Serial input

Also called: SI, serial in, serial data input, SIR, SIL, shift input

The input of a shift register where a new bit enters, one bit per clock edge. It drives the D input of the first flip-flop in the chain.

The serial input (SI) of a shift register is the single wire through which new bits enter, one per clock edge. It drives the D input of the first flip-flop in the chain.

In a shift right register, SI feeds Q3.
In a shift left register, SI feeds Q0.
A universal shift register has two: SIR for shifting right (into Q3) and SIL for shifting left (into Q0).

What you connect to SI decides what the register does:

external data: serial reception, as in a SIPO register,
0: a logical shift,
the register's own MSB: an arithmetic shift right,
its own serial output: a rotate, or a ring counter if it runs forever,
the complement of its serial output: a johnson counter,
another register's serial output: a serial transfer.

Like any synchronous input, SI only matters at the rising edge. Its value just before the edge is what enters.

Worked examples

Example

Rotating left with a universal register

A 4-bit universal shift register holds 0110. Its SIL input is wired to its own Q3, and it is put in mode 10 (shift left) for one edge.

1.
Shifting left, every bit takes its right neighbor's value, and Q0 takes SIL.
2.
SIL = old Q3 = 0.
3.
Result: 1100. The bit that left Q3 re-entered at Q0, so this is a rotate left.

Example

SI wired to the register itself

A 4-bit shift-right register holds 1010. What does one edge give with SI = 0, SI = Q3 and SI = Q0?

1.
SI = 0: 0101 (logical shift right).
2.
SI = Q3 = 1: 1101 (arithmetic shift right).
3.
SI = Q0 = 0: 0101 (rotate right). Same as the logical shift here, because the bit that left was a 0.

Common mistakes

Connecting SI to the wrong end of the register for the shift direction.
Using SI's value after the edge. The value just before the edge is what enters.

Practice Serial input

Interactive questions with instant feedback and a worked solution for every wrong answer.

Registers lesson full course

In a 4-bit universal shift register, what is connected to the shift-right input (input 1) of Q3's MUX?

Learn it step by step

Serial input is taught in Registers.

Registers →

All terms