S/R Flip-Flop with Set/Reset

The S/R Flip-Flop with Set/Reset models a generic clocked S/R Flip-Flop with either asynchronous or synchronous set and reset inputs. The Q and QN outputs can change state only on the specified clock edge unless the asynchronous set or reset is asserted. The clock edge trigger can be set with the Trigger Condition parameter to be either rising edge ( 0_TO_1 ) or falling edge ( 1_TO_0 ). If set and reset inputs are not required, the S/R Flip-Flop can be used.

In this topic:

Model Name: S/R Flip-Flop with Set/Reset
Simulator: This device is compatible with the SIMPLIS simulator.
Parts Selector Menu Location: Digital Functions > Flip-Flops
Symbol Library: None - the symbol is automatically generated when placed or edited.
Model Library: SIMPLIS_DIGI1.LB
Subcircuit Names:
  • SIMPLIS_DIGI1_SRFF_SR_N : Without Ground Reference
  • SIMPLIS_DIGI1_SRFF_SR_Y : With Ground Reference
Symbols:
Rising edge triggered, asychronous set/reset, without ground reference.
Rising edge triggered, asychronous set/reset, with ground reference.
Rising edge triggered, sychronous set/reset, without ground reference.
Rising edge triggered, sychronous set/reset, with ground reference.
Falling edge triggered, asychronous set/reset, without ground reference.
Falling edge triggered, asychronous set/reset, with ground reference.
Falling edge triggered, sychronous set/reset, without ground reference.
Falling edge triggered, sychronous set/reset, with ground reference.
Multiple Selections: Only one device at a time can be edited.

Editing the S/R Flip-Flop with Set/Reset

To configure the S/R Flip-Flop with Set/Reset, follow these steps:

  1. Double click the symbol on the schematic to open the editing dialog to the Parameters tab.
  2. Make the appropriate changes to the fields described in the table below the image.
Label Parameter Description
Clock to Output Delay Delay from the triggering clock event until the Flip-Flop outputs change
Minimum Clk Width Minimum valid clock width. Clock widths less than this parameter will not trigger the Flip-Flop.
Trigger Condition Determines the triggering condition of the Flip-Flop clock pin:
  • 0_TO_1 for rising edge triggered
  • 1_TO_0 for falling edge triggered
Initial Condition Initial condition of the Flip-Flop output at time=0
Setup Time Minimum time before the triggering clock event that the input signals must remain steady so that a valid change in each input state is recognized.
Hold Time Minimum time after the triggering clock event that the input signals must remain steady so that a valid change in each input state is recognized.
Ground Ref Determines whether or not a device has a ground reference pin. Any digital component that has an input or output pin connected to an analog circuit node must have its Ground Ref pin connected to an analog node. This is usually the ground on the schematic.
Set/Reset Delay Delay from when the SET or RST pin goes active until the Q output is actually set or reset.
Set/Reset Level Determines the Set/Reset level of a device:
  • 1 means active high
  • 0 means active low
Set/Reset Type Determines whether or not output events are synchronized with a clock event:
Set/Reset Type Description
SYNC Set/reset events are synchronized to the clock edge defined by the Trigger Condition parameter.
ASYNC Set/reset events are asynchronous to the clock edge.
.

To define the parameters for the interface between this digital component and each analog component connected directly to an input or output pin, follow these steps:

  1. From the Edit S/R Flip-Flop with Set/Reset dialog box, click on the Interface tab.
  2. Make the appropriate changes to the fields described in the table below the image.
Label Parameter Description
Input Resistance Input resistance of each Flip-Flop input pin
Hysteresis, Threshold Hysteresis and Threshold of the inputs. The hysteretic-window width, HYSTWD is centered around Threshold (TH) voltage. To determine the actual threshold ( TL , THI ), substitute Threshold (TH) and Hysteresis (HYSTWD) in each of the following formulas:
Input Logic Level Actual Threshold
1 Threshold + 0.5 * Hysteresis
0 Threshold - 0.5 * Hysteresis
Output Resistance Output resistance of Q and QN pins
Output High Voltage Output high voltage for Q and QN pins
Output Low Voltage Output low voltage for Q and QN pins

Truth Table

The following truth table assumes a Trigger Condition=0_TO_1 which represents a rising edge clocked Flip-Flop, Set/Reset level=1, and Set/Reset Type=ASYNC, representing asynchronous set/reset.

Asynchronous Set/Reset

Inputs Outputs Action
S R CLK SET RST Q QN
1 0
0 0 1 0 Set the Flip-Flop
0 1
0 0 0 1 Reset the Flip-Flop
0 or 1 0 or 1 0 or 1 1 0 1 0 Asynchronously set the Flip-Flop
0 or 1 0 or 1 0 or 1 0 1 0 1 Asynchronously reset the Flip-Flop
0 or 1 0 or 1 0 or 1 1 1 Last Q Last QN Illegal concurrent SET and RST

The following truth table assumes a Trigger Condition=0_TO_1 which represents a rising edge clocked Flip-Flop, Set/Reset level=1, and Set/Reset Type=SYNC, representing synchronous set/reset.

Synchronous Set/Reset

Inputs Outputs Action
S R CLK SET RST Q QN
1 0
0 0 1 0 Set the Flip-Flop
0 1
0 0 0 1 Reset the Flip-Flop
0 or 1 0 or 1
1 0 1 0 Synchronously set the Flip-Flop
0 or 1 0 or 1
0 1 0 1 Synchronously reset the Flip-Flop
0 or 1 0 or 1
1 1 Last Q Last QN Illegal concurrent SET and RST

Examples

The test circuit used to generate the waveform examples in the next section can be downloaded here: simplis_015_srflipflopwsetrst_example.sxsch.

Waveforms

The following waveforms assume Trigger Condition=0_TO_1 which represents a rising edge clocked Flip-Flop, Set/Reset level=1, and Set/Reset Type=ASYNC, representing asynchronous set/reset.

Subcircuit Parameters

The subcircuit parameters, parameter names, data types, ranges, units, and descriptions are in the following table. The parameter names can be used to directly generate netlist entries for the device. For example, an S/R Flip-Flop with Set/Reset netlist entry without ground reference would be: 
X$U1 3 5 2 4 6 7 SIMPLIS_DIGI1_DFF_SR_N vars: IC=0 MIN_CLK=10p TRIG_COND='0_TO_1' CLK_TO_OUT_DELAY=20p SETUP_TIME=10p HOLD_TIME=1p SET_RESET_DELAY=15p SET_RESET_TYPE='ASYNC' SET_RESET_LEVEL=1 GNDREF='N'
Parameter Name Label Data Type Range Units Parameter Description
CLK_TO_OUT_DELAY Clock to Output Delay Number 1f to 1024 s Delay from the triggering clock event until the Flip-Flop outputs change
GNDREF Ground Ref String
  • 'Y'
  • 'N'
 none Determines whether or not a device has a ground reference pin. Any digital component that has an input or output pin connected to an analog circuit node must have its Ground Ref pin connected to an analog node. This is usually the ground on the schematic.
HOLD_TIME Hold Time Number 1f to 1024 s Minimum time after the triggering clock event that the input signals must remain steady so that a valid change in each input state is recognized.
HYSTWD,
TH		 Hysteresis,
Threshold		 Number min: 1f V Hysteresis and Threshold of the inputs. The hysteretic-window width, HYSTWD is centered around Threshold (TH) voltage. To determine the actual threshold ( TL , THI ), substitute Threshold (TH) and Hysteresis (HYSTWD) in each of the following formulas:
Input Logic Level Actual Threshold
1 Threshold + 0.5 * Hysteresis
0 Threshold - 0.5 * Hysteresis
IC Initial Condition Number
  • 0
  • 1
 none Initial condition of the Flip-Flop output at time=0
MIN_CLK Minimum Clk Width Number 1f to 1024 s Minimum valid clock width. Clock widths less than this parameter will not trigger the Flip-Flop.
RIN Input Resistance Number min: 100 Ω Input resistance of each Flip-Flop input pin
ROUT Output Resistance Number min: 1m Ω Output resistance of Q and QN pins
SETUP_TIME Setup Time Number 1f to 1024 s Minimum time before the triggering clock event that the input signals must remain steady so that a valid change in each input state is recognized.
SET_RESET_DELAY Set/Reset Delay Number 1f to 1024 s Delay from when the SET or RST pin goes active until the Q output is actually set or reset.
SET_RESET_LEVEL Set/Reset Level Number
  • 0
  • 1
 none Determines the Set/Reset level of a device:
  • 1 means active high
  • 0 means active low
SET_RESET_TYPE Set/Reset Type String
  • 'SYNC'
  • 'ASYNC'
 none Determines whether or not output events are synchronized with a clock event:
Set/Reset Type Description
SYNC Set/reset events are synchronized to the clock edge defined by the Trigger Condition parameter.
ASYNC Set/reset events are asynchronous to the clock edge.
.
TRIG_COND Trigger Condition String
  • '0_TO_1'
  • '1_TO_0'
 none Determines the triggering condition of the Flip-Flop clock pin:
  • 0_TO_1 for rising edge triggered
  • 1_TO_0 for falling edge triggered
VOH Output High Voltage Number any V Output high voltage for Q and QN pins
VOL Output Low Voltage Number any V Output low voltage for Q and QN pins