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SR flip-flop

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This block describes the simplest and the most fundamental latch the SR flip flop. The output Q depends of the state of the inputs S and R. The output !Q is the logical negation of Q

  • If S (Set) is pulsed high while R is held low, then the Q output is forced high, and stays high when S returns low.

  • if R (Reset) is pulsed high while S is held low, then the Q output is forced low, and stays low when R returns low.

  • When S and R are low, Q(t) takes the value of the previous output state Q(t-1).

  • When S and R are both high, both Q and !Q take the low or high values; the state is unstable. Practically this case is forbidden.

The user can set the initial output state with Initial Value parameter.

This block is almost used as a memory

The truth table of this block is:

S R Qn !Qn
0 0 Qn !Qn Hold
0 1 0 1 Reset
1 0 1 0 Set
1 1 U U Forbidden state

where U stands for "Unknown".

Data types

The block supports the following types :

  • Inputs:

    • R: scalar. Scilab's int8 data type only.

    • S: scalar. Scilab's int8 data type only.

    A positive input is considered as logical 1, a negative or a null input as logical 0.

  • Outputs: scalar. Scilab's int8 data type.


  • Initial Value

    Initial Value of the state Q. It must be int8 data type.

    Properties : Type 'vec' of size 1.

Default properties

  • always active: no

  • direct-feedthrough: yes

  • zero-crossing: no

  • mode: no

  • regular inputs:

    - port 1 : size [1,1] / type 5

    - port 2 : size [1,1] / type 5

  • regular outputs:

    - port 1 : size [1,1] / type 5

    - port 2 : size [1,1] / type 5

  • number/sizes of activation inputs: 0

  • number/sizes of activation outputs: 0

  • continuous-time state: no

  • discrete-time state: no

  • object discrete-time state: no

  • name of computational function: csuper


The following example presents a typical anti-bouncing application of the SR flipflop. The output graph shows the memory effect of the flipflop.

Interfacing function

  • SCI/modules/scicos_blocks/macros/IntegerOp/SRFLIPFLOP.sci

See also

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Last updated:
Tue Mar 07 09:29:00 CET 2023