APPLICATION NOTE
Increase the Number of Inputs on a Microcontroller
Microcontrollers often have a very limited number of GPIO pins. Parallel-in serial-out shift registers can be used to read in a large number of input signals while only utilizing a few GPIO pins from the controller.
See more about this use case in the Logic Minute video Increase the Number of Inputs on a Microcontroller.
Design Considerations
- The clock input controls the rate at which data is read out of the shift register
- The clock (Fclk) must be faster than the parallel input data (BRin) based on the number of input bits (N) by this equation: Fclk (MHz) = BRin (Mbps) × N
- For more inputs, shift registers can be daisy-chained together
- [FAQ] What is the default output of a latched device? (Flip-Flop, latch, register)
- [FAQ] Where do I find maximum power dissipation for a device?
- Ask a question on our Engineer-to-Engineer (E2E™) forum
Recommended Parts
| Part Number | Automotive Qualified | VCC Range | Bits | Features |
|---|---|---|---|---|
| SN74HCS165 | 2 V to 6 V | 8 | Schmitt-trigger inputs Inverted and standard serial outputs | |
| SN74HCS165-Q1 | ✓ | |||
| SN74HCS166 | 2 V to 6 V | 8 | Schmitt-trigger inputs Asynchronous clear input Standard serial output | |
| SN74HCS166-Q1 | ✓ | |||
| CD4014B | 3 V to 18 V | 8 | Synchronous parallel input Outputs from Q6, Q7, and Q8 | |
| CD4021B | 3 V to 18 V | 8 | Asynchronous parallel input Outputs from Q6, Q7, and Q8 |
For more devices, browse through the online parametric tool where you can sort by desired voltage, channel numbers, and other features.