S-R Logic Latches
Last Updated: January 16, 2025 Reviewed by: Mohsen Mostafaei, consulting engineer
Description
S-R Logic Latches, also known as Set-Reset latches, are digital storage devices used to store a single bit of data. They have two inputs, labeled S (Set) and R (Reset), and two outputs, Q and Q' (the complement of Q). The primary function of an S-R latch is to maintain a stable output state (either 0 or 1) until an input signal causes it to change.
Working Principle
An S-R latch operates using two cross-coupled NAND or NOR gates. When the Set input (S) is activated, the latch sets the output Q to 1. Conversely, when the Reset input (R) is activated, the output Q is reset to 0. If both inputs are inactive, the latch maintains its current state, effectively "remembering" the last input. This makes S-R latches useful for creating memory elements in digital circuits, as they can store and maintain a state without continuous input signals.
Applications
S-R latches are used in various specific applications, including:
- Debouncing Switches: S-R latches can be used to eliminate the noise or "bounce" that occurs when mechanical switches are pressed, ensuring a clean and stable signal.
- Simple Memory Storage: They are used in simple memory storage applications where a single bit of data needs to be stored and maintained.
- Control Circuits: S-R latches are often used in control circuits to maintain the state of a system, such as in alarm systems where the latch can maintain an alarm state until it is reset.
Advantages over other Logic Latches
- Simplicity: S-R latches are simpler to design and understand compared to other types of latches like D-type or J-K latches.
- Low Power Consumption: Due to their simple design, S-R latches generally consume less power, making them suitable for low-power applications.
- Fast Operation: They can operate quickly as they do not require a clock signal, unlike some other types of latches and flip-flops.
Limitations
- Invalid State: One significant limitation of S-R latches is the invalid state that occurs when both Set and Reset inputs are active simultaneously. This condition must be avoided to ensure proper operation.
- Limited Functionality: S-R latches are relatively basic and may not be suitable for more complex applications that require additional functionality, such as edge-triggered operations.
Considerations
When considering the use of S-R latches, several factors should be taken into account:
- Initial Costs: S-R latches are generally inexpensive due to their simple design and widespread availability.
- Operating Expense: They have low operating expenses, primarily due to their low power consumption.
- Durability: S-R latches are durable and reliable, provided they are used within their specified operating conditions.
- Accuracy: They offer high accuracy in maintaining the stored state, as long as the invalid state condition is avoided.
- Replacement and Maintenance Costs: These costs are minimal, given the simplicity and robustness of S-R latches.
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-SO [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 15V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 3
from RS Components, Ltd.
CMOS Quad NOR R/S Latch CD4043BE [See More]
- Latch Type: S-R; Transparent-D; Sr type
- Package Type: DIP; Pdip
- Output Characteristics: 3 state
- Logic Family: CMOS; 4000
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-SO [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 6.8
from RS Components, Ltd.
MC14044BDG, LOG CMOS LATCH QUAD NAND [See More]
- Latch Type: S-R; Transparent-D; Sr type
- Package Type: Soic; SOIC
- Output Characteristics: 3 state
- Logic Family: CMOS; 4000
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-TSSOP [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 6.8
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-SOIC [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 6.8
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-PDIP [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 6.8
from DigiKey
S-R Latch 4 Channel 1:1 IC Standard 16-SOIC [See More]
- Latch Type: S-R
- Supply Voltage: 4.75V ~ 5.25V
- Output Characteristics: Standard
- Low-level Output Current (Sink): 8
from DigiKey
IC LATCH QUAD R-S 16-SOIC [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 8.8
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-DIP [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 15V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 3
from DigiKey
S-R Latch 4 Channel 4:4 IC Tri-State 16-SO [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 15V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 3
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-PDIP [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 15V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 6.8
from DigiKey
S-R Latch 4 Channel 1:1 IC Standard 16-SO [See More]
- Latch Type: S-R
- Supply Voltage: 2V ~ 6V
- Output Characteristics: Standard
- Low-level Output Current (Sink): 5.2
from DigiKey
S-R Latch 2 Channel 1:1 IC Differential 20-PLCC (9x9) [See More]
- Latch Type: S-R
- Supply Voltage: 4.94V ~ 5.46V
- Output Characteristics: Differential
- Propagation Delay: 1
from DigiKey
S-R Latch 2 Channel 1:1 IC Differential 16-CDIP [See More]
- Latch Type: S-R
- Supply Voltage: 4.94V ~ 5.46V
- Output Characteristics: Differential
- Propagation Delay: 1
from DigiKey
S-R Latch 2 Channel 1:1 IC Differential 16-PDIP [See More]
- Latch Type: S-R
- Supply Voltage: 4.94V ~ 5.46V
- Output Characteristics: Differential
- Propagation Delay: 1
from DigiKey
S-R Latch 4 Channel 1:1 IC Tri-State 16-SOEIAJ [See More]
- Latch Type: S-R
- Supply Voltage: 3V ~ 18V
- Output Characteristics: 3-State
- Low-level Output Current (Sink): 8.8
from LCSC Electronics Technology (HK) Limited
3V~15V S-R Latch 15ns SO-16 Latches ROHS [See More]
- Latch Type: S-R
- Supply Voltage: 3V~15V
- Output Characteristics: Tri-State
- Propagation Delay: 15
from Lansdale Semiconductor, Inc.
The 5400/7400 series of transistor-transistor logic is a medium-speed, high-noise-immunity family of saturating integrated logic circuits designed for general digital logic applications requiring clock frequencies to 30MHz and switching speeds in the 12-15 ns range under moderate capacitive loading. [See More]
- Latch Type: S-R
- Operating Temperature: -55 to 125
- Supply Voltage: 7 VDC
- Package Type: DIP (optional feature); Lead Flat Pack
from Lingto Electronic Limited
74HC279 QUAD SET - RESET LATCH [See More]
- Latch Type: S-R
- High-level Output Current (Source): 5.2
- Low-level Output Current (Sink): 5.2
- Propagation Delay: 13
from Lingto Electronic Limited
IC LATCH TRI R/S QUAD 3ST 16-DIP [See More]
- Latch Type: S-R
- High-level Output Current (Source): 3
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 60
from Lingto Electronic Limited
IC NOR R/S LATCH 3ST QUAD 16SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
IC NOR R/S LATCH 3ST QUAD 16SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
IC NOR R/S LATCH 3ST QUAD 16-DIP [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
R-S LATCH [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
QUAD NOR R/S LATCH [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
IC NOR R/S LATCH 3ST QUAD 16SO [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
IC NOR R/S LTCH 3ST QUAD 16TSSOP [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
R-S LATCH, LOW LEVEL TRIGGERED [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6
- Low-level Output Current (Sink): 6
- Propagation Delay: 60
from Lingto Electronic Limited
R-S LATCH, LOW LEVEL TRIGGERED [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
CMOS QUAD NAND R/S LATCH [See More]
- Latch Type: S-R
- High-level Output Current (Source): 6.8
- Low-level Output Current (Sink): 6.8
- Propagation Delay: 50
from Lingto Electronic Limited
IC R/S LATCH 3-STATE QUAD 16SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 3
- Low-level Output Current (Sink): 3
- Propagation Delay: 15
from Lingto Electronic Limited
IC R/S LATCH 3-STATE QUAD 16SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 3
- Low-level Output Current (Sink): 3
- Propagation Delay: 30
from Lingto Electronic Limited
54LS279 - QUAD SET - RESET LATCH [See More]
- Latch Type: S-R
- High-level Output Current (Source): 4
- Low-level Output Current (Sink): 0.4000
- Propagation Delay: 27
from Lingto Electronic Limited
IC LATCH QUAD S-R 16-SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 5.2
- Low-level Output Current (Sink): 5.2
- Propagation Delay: 13
from Lingto Electronic Limited
IC S-R LATCH DUAL 20PLCC [See More]
- Latch Type: S-R
- Propagation Delay: 1
from Lingto Electronic Limited
IC LATCH DUAL MECL 10H 16-CDIP [See More]
- Latch Type: S-R
- Propagation Delay: 1
from Lingto Electronic Limited
IC LATCH R-S QUAD NOR 16SOIC [See More]
- Latch Type: S-R
- High-level Output Current (Source): 8.8
- Low-level Output Current (Sink): 8.8
- Propagation Delay: 60
from Lingto Electronic Limited
IC LATCH R-S QUAD P/N 16SOEIAJ [See More]
- Latch Type: S-R
- High-level Output Current (Source): 8.8
- Low-level Output Current (Sink): 8.8
- Propagation Delay: 60
from Lingto Electronic Limited
IC QUAD NOT-S/NOT-R LATCH 16-DIP [See More]
- Latch Type: S-R
- High-level Output Current (Source): 0.4000
- Low-level Output Current (Sink): 8
- Propagation Delay: 12
from Lingto Electronic Limited
IC QUAD S-R LATCHES 16SO [See More]
- Latch Type: S-R
- High-level Output Current (Source): 0.4000
- Low-level Output Current (Sink): 8
- Propagation Delay: 12
from Lingto Electronic Limited
R-S LATCH, LS SERIES, 4 FUNC, LO [See More]
- Latch Type: S-R
- High-level Output Current (Source): 8
- Low-level Output Current (Sink): 0.4000
- Propagation Delay: 27