later time.

A one-bit memory would be similar to a cheap calculator's Min key:

• Pressing the Min key will update the memory value
• The calculator will keep the same stored value as long as the Min key is not pressed again.

The ability to indicate precisely when the value stored in a memory element will be updated is very important because a memory element that "updates itself at the wrong moment" is a "forgetful" memory and is not very useful...

• The most simple memory element would be a device that can store 1 bit information.

A memory device will need to support 2 capabilities:

• The ability to remember an input value when it is told to do so (and only when it is told to do so !)
• The ability to return the value stored

• Symbolically, a one-bit memory device will look something like this:

• When Write = 0, the output of the 1-bit memory will not change (eventhough the input value to the 1-bit memory may change - the memory must remember the store value, i.e., the output must remain the same because the output of the memory represents the value stored inside the memory).
• When Write = 1, the output of the 1-bit memory will be equal to the input value, i.e., the memory is now being instructed to remember a new value.

• We will first explore an interesting circuit and then return to the problem of constructing a 1-bit memory.