Difference between revisions of "Carsten Milkau/RS-Latch"
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| − | + | An RS-latch is a circuit that stores which of its inputs was last "on". It has typically two inputs: input ''reset'' storing value "0" and input ''set'' storing value "1", hence the name. It can have multiple outputs (possibly inverted), which typically differ mostly in their behavior when multiple inputs are "on" at the same time. Usually, it is considered illegal to set more than one input to "on", but sometimes this is used deliberately (e.g. when constructing an [[Edge-Triggered RS-Latch]]). | |
| − | + | == Definition == | |
{| class="wikitable" | {| class="wikitable" | ||
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| − | It is possible to define a variant with more than two values. | + | It is possible to define a variant with more than two values. Also, the values stored need not necessarily be 0 and 1. |
=== Connectors === | === Connectors === | ||
| − | * | + | * Reset: Stores the first value (typically 0) |
| − | * | + | * Set: Stores the second value (typically 1) |
| − | * | + | * Output1: Stored value (first value if in illegal state) |
| + | * Output2: Stored value (second value if in illegal state) | ||
| − | === | + | === Illegal State === |
| − | + | If multiple inputs are on at the same time, the latch is considered in "illegal state". In principle, the RS-latch assumes both values at once, which circulate through the latch when both inputs turn off immediately (see value table), i.e. the latch becomes a [[Clock]]. This is usually undesired, but can be useful in some cases. | |
| − | + | == Implementation Using Logic Select Units == | |
| − | + | [[File:RS-Latch Using Select Units.png|thumb|An RS-Latch implemented using two Logic Select Units and two Logic Memory units]] | |
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| + | This RS-latch can store arbitrary values. It can also easily be extended to support more than two different values. It consists of a loop of [[Kit (Logic Processor)#Logic Select Unit|Logic Select Units]], each connected to a dedicated memory cell (or other data source). If all inputs are off, the values circulate in the loop. If one input is on, its corresponding value is read from the respective memory cell and replaces all values in the loop (unless an additional input is on). | ||
{| class="wikitable" | {| class="wikitable" | ||
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! Unit name || Unit type || Connector / Setting || Source / Value | ! Unit name || Unit type || Connector / Setting || Source / Value | ||
|- | |- | ||
| − | | select 1 || Logic Select Unit || select | + | | select 1 || Logic Select Unit || select || '''reset''' (input) |
|- | |- | ||
| − | | || || 0 (input 1) | + | | || || 0 (input 1) || select 2 |
|- | |- | ||
| − | | || || 1 (input 2) | + | | || || 1 (input 2) || "0" (constant) |
|- | |- | ||
| − | | | + | | || || output || '''output1''' (output) |
| + | |- | ||
| + | | select 2 || Logic Select Unit || select || '''set''' (input) | ||
| + | |- | ||
| + | | || || 0 (input 1) || select 1 | ||
|- | |- | ||
| − | | || || | + | | || || 1 (input 2) || "1" (constant) |
|- | |- | ||
| − | | || || | + | | || || output || '''output2''' (output) |
|} | |} | ||
Latest revision as of 16:08, 18 July 2018
An RS-latch is a circuit that stores which of its inputs was last "on". It has typically two inputs: input reset storing value "0" and input set storing value "1", hence the name. It can have multiple outputs (possibly inverted), which typically differ mostly in their behavior when multiple inputs are "on" at the same time. Usually, it is considered illegal to set more than one input to "on", but sometimes this is used deliberately (e.g. when constructing an Edge-Triggered RS-Latch).
Definition[edit]
| inputs | outputs | ||||||
|---|---|---|---|---|---|---|---|
| reset | set | output1 (previous) | output2 (previous) | output1 | output2 | ||
| 0 | 0 | x | y | y | x | ||
| 0 | 1 | x | y | 0 | 0 | ||
| 1 | 0 | x | y | 1 | 1 | ||
| 1 | 1 | x | y | 0 | 1 | ||
It is possible to define a variant with more than two values. Also, the values stored need not necessarily be 0 and 1.
Connectors[edit]
- Reset: Stores the first value (typically 0)
- Set: Stores the second value (typically 1)
- Output1: Stored value (first value if in illegal state)
- Output2: Stored value (second value if in illegal state)
Illegal State[edit]
If multiple inputs are on at the same time, the latch is considered in "illegal state". In principle, the RS-latch assumes both values at once, which circulate through the latch when both inputs turn off immediately (see value table), i.e. the latch becomes a Clock. This is usually undesired, but can be useful in some cases.
Implementation Using Logic Select Units[edit]
This RS-latch can store arbitrary values. It can also easily be extended to support more than two different values. It consists of a loop of Logic Select Units, each connected to a dedicated memory cell (or other data source). If all inputs are off, the values circulate in the loop. If one input is on, its corresponding value is read from the respective memory cell and replaces all values in the loop (unless an additional input is on).
| Unit name | Unit type | Connector / Setting | Source / Value |
|---|---|---|---|
| select 1 | Logic Select Unit | select | reset (input) |
| 0 (input 1) | select 2 | ||
| 1 (input 2) | "0" (constant) | ||
| output | output1 (output) | ||
| select 2 | Logic Select Unit | select | set (input) |
| 0 (input 1) | select 1 | ||
| 1 (input 2) | "1" (constant) | ||
| output | output2 (output) |