What is a latch circuit and what it consists of?

[epp]

race around conditions in latches

What is latch circuit and from which it consist of?
I have schematic where inputs of latch are control signal of 3phase RDCL inverter and outputs from latch are gating signals to its transistors.

Regards.

 

[mazelk]

Re: Latch

Hi...

Typically, latch consists in a transistor configuration that 'store' in its output, the logical level of one from input signals. The input signal in 'set input' can maintain a high level during a bite of time, but output signal 'store' this level until a high level be in 'reset input'.

Latch is like a flip-flop without a control as clock. Maybe contain an enable pin. For example, latch can be used as a noise supressor by switching in digital discrete circuits

 

[Hero]

Re: Latch

Hi,

There are many confusions between latches and flip-flops esspecially in the old books.

Latch is asynchronous element with one or two feedback loops sensitive to propagation delays and race-around conditions.

Flip-flops are designed to have two latches one master and one slave. FFs are less sensitve to propagation delays and don't have race around problems but they are slower than latches and recquire more power.

D-latch is transparent when (LE==1) and holds input when LE is going from high to low state. Exact Boole equation for D-Latch is Q=(D&LE) | (Q&D) | (Q&!LE). Q(LE==1)=D | (Q&D)=D, Q(LE==0)=(Q&D) | Q=Q.

D-FF contains master and slave D-latch and one inverter between LE inputs this common input to master and slave LE inputs is CLK signal.

These are functional descirptions you can find many different implementations in many different books and also some serious mistakes in some books of digital electronics.

Optimal implementations for lathes and FFs are technology dependant.

 

[leo83]

Re: Latch

Latches and flip-flops
In the same way that gates are the building blocks of combinatorial circuits, latches and flip-flops are the building blocks of sequential circuits.
While gates had to be built directly from transistors, latches can be built from gates, and flip-flops can be built from latches. This fact will make it somewhat easier to understand latches and flip-flops.

Both latches and flip-flops are circuit elements whose output depends not only on the current inputs, but also on previous inputs and outputs. The difference between a latch and a flip-flop is that a latch does not have a clock signal, whereas a flip-flop always does.

 

[epp]

Re: Latch

Could anyone give some schematic of a latch or some datasheet?
How to simulate it in Simulink or Pspice?
Regards

 

[Kevin Weddle]

Re: Latch

A flip flop is just a latch with a clock enable. A latch is just 4 nand gates. When you have two highs you get a low which causes the input to go low because of the feedback. This low stays there and the ouput goes high. But the high is just an enable and the input stays low. So you took the two highs and made one of the inputs go low and it stays low. The output went high but it is just an enable.