Octal D-type Transparent Latches (with inverted 3-state outputs) # Technical Documentation: HD74HCT373FPEL Octal D-Type Latch with 3-State Outputs
Manufacturer : Hitachi (HIT)
Component Type : High-Speed CMOS Logic, Octal Transparent Latch
Package : FPEL (Plastic SOP, 20-pin)
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## 1. Application Scenarios
### Typical Use Cases
The HD74HCT373FPEL is an octal transparent latch featuring 3-state outputs, primarily designed for temporary data storage and bus interfacing in digital systems. Its core function is to capture and hold data present at its inputs (D0-D7) when the Latch Enable (LE) signal is high, and then output this data when the Output Enable (OE) signal is active low. When OE is high, the outputs enter a high-impedance (Hi-Z) state, allowing the bus to be shared with other devices.
Primary use cases include:
* Data Bus Buffering/Isolation: Serves as an interface between a microprocessor's data bus and multiple peripheral devices (memory, I/O ports, other ICs). It prevents bus contention by isolating the CPU bus from peripheral buses when outputs are disabled.
* Temporary Data Storage (Latching): Holds data from a transient source (like an analog-to-digital converter or a multiplexer) until the central processing unit is ready to read it, solving timing/synchronization issues.
* Address Latching: In microprocessor systems with multiplexed address/data buses (common in 8086/8088, 8051 architectures), it is used to demultiplex and latch the address information presented during the first part of a bus cycle.
* Port Expansion: Can be used to increase the number of output ports from a microcontroller by latching data sent over a shared bus.
### Industry Applications
* Industrial Control Systems: Used in PLCs (Programmable Logic Controllers) and industrial automation for I/O port expansion and sensor data capture.
* Telecommunications Equipment: Employed in routers, switches, and network interface cards for data routing and buffering.
* Automotive Electronics: Found in engine control units (ECUs) and infotainment systems for interfacing between microcontrollers and various sensors/actuators.
* Consumer Electronics: Used in printers, set-top boxes, and gaming consoles for memory interfacing and general-purpose logic.
* Test & Measurement Equipment: Utilized for capturing and holding digital signals from devices under test for analysis.
### Practical Advantages and Limitations
Advantages:
* Bus Driving Capability: The 3-state outputs allow direct connection to a bus-oriented system without external pull-up/pull-down resistors when in Hi-Z mode.
* High Noise Immunity: The HCT family offers CMOS-level power consumption with TTL-compatible input thresholds (V_IH = 2.0V min), ensuring robust operation in noisy environments.
* Wide Operating Voltage: Typically 4.5V to 5.5V, compatible with standard 5V logic systems.
* Balanced Propagation Delays: Ensures stable synchronous operation.
Limitations:
* Voltage Restriction: Requires a regulated 5V supply. Not suitable for modern low-voltage (3.3V, 1.8V) systems without level shifters.
* Transparent Nature: While LE is high, outputs follow inputs. This can cause bus instability if not properly timed; edge-triggered flip-flops are better for pure synchronous storage.
* Limited Current Sink/Source: Output drive current (typically ±6mA for HCT) is sufficient for driving a few TTL inputs or CMOS loads but not for directly driving LEDs or relays without a buffer.
* Package Density: The
