SIMPLIS Parts

Shift Register

The Shift Register models a combination of a clocked data type register and a shift register. The clock edge for the register can be set with the Trigger Condition parameter to be a rising edge (0_TO_1) or a falling edge (1_TO_0). The set and reset inputs can be either asynchronous or synchronous, depending on the Set/Reset Type parameter. The active logic level of the inputs can be configured with the Set/Reset Level and Load/Shift Level parameters.

For the shift register with both asynchronous and synchronous set and reset inputs, see Shift Register with both Async and Sync Set/Reset.

Related topics:

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Editing the Shift Register
Truth Table
- Asynchronous Set/Reset
- Synchronous Set/Reset
Examples
Waveforms
Subcircuit Parameters

Model Name:	Shift Register
Simulator:		This device is compatible with the SIMPLIS simulator.
Parts Selector Menu Location:	Digital Functions \| Registers
Symbol Library:	None - the symbol is automatically generated when placed or edited.
Model File:	None - the device model is generated before simulation.
Subcircuit Name:	SIMPLIS_DIGI1_D_SHIFT_REG_SOAS_N: With Ground Reference SIMPLIS_DIGI1_D_SHIFT_REG_SOAS_Y: With Ground Reference
Symbol:
Multiple Selections:	Only one device at a time can be edited.

Editing the Shift Register

To configure the shift register, follow these steps:

Double click the symbol on the schematic to open the editing dialog to the Parameters tab.
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 edge of the clock until the shift register output changes
Number of Bits	Number of input and output bits for the shift register
Trigger Condition	Determines the triggering condition of the shift register clock pin: 0_TO_1 for rising edge triggered 1_TO_0 for falling edge triggered
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 ground on the schematic. For more information on the use of Ground Ref for digital components, see When is Ground Ref Required?
Minimum Clock Width	Minimum valid clock width. Clock widths less than this parameter will not trigger the shift register.
Setup Time	Minimum time that input signals must remain constant before the triggering clock edge in order to register as a valid change in the inputs
Hold Time	Minimum time that input signals must remain constant after the triggering clock edge in order to register as a valid change in the inputs
Initial Condition	Initial condition of the shift register output in decimal
Left/Right Level	Determines the logic level of the shift register left/right (L/R) pin: Left_0/Right_1 means shift left on low and shift right on high. Left_1/Right_0 means shift left on high and shift right on low.
Load/Shift Level	Determines the logic level of the shift register load/shift (LD/SH) pin: Load_0/Shift_1 means load on low and shift on high. Load_1/Shift_0 means load on high and shift on low.

To define the set/reset behavior of the shift register, follow these steps from the Shift Register dialog box:

Click on the Set/Reset tab.
Make the appropriate changes to the fields described in the table below the image.

Label

Parameter Description

Set/Reset Delay

Delay from when the set or reset pin goes active until the output is actually set or reset. The Set/Reset Delay is used only for asynchronous set/reset. Registers with synchronous set/reset use the Clock to Output Delay.

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	Set/Reset Behavior
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.

Set To

Determines the shift register output value when the set pin goes active

Reset To

Determines the shift register output value when the reset pin goes active

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 from the Shift Register dialog box:

Click on the Interface tab.
Make the appropriate changes to the fields described in the table below the image.

Label

Parameter Description

Input Resistance

Input resistance of each input pin

Threshold

Hysteresis

The Threshold (T) and Hysteresis (H) of the Schmitt trigger input buffer on each Register input. To determine the low-to-high threshold (TH) and the high-to-low threshold (TL), substitute Threshold (T) and Hysteresis (H) in each of the following formulas :

Input Logic Transition	Actual Threshold
0 ➞ 1	TH = Threshold + 0.5 * Hysteresis
1 ➞ 0	TL = Threshold - 0.5 * Hysteresis

Output Resistance

Output resistance of the shift register output pins

Output High Voltage

Output high voltage for the shift register output pins

Output Low Voltage

Output low voltage for the shift register output pins

Truth Table

The two truth tables below show the logic behavior with asynchronous and synchronous set/reset.

Asynchronous Set/Reset

The following truth table assumes these parameter values:

Set/Reset Type=ASYNC
Trigger Condition=0_TO_1 which represents a rising edge clocked shift register
Set/Reset level=1
Load/Shift Level=Load_1/Shift_0.

When the EN input is high, and the Load/Shift Level=Load_1/Shift_0, the Register will Load the D input when the LD/SH pin is high, and Shift the Q output when the LD/SH pin is low.

Inputs							Output	Action
LD/SH	ASET	ARST	EN	L/R	D	CLK	Q	Action
0 or 1	0	0	0	0 or 1	0 or 1		Last Q	Retain state
1	0	0	1	0 or 1	0 or 1		Data input	Load data
0	0	0	1	0	0		Q = Last Q shifted left, LSB=0	Shift left
0	0	0	1	1	0		Q = Last Q shifted right, MSB=0	Shift Right
0	0	0	1	0	1		Q = Last Q shifted left, LSB=1	Shift left
0	0	0	1	1	1		Q = Last Q shifted right, MSB=1	Shift Right
0 or 1	1	0	0 or 1	0 or 1	0 or 1	0 or 1	Asynchronous Set To value	Asynchronous set
0 or 1	0	1	0 or 1	0 or 1	0 or 1	0 or 1	Asynchronous Reset To value	Asynchronous reset
0 or 1	1	1	0 or 1	0 or 1	0 or 1	0 or 1	Last Q	Illegal concurrent ASET and ARST

Synchronous Set/Reset

The following truth table assumes these parameter values:

Set/Reset Type=SYNC
Trigger Condition=0_TO_1, which represents a rising edge clocked shift register
Set/Reset level=1
Load/Shift Level=Load_1/Shift_0.

When the EN input is high and the Load/Shift Level=Load_1/Shift_0, the register will load the D input when the LD/SH pin is high, and shift the Q output when the LD/SH pin is low.

Inputs							Output	Action
LD/SH	SET	RST	EN	L/R	D	CLK	Q	Action
0 or 1	0	0	0	0 or 1	0 or 1		Last Q	Retain state
1	0	0	1	0 or 1	0 or 1		Data input	Load data
0	0	0	1	0	0		Q = Last Q shifted left, LSB=0	Shift left
0	0	0	1	1	0		Q = Last Q shifted right, MSB=0	Shift Right
0	0	0	1	0	1		Q = Last Q shifted left, LSB=1	Shift left
0	0	0	1	1	1		Q = Last Q shifted right, MSB=1	Shift Right
0 or 1	1	0	0 or 1	0 or 1	0 or 1		Synchronous Set To value	Synchronous set
0 or 1	0	1	0 or 1	0 or 1	0 or 1		Synchronous Reset To value	Synchronous reset
0 or 1	1	1	0 or 1	0 or 1	0 or 1		Last Q	Illegal concurrent SET and RST

Examples

The test circuit used to generate the waveform examples in the next section can be downloaded here: simplis_051_shiftregistersoas_example.zip.

To simulate this design, follow these steps:

Unzip the archive to a location on your computer.
To open the schematic, double click the .sxsch file or drag that file into the SIMetrix/SIMPLIS Command Shell.

Waveforms

This example of the shift register uses two Digital Signal Sources to generate the input pulses to the shift register. The input source waveforms are defined in a text file with each state transition defined on a single line. These two sources excite every state transition in the truth table. The example uses asynchronous set/reset.

For clarity, the simulation waveforms taken from the circuit example have been divided into three sections.

Load and Shift operations when D=0: 0-1us
Load and Shift operations when D=1: 1-2us
Asynchronous Set/Reset: 2-3us

Load and Shift operations when D=0: 0-1us

The image below shows the Load and Shift behavior when the D input is 0. During a Left shift operation, the D input is shifted into the LSB, and since D=0, the new Q output is twice the old Q. For a Right shift the D input is shifted into the MSB, and since D=0, the new Q output is half the old Q. The initial condition of the Shift Register is set to 15 decimal in the example.

Load and Shift with D=0
Time	Event	Q Output
100n	Load	4
200n	Retain state	4
300n	Shift left	8
400n	Shift right	4
500n	Shift right	2
600n	Shift left	4
700n	Shift left	8
800n	Shift left	0

Load and Shift operations when D=1: 1-2us

The image below shows the Load and Shift behavior when the D input is 1. During a Left shift operation, the D input is shifted into the LSB, effectively adding one to the result. For a Right shift the D input is shifted into the MSB, adding 2^n-1 to the result. For this example, n=4, so the MSB is equal to 8.

Load and Shift with D=1
Time	Event	Q Output
1.1u	Load	4
1.2u	Retain state	4
1.3u	Shift left	9
1.4u	Shift right	12
1.5u	Shift right	14
1.6u	Shift left	13
1.7u	Shift left	11
1.8u	Shift left	7

Asynchronous Set/Reset: 2-3us

The image below shows the set/reset behavior of the register.

Asynchronous Set/Reset
Time	Event	Q Output
2.32u	Asynchronous set	11
2.42u	Asynchronous reset	1
2.5u	Load	4
2.62u	Illegal concurrent ASET and ARST	4

Subcircuit Parameters

Because the shift register model is generated by a template script when the simulation is executed, a fixed model cannot be inserted into a netlist. The template script for this device is simplis_make_register_model.sxscr, which you, as a licensed user, can download in a zip archive of all built-in scripts.

To download this zip file, follow these steps:

Note: You will be prompted to log in with the user name and password given to you when you registered.

Click here to go to the product-installation page on the SIMetrix website.
Click Download Links in the first paragraph of the product-installation web page.
Scroll down to the Built-in Scripts section.
Right click on Download and select Save target as..., and then navigate to a location on your computer to save the zip file.