Back Annotation and Delay Calculation

Calculating Delays

Libraries with Mixed Cell Timing Descriptions

If you are calculating delays for a description that contains cells with path delays and distributed delays, you must use some mechanism to determine which objects to scan on a cell-by-cell basis. Following are some of the methods you can use to accomplish this:

■scan method—Use acc_next_modpath to check for path delays within a cell, scanning cell outputs if none are found.

■attribute method—Declare a specparam in all cell descriptions that indicate the type of delay methodology used within the cell, and retrieve this specparam value using acc_fetch_attribute during delay calculation.

■table lookup method—Use acc_fetch_defname to retrieve the cell -type name during delay calculation and use this name in conjunction with an external (file) or internal (data structure) table lookup mechanism to determine which objects to scan.

Once you have determined the delay type for a particular cell instance, use the methods previously described in this section to scan the path delays or output ports within the instance.

Providing and Retrieving Delay

Calculation Data

Types of Data Needed

Typically, several pieces of data are needed to calculate delays. Some of this information, such as cell I/O loading factors and cell output drive factors, is the same for all instances of a given cell type. Other data, such as the loading capacitance due to interconnect wire, may be different for every cell instance in the design. This section describes methods of providing and retrieving both classes of data. Note that, although the examples in this section use data for a specific delay calculation formula to illustrate these methods, delay calculation data can always be placed into one of these two classes and thus handled with the generalized approaches described here.

Coding Data into Cell Descriptions (Attributes)

In most delay calculation processes, a subset of the data is constant across all instances of a given cell type. Typical pieces of data that fall into this category are as follows:

■cell I/O load factors

■cell output drive factors

Back Annotation and Delay Calculation

Calculating Delays

■delay scaling constants

Usually, each piece of data is associated with a particular object in the cell, such as a port, net or path delay. By using a special specparam naming convention in conjunction with the access routine acc_fetch_attribute, you can code this data directly into the Verilog-XL

HDL description of the cell and retrieve it during delay calculation. The pieces of data that you provide in the cell description are known as cell attributes.

To attach an attribute to an object within a cell, define a specparam in the cell’s specify block. This specparam must contain the name of the object as part of its full name using the convention described in Figure 4-12 on page 57 and Figure 4-13 on page 57 .

Figure 4-12 Specparam attribute naming conventions

Figure 4-13 Definition of terms used in Figure 4-12 on page 57

Figure 4-2 on page 45 , repeated next as Figure 4-14 on page 58 , shows an example VerilogXL HDL cell module description with path delay timing that contains several attributes coded as specparams. The attributes are associated with path delays and nets within the module.

Back Annotation and Delay Calculation

Calculating Delays

Figure 4-14 Cell module with path delay timing and attributes coded as specparams

Figure 4-15 on page 59 shows an example Verilog-XL HDL cell module description with lumped (primitive output) delay timing that contains several attributes coded as specparams.

In this case, all of the attributes are associated with nets within the module.

Back Annotation and Delay Calculation

Calculating Delays

Figure 4-15 Cell module with lumped (primitive output) delay timing and attributes coded as specparams

// D flip-flop lumped delay timing model ‘celldefine

module dflop(q,qb,clk,d); input clk, d;

output q, qb;

To retrieve specparam attributes from within your delay calculation facility, call acc_fetch_attribute with a handle to the object and the appropriate <attribute_string>. Figure 4-16 on page 59 shows a C-language routine fragment that retrieves attribute values that are attached to path delays with the attribute string RiseStrength$, as in the example design of Figure 4-2 on page 45 , Figure 4-3 on page 46 and Figure 4-4 on page 46 . Note that no call to acc_configure is needed because the attribute specparams use the default path delimiter string $.

Figure 4-16 Retrieving attribute information with acc_fetch_attribute

static void dl_calc_path_delays(start_mod,estimate_flag)

handle mod, path; handle pathout_net; int fanout_count;

Back Annotation and Delay Calculation

Calculating Delays

double fanout_load, wire_load;

double rise_strength, fall_strength, rise, fall; /**********************************************************/ /*** Process each cell module instance within the scope ***/

/**********************************************************/ mod = null;

while (mod = acc_next_cell(start_mod, mod))

{

/*********************************************/ /*** Process each path in this cell module ***/

/*********************************************/ path = null;

while (path = acc_next_modpath(mod,path))

{

. . .

/*************************************************/ /*** Determine attributes associated with path ***/

/*************************************************/ rise_strength = acc_fetch_attribute(path, "RiseStrength$"); fall_strength = acc_fetch_attribute(path, "FallStrength$");

. . .

}

}

}

If acc_fetch_attribute does not find the attribute associated with the indicated object, it will look for a specparam attribute with a name identical to <attribute_string>. This provides a convenient way to describe an attribute that is associated with several objects in a cell. Figure 4-17 on page 61 shows an example cell description containing an attribute value described with the specparam FanoutLoad$ that is not attached to any specific object. The routine acc_fetch_attribute will return this value for the attribute when called with a handle to any object in the cell except input sel and output y. These two objects have their own explicitly associated attribute values.