Stitch Flow Use Model (stitch)

Description

Stitching enables Spectre APS to plug in the parasitic elements while simulation is running. Compared to the flat RC netlist and the hierarchical RC netlist approaches, the Stitching flow has the following advantages:

• Reuse of the pre-layout simulation test-bench. There is no need to change the probe and the measure statements.
• What-if analysis powered by selective stitching.

Spectre APS stitching is enabled by options.

Parasitic File Loading Parameters

• spf
  This option specifies the to-be-stitched DSPF file and its stitching scope. The syntax is spf="scope filename". The scope can be a subcircuit or an instance. When a subcircuit is specified as the scope, the DSPF file is stitched to all the instances of that subcircuit. When an instance is specified as the scope, the DSPF file is stitched to that instance only. Multiple DSPF files can be specified for stitching by using the option multiple times.
  Example:

  spf="mem mem.dspf"

• dpf
  This option specifies the to-be-stitched DPF file and its stitching scope. The syntax is dpf="scope filename". The scope can be a subcircuit or an instance. When a subcircuit is specified as the scope, the DPF file is stitched to all the instances of that subcircuit. When an instance is specified as the scope, the DPF file is stitched to that instance only. Multiple DPF files can be specified for stitching by using the option multiple times.
  Example:

  dpf="X1.XPLL PLL.dpf" dpf="X1.XMEM mem.dpf"

  This means that the PLL.dpf file needs to be stitched to the X1.XPLL instance and the mem.dpf file needs to be stitched to the X1.XMEM instance.
• spef
  This option specifies the to-be-stitched SPEF file and its stitching scope. The syntax is spef="scope filename". The scope can be a subcircuit or an instance. When a subcircuit is specified as the scope, the SPEF file is stitched to all the instances of that subcircuit. When an instance is specified as the scope, the SPEF file is stitched to that instance only. Multiple SPEF files can be specified for stitching by using the option multiple times.
  Example:

  spef="adc a.spef"

Stitching Parsing Options

• spfscale
  This option specifies the scaling factor of all the elements in the parasitic files (DSPF/SPEF/DPF). For example, consider that the width of a device in the DSPF file is w=2. If spfscale=1.0e-6 is used, the actual width will be w=2.0e-6.
• spfscalec
  This option specifies the scaling factor of all the capacitors in the parasitic files (DSPF/SPEF/DPF). Example, suppose a capacitor's value in the DSPF file is 2. If spfscalec=1.0e-15 is used, the actual capacitor value will be 2.0e-15.
• spfscaler
  This option specifies the scaling factor of all the resistors in the parasitic files (DSPF/SPEF/DPF). Example, suppose a resistor’s value in the DSPF file is 2. If spfscaler=1.0e-3 is used, the actual capacitor value will be w=2.0e-3.
• spfswapterm
  This option specifies the swappable terminals of a subcircuit macro-model. The syntax is spfswapterm="terminal1 terminal2 subcktname".
  Example:

  spfswapterm="n1 n2 nch_mac"

  This indicates that terminals n1 and n2 of subckt nch_mac are swappable. In general, this is applicable to devices that are modeled by subcircuits. Multiple spfswapterm statements are supported.
• spfxtorprefix
  This option specifies the prefix in the names (devices and nets) in the DSPF/SPEF/DPF file. The device names in the prelayout netlist and the DSPF/SPEF file often do not match. The spfxtorprefix option can be used to help match the device names. The syntax is spfxtorprefix="<substring> [<replace_substring>]".
  Example:

  spfxtorprefix="XM X"

  XX1/XM1 exists in the prelayout netlist but the corresponding device name in the DSPF file is XMX1/XM1. This option will change XM to X.
• spfaliasterm
  Sometimes the terminal names of devices in DSPF/SPEF/DPF files are different from those in the simulation model library. This happens often in the technology nodes that uses subcircuits to model devices. The syntax is spfaliasterm="<model|subckt> <prelayout_term1>=<spf_alias1> <prelayout_term2>=<spf_alias2>... <prelayout_termN>=<spf_aliasN>". Multiple statements are supported.
  Example:

  spfaliasterm="nfet_mac n1=D n2=G n3=S n4=B"

  This means that in subckt nfet_mac, terminal n1 corresponds to terminal D in the DSPF file, n1 corresponds to terminal G, n3 corresponds to terminal S and n4 corresponds to terminal B.
• speftriplet
  This option specifies the value that should be used for stitching in the SPEF file. This is effective only when the values in the SPEF file are represented by triplets (for instance, 0.325:0.41:0.495). Default value is 2. Possible values are 1, 2 and 3.
• spfinstancesection
  This option controls the backannotation of device parameters in the instance section of the DSPF file. If spfinstancesection is turned off, the instance section is ignored (that is, the device parameters are not changed during stitching). Default is on.
• spfbusdelim = busdelim_schematic [busdelim_parasitic]
  This option maps the bus delimiter between schematic netlist and parasitic file (i.e. DSPF, SPEF, or DPF). The option defines the bus delimiter in the schematic netlist, and optionally the bus delimiter in the parasitic file. By default, the bus delimiter of the parasitic file is taken from the parasitic file header (i.e. *|BUSBIT [], *|BUS_BIT [], or *|BUS_DELIMITER []). If the bus delimiter is not defined in the parasitic file header, you need to specify it by using the spfbusdelim option in schematic netlist.
  Example:
  • spfbusdelim=<> - A<1> in the schematic netlist is mapped to A_1 in the DSPF file, if the bus delimiter header in the DSPF file is "_".
  • spfbusdelim=@ [] - A@1 in the schematic netlist is mapped to to A[1] in the DSPF file (the bus delimiter in DSPF header will be ignored).
• spfinstarraydelim
  This option specifies the supplemental bus delimiter, if more than one bus delimiter is used in the pre-layout netlist (usually for instance array indexing). This option is similar to spfbusdelim.

Selective Stitching Options

• spfcnet
  This option specifies the net that has its total capacitance stitched. All other parasitic components, say parasitic resistors, associated with this net are ignored. The full hierarchical names are required. Multiple statements are supported. Wildcards are supported.
  Example:

  spfcnet=X1.netA

• spfcnetfile
  This option has the same functionality as spfcent.However, it accepts a text file in which all the C-only stitched nets are listed. Only one file can be specified. The syntax is spfcnetfile="filename". The format in the file is one line per net.
  Example:

  spfcnetfile="nets.tex"

  The format in the nets.tex is:

  netA

  netB

  netC

• spfrcnet
  This option specifies the name of the net to be stitched with parasitic resistors and capacitors. The other nets are stitched with lumped total capacitances. Multiple statements are supported. Wildcards are supported and you can specify multiple nets. Full hierarchical names are required.
  Example:

  spfrcnet=netA

• spfrcnetfile
  This option has the same functionality as spfrcnet. However, it accepts a text file in which all the RC stitched nets are specified. Only one file can be specified. The syntax is spfrcnetfile="filename". The format in the file is one line per net.
  Example:

  spfrcnetfile="nets.tex"

  The format in the nets.tex is:

  netA

  netB

  netC

• spfnetcmin
  This option allows you to select the net for stitching by the value of its total node capacitance. If a net’s total node capacitance exceeds spfnetcmin, all parasitics associated with the net are stitched correctly; otherwise, only the total capacitance is added to the net node.
  Example:

  spfnetcmin=1.0e-6

• spfskipnet
  This option specifies the net to be skipped for stitching, that is, all parasitic components of the net are not stitched. Wildcards and multiple statements are supported.
  Example:

  spfskipnet=X1.nodeA

• spfskipnetfile
  This option allows you to specify the nets to be skipped as a list in the text file called file_name. The syntax is spfskipnetfile="filename". Only one file can be specified. The format in the file is one line per net.
  Example:

  spfskipnetfile="nets.tex"

  nets.tex file format is:

  netA

  netB

  netC

Stitching Message Control Option

• spfmsglimit
  This option specifies the maximum number of messages to be printed in the spfrpt file. The messages in the spfrpt file are categorized by their ID number (STITCH-ID). This option specifies the maximum number of messages for a particular type of messages by using their STITCH-ID. The syntax is spfmsglimit="number STITCH-ID_1 STITCH-ID_2". When STITCH-ID is not specified, the tool assigns the maximum message number limit to all messages categories (STITCH-IDs).
  Example:

  spfmsglimit="10 STITCH-0010"

  This tells the tool to print not more than 10 messages for the STITCH-0010 message category, in the meantime, for the other message categories, the default maximum limit of 50 messages will apply.

For additional information, see Parasitic Backannotation of DPSF/SPEF/DPF Files in the Spectre Classic Simulator, Spectre APS, Spectre X, and Spectre XPS User Guide.

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