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Serial output

Also called: SO, serial out, serial data output, shift output

The output of the last flip-flop in a shift register. It shows the bit that will leave the register at the next shift.

The serial output of a shift register is the Q output of the last flip-flop in the chain. It always shows the bit that is about to leave.

At each edge the current bit leaves (unless it is fed back) and the next one moves into place, so watching the serial output over several edges gives the stored word one bit at a time. For a shift-right register that is LSB first.

Uses:

  • PISO transmission: load a word in parallel, then read it bit by bit on the serial output. See siso sipo piso pipo.
  • Chaining: wire one register's serial output to the next register's serial input to make a longer register, or to perform a serial transfer.
  • Feedback: wire it back to the same register's serial input to rotate.

Remember the timing: the bit is on the serial output straight after it arrives in the last stage, without waiting for another edge.

Worked example

Example

Reading a word out serially

A 4-bit shift-right register holds 1011, with SI = 0. List the serial output now and after each of three edges.

  1. 1.

    Now: Q0 = 1.

  2. 2.

    Edge 1: contents 0101, Q0 = 1.

  3. 3.

    Edge 2: 0010, Q0 = 0.

  4. 4.

    Edge 3: 0001, Q0 = 1.

  5. 5.

    Sequence 1, 1, 0, 1: the word 1011 read LSB first.

Common mistakes

  • Waiting one extra edge for the first bit. The bit in the last stage is already on the output.

  • Reading the output sequence as MSB first. A shift-right register sends the LSB first.

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Serial output is taught in Registers.

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