Memory circuit

This article is about a specific category of redstone circuits. For other circuits, see redstone circuits.

Note: This page uses many schematics, which are loaded individually for performance reasons. [Schematic Help]

Latches and flip-flops are effectively 1-bit memory cells. They allow circuits to store data and deliver it at a later time, rather than acting only on the inputs at the time they are given. As a side-effect of this, they can turn an impulse into a constant signal, "turning a button into a lever".

Devices using latches can be built to give different outputs each time a circuit is activated, even if the same inputs are used, and so circuits using them are referred to as "sequential logic". They allow for the design of counters, long-term clocks, and complex memory systems, which cannot be created with combinatorial logic gates alone. Latches are also used when a device needs to behave differently depending on previous inputs.

There are several basic categories of latches, distinguished by how they are controlled. For all types, the input lines are labeled according to their purpose (Set, Reset, Toggle, Data, Clock). There are also more arbitrary labels: The output is commonly labeled Q for historical reasons. Sometimes there is also an "inverse output" Q̅, which is always ON when Q is OFF and vice versa. If both Q and Q̅ are available, we say the circuit has "dual outputs". Most of the following types can be built as a "latch" that responds to the level of a signal, or as a "flip-flop" triggered by a change in the signal.

• A RS latch has separate control lines to set (turn on) or reset (turn off) the latch. Many also have dual outputs. The oldest form of RS latch in Minecraft is the RS-NOR latch, which forms the heart of many other latch and flip-flop designs.
• A T latch has only one input, the toggle. Whenever the toggle is triggered, the latch changes its state from OFF to ON or vice versa.
  • There are also SRT latches, combining the inputs and abilities of the RS and T latches.
• A D latch has a data input and a clock input. When the clock is triggered, the data input is copied to the output, then held until the clock is triggered again.
• A JK latch has three inputs: A clock input, and the J and K inputs. (J and K don't stand for anything.) When the clock is triggered, the latch's output can be set, reset, toggled, or left as is, depending on the combination of J and K. While these are common in real-world electronics, in Minecraft they tend to be bulky and impractical -- most players would use an SRT latch instead.

[edit] RS Latches

[edit] About RS Latches

An RS latch has 2 inputs, S and R. The output is conventionally labeled Q, and there is often an optional "inverse output" Q̅. (Having both Q and Q̅ is called "dual outputs"). When a signal comes into S, Q is set on and stays on until a similar signal comes into R, upon which Q is reset to "off". Q̅ indicates the opposite of Q -- when Q is high, Q̅ is low, and vice versa. Where a Q̅ output is available, you can often save a NOT gate by using it instead of Q.

Note that the proper name for this category of latch is "SR latch". However, in real-world electronics as in Minecraft, the classic implementation of such latches starts by inverting the inputs; such a latch is the proper "RS latch", but they're so common that the term is commonly used also for what "should" be called SR latches.

Typical uses include an alarm system in which a warning light stays on after a pressure plate is activated until a reset button is pushed, or a rail T-junction being set and reset by different detector rails. RS latches are common parts of other circuits, including other sorts of latches.

Setting both inputs high simultaneously is a "forbidden" condition, generally something to avoid. In the truth table, S=1, R=1 breaks the inverse relationship between Q and Q̅. If this happens, you will get "undefined behavior" -- various designs can do different things, and especially, Q and Q̅ can be high or low at the same time.

If the forbidden state is co-opted to toggle the output, the circuit becomes a JK latch, described in its own section. If there is instead a third input which toggles the output, the circuit becomes an "RST latch".

Any RS latch with dual outputs is functionally symmetrical: pulsing each input turns on "its" output, and turns off the other one. Thus R and S are interchangeable, if you also swap the outputs: Which input you pick as S chooses which of the outputs is Q, then the other input will be R and the other output will be Q̅ . (If the original circuit only had a Q output, then swapping the inputs will turn it into Q̅.) In several designs (A, B, D, F, I) the functional symmetry is reflected by the circuit's physical symmetry, with each input energizing the torch it leads to, while turning off the other.

RS latches can be built in a number of ways:

• Two NOR gates can be linked so that whichever is lit, the other will be off. The RS NOR latch is the "original" RS latch, and still among the smallest memory devices that can be made in vanilla Minecraft. While they can be built with just torches and redstone dust, repeaters can also be used.
• It is also possible to construct an RS NAND latch, using NAND gates instead of NOR gates. These will be larger and more complex than an RS NOR latch, but may be useful for specialized purposes.
• Other RS latches can be created by fitting an "input sustaining circuit" with a reset switch, say by adding a pair of NOT gates or a piston, placed so as to interrupt the circuit when triggered. Such a construction can be nearly as compact as an RS NOR latch (and often with better I/O isolation and/or timing), but they will usually not have a natural Q̅ output.
• Pistons can be used to physically toggle a block's location. Use of a Block of Redstone allows 4×1×1 design that is fast and flexible, but expensive -- besides the redstone block, it requires two pistons.

An RS latch can easily be expanded into a monostable circuit, which automatically disables itself some time after being activated. To do this, split the output redstone path into 2 parts. The new path should run through some repeaters, and in to the reset input. When you turn on the latch, the redstone will run through the delay before turning off the latch. (This works not only for Q and R, but for Q̅ and S as well.) You can also use a more complex delay mechanism instead of repeaters, e.g. a water clock.)

An "Enable/Disable RS latch" can be made by adding a pair of AND gates in front of the inputs, testing each of them against a third input, E. Now if E is true, the memory cell works as normal. If E is false, the memory cell will not change state. That is, E latches the RS latch itself.

[edit] D Latches and Flip-Flops

A D or "data" flip-flop, or a gated D latch, has two inputs: The data line D, and the "clock" input C. When triggered by C, the circuits set their output (Q) to D, then retain that output state between triggers. A D flip-flop is edge triggered; it sets the output to D only when its clock input changes from "off" to "on" (positive edge) or vice versa (negative edge), according to the circuit. A D gated latch is level triggered, changing the output to match D as long as the clock is either high or low (again, according to the circuit). You can often turn a gated D latch into a D flip flop by adding an edge trigger. Some of the designs also have the additional inputs and inverse output of an RS Latch.

Building these devices with torches is fairly unwieldy, but since version 1.4 of Minecraft, repeaters have gained latching functionality, which reduces the basic D gated latch to two repeaters, and the D flipflop to four repeaters:

		Q
C
		D

Modern Gated D Latch
(High level)

			Q
C
			D

Modern D Flip-flop
(rising edge)

Design G uses the repeater's new latching feature, added to the game in version 1.4. It holds its state while the clock is high, and is by far the most compact of the D latch designs. Design H combines two G latches, one high and one low triggered, to create a positive edge-triggered flip-flop. The block and redstone torch can be reversed for a negative edge-triggered design.

[edit] Torch-based Designs

For historical interest, several older designs, not dependent on latched repeaters, are provided, along with a table of their resource needs and other characteristics.

[edit] JK Flip-Flops and Latches

A JK flip-flop is another memory element which, like the D flip-flop, will only change its output state when triggered by a clock signal C. They can be edge-triggered (designs A, D, E) or level-triggered (C).

When the flip-flop is triggered the effect on the output Q will depend on the values of the two inputs J and K:

• If the input J = 1 and the input K = 0, the output Q = 1.
• When J = 0 and K = 1, the output Q = 0.
• If both J and K are 0, then the JK flip-flop maintains its previous state.
• If both are 1, the output will complement itself — i.e., if Q = 1 before the clock trigger, Q = 0 afterwards.

The table below summarizes these states — note that Q(t) is the new state after the trigger, while Q(t-1) represents the state before the trigger.

The JK flip-flop's complement function (when J and K are 1) is only meaningful with edge-triggered JK flip-flops, as it's an instantaneous trigger condition. With level-triggered flip-flops (e.g. design C), maintaining the clock signal at 1 for too long causes a race condition on the output. Although this race condition is not fast enough to cause the torches to burn out, it makes the complement function unreliable for level-triggered flip-flops.

The JK flip-flip is a "universal flip-flop", as it can be converted to any of the other types: It's already an RS latch, with the "forbidden" input used for toggling. To make it a T flip flop, set J = K = T, and to make it a D flipflop, set K to the inverse of J, that is J = K̅ = D. In the real world, mass production makes JK latches useful and common: a single circuit to produce in bulk, that can be used as any other sort of latch. In Minecraft, however, JK latches are invariably larger and more complex than the other types, and it's almost always easier to just build the specific latch type needed, including an "SRT Latch", which has all the same abilities but gets the toggle function from a separate input.

Aside from these redstone designs, it is also possible to make a JK flip-flop by modifying a rail toggle.

[edit] Design Table

[edit] T Flip-Flops and Toggles

T flip-flops are also known as "toggles." Whenever T changes from OFF to ON, the output will toggle its state. A useful way to use T flip-flops in Minecraft could for example be a button connected to the input. When you press the button the output toggles (a door opens or closes), and does not toggle back when the button pops out. These are also the core of all binary counters and clocks, as they function as a "period doubler", releasing one pulse for every two received.

[edit] T Flip-flop Designs

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