What is 74HC165 Shift Register IC? Pin Diagram and Circuit
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The 74HC165 is an 8-bit parallel-in serial-out (PISO) shift register that converts multiple parallel inputs into a serial data stream. It is widely used for GPIO expansion, keypad interfacing, industrial control, and embedded system applications.
Key features of 74HC165
- Wide Supply Voltage Range: Operates from 2.0 V to 6.0 V.
- Low Power Consumption: CMOS low power dissipation technology.
- High Noise Immunity: Reliable operation in noisy environments.
- Overvoltage Tolerant Inputs: Inputs can tolerate up to 15 V, enabling use in HIGH to LOW level shifting applications.
- Input Compatibility:
- 74HC165: CMOS level inputs.
- 74HCT165: TTL level inputs.
- Operating Modes: Features an asynchronous 8-bit parallel load and a synchronous serial input.
- Standard Compliance & Protection: Complies with JEDEC standards (JESD8C, JESD7A) and features robust ESD protection (exceeds 2000 V HBM and 1000 V CDM).
74HC165 Pin Diagram
| Pin | Symbol | Description |
|---|---|---|
| 1 | PL | Asynchronous parallel load input (active LOW). When LOW, data from D0–D7 is loaded into the register. |
| 2 | CP | Clock input. Data is shifted on the LOW-to-HIGH transition. |
| 3, 4, 5, 6 | D4, D5, D6, D7 | Parallel data inputs. |
| 7 | Q7̅ | Complementary serial output from the last stage. |
| 8 | GND | Ground (0 V). |
| 9 | Q7 | Serial output from the last stage. |
| 10 | DS | Serial data input. Used when shifting data in serially (when PL is HIGH). |
| 11, 12, 13, 14 | D0, D1, D2, D3 | Parallel data inputs. |
| 15 | CE̅ | Clock enable input (active LOW). A HIGH on this pin disables the CP input. |
| 16 | VCC | Positive supply voltage (2.0 V to 6.0 V). |
Example Use Case : Microcontroller GPIO Expansion (Input)
Scenario: You are building a control panel with 8 physical toggle switches, but your Microcontroller Unit (MCU) only has 3 available GPIO pins.
How the 74HC165 solves this :
Connect the 8 toggle switches to the parallel data inputs (D0 to D7) of the 74HC165. Connect three MCU pins to the shift register as follows :
- One pin to the Parallel Load (PL’).
- One pin to the Clock (CP).
- One pin to read the Serial Output (Q7).
Read 8 digital inputs using only 3 pins on your microcontroller by software, successfully utilizing the IC’s Parallel to serial data conversion capability.
Applications of 74HC165
- Parallel to serial data conversion: Expanding data inputs by reading multiple parallel signals and converting them into a single serial data stream.
- HIGH to LOW level shifting: Leveraging the 15V overvoltage tolerance on inputs to interface higher voltage logic with lower voltage logic.
Summary
74HC165 expands digital inputs using serial communication while reducing required microcontroller pins.