8.5  Problems

* = Difficult, ** = Very difficult, *** = Extremely difficult

8.1 (Files, 60 min.) Create a version of Table 8.1 for your design system.

8.2 (Scripts, 60 min.) Create a version of Table 8.5 for your design system.

8.3 (Halfgate, 60 min.) 

  • a. Using an FPGA of your choice, estimate the preroute delay of a single inverter (including I/O delays).
  • b. Complete a halfgate design and explain the postroute delays (make sure you know what conditions are being used—worst-case commercial, for example).

8.4 (***Xilinx die analysis, 120 min.) The data in Table 8.10 shows some information derived from a die photo of an ATT3020 (equivalent to a Xilinx 3020) in the AT&T data book. The die photo shows the CLBs clearly enough that we can measure their size. Then, knowing the actual die size, we can calculate the CLB size and other parameters. From your knowledge of the contents of the XC3020 CLB, as well as the programming and interconnect structures, make an estimate (showing all of your approximations and explaining all of your assumptions) of the CLB area and compare this to the value of 277 mils 2 shown in Table 8.10 . You will need to calculate the number of logic gates in each CLB including the LUT resources. Estimate how many pass transistors and memory elements are required as well as calculate how many routing resources are assigned to each CLB. Hint: You may need to use the Xilinx software, look at the Xilinx data books, or even the AT&T (Lucent) Orca documentation.

TABLE 8.10  ATT3020 die information (Problem 8.4).


Specified in data book

Measured on die photo

Calculated from die photo

3020 die width

183.5 mil

4.1 cm

3020 die height

219.3 mil

4.9 cm

3000 CLB width

0.325 cm

14.55 mil = 370 m m

3000 CLB height

0.425 cm

19.02 mil = 483 m m

3000 CLB area

277 mils 2

3020 pad pitch

1.61 mm/pad

7.21 mil/pad

Source: AT&T Data Book, July 1992, p. 3-76, MN92-024FPGA.

8.5  (***FPGA process, 120 min.) Table 8.11 describes AT&T’s 0.9 m m twin-tub V CMOS process, with 0.75 m m minimum design rules and 0.6 m m effective channel length and silicided (TiS 2 ) poly, source, and drain. This is the process used by AT&T to second-source the Xilinx XC3000 family of FPGAs. Calculate the parasitic resistance and capacitance parameters for the interconnect.

TABLE 8.11  ATT3000 0.9 m m twin-tub V CMOS process (Problem 8.5).



Die thickness, t die

21 mil

Wafer diameter, W D

5 inch

Wafer thickness, W t

25 mil

Minimum feature size, 2 l

0.75 m m

Effective gate length, L eff ( n -channel and p -channel)

0.6 m m

First-level metal, m1


Second-level metal, m2


m1 width

0.9 m m

m2 width

1.2 m m

m1 thickness

0.5 m m

m2 thickness

1.0 m m

m1 spacing

1.0 m m

m2 spacing

1.3 m m

D1 dielectric thickness, boron/phosphorus doped glass

3500 Å

D2 dielectric thickness, undoped glass

9000 Å

Minimum contact size

1.0 m m

Minimum via size

1.2 m m

Isolation oxide, FOX

3500 Å

Gate oxide

150 Å

Source: AT&T Data Book, July 1992, p. 2-37 and p. 3-76, MN92-024FPGA.

8.6 (Xilinx die costs, 10 min.) Table 8.12 shows the AT&T ATT3000 series die information. Assume a 6-inch wafer that costs $2000 to fabricate and has a 90 percent yield. (a)  What are the die costs? (b)  Compare these figures to the costs of XC3020 parts in 1992 and comment.

TABLE 8.12  ATT3000 die information (Problem 8.6).


Die height

Die width

Die area

Die area


Die perimeter

I/O pads






8 ¥ 8








10 ¥ 10








12 ¥ 12








16 ¥ 14








16 ¥ 20



Source: AT&T Data Book, July 1992, p. 3-75, MN92-024FPGA. 1 mil 2 = 2.54 2 ¥ 10 –6 cm 2 = 6.452 ¥ 10 –6 cm 2 .

8.7 (Pad density) Table 8.12 shows the number of pads on each of the AT&T 3000 (equivalent to the Xilinx XC3000) die. Calculate the pad densities in mil/pad for each part and compare with the figure for the ATT3020 in Table 8.10 .

8.8 (Xilinx HardWire, 10 min.) Xilinx manufactures nonprogrammable versions of its LCA family of FPGAs. These HardWire chips are useful when a customer wishes to convert to high-volume production. The Xilinx 1996 Product overview (p. 16) shows two die photographs: one, an XC3090 (with the four quadrants of 8 ¥ 10 CLB matrices visible), which is 32 mm ¥ 47 mm; the other shows the HardWire version (24 mm ¥ 29 mm). Estimate the die size of the HardWire version from the data in Table 8.12 and estimate the percentage of a Xilinx LCA that is taken up by SRAM.

Answer: 60,500 mils 2 ; 50 %.

8.9 (Xilinx XDE, 10 min.) During his yearly appraisal Dewey explains to you how he improved three Xilinx designs last year and managed to use 100 percent of the CLBs on these LCA chips by means of the XDE manual place-and-route program. As Dewey’s boss, rank Dewey from 1 (bad) to 5 (outstanding) and explain your ranking in a space that has room for no more than 20 words.

8.10 (Clocks, 60 min) (From a discussion on an Internet newsgroup including comments from Peter Alfke of Xilinx) “Xilinx guarantees that the minimum value for any delay parameter is always more than 25 % of the maximum value for that same parameter, as published for the fastest speed grade offered at any time. Many parameters have been reduced significantly over the years, but the clock delay has not. For example, comparing the fastest available XC3020-70 in 1988 with the fastest available XC3020A-6 (1996):

  • logic delay ( t ILO ) decreased from 9 ns to 4.1 ns
  • output-to-pad delay decreased from 10 ns to 5 ns
  • internal-clock-to-output pad delay decreased from 13 ns to 7 ns

The internal speed has more than doubled, but the worst-case clock distribution delay specification has only changed from 6.0 ns (1988) to 5.7 ns (1996).”

Comment on the reasons for these changes and their repercussions.

8.11 (State-machine design)

  • a. (10 min.) Draw the state diagram for the LOG/iC code in Table 8.2 .
  • b. (10 min.) Show, using an example input sequence, that the detector works.
  • c. (10 min.) Show that the state equations and the encoding for the PALASM code in Table 8.2 correctly describe the sequence detector state machine.
  • d. (30 min.) Convert this design to a different format of your choice: schematic, low-level design language, or HDL.
  • e. (30 min.) Simulate and test your design.

8.12 (FPGA software, 60 min.) Write a minitutorial (less than 2 pages) on using your FPGA design system. An example set of instructions for the Altera MAX PLUS II software on a Unix system are shown below:


  1. Copy ~altera/M+2/maxplus2.ini into ~you/yourDirectory (call this the working directory).
  2. Edit maxplus2.ini and point the DESIGN_NAME to your design
  3. Copy ~altera/M+2/compass.lmf and ~altera/M+2/compass.edc into your working directory.
  4. Copy ~altera/M+2/foo.acf into your working directory and rename it mydesign.acf if your design name is mydesign.edf .
  5. Set the environment as follows:

setenv LM_LICENSE_FILE ~altera/maxplus2/adm/license.altera

set path=( ~altera/maxplus5.1/bin)

and run the programs in batch mode: maxplus2 -c mydesign.edf . Add to this information on any peculiarities of the system you are using (handling of overwriting of files, filename extensions and when they are created, arguments required to run the programs, and so on).

8.13 (Help, 20 min.) Print the “help” for the key programs in your FPGA system and form it into a condensed “cheat-sheet.” Most programs echo help instruction when called with a '-help' or '?' argument (this ought to be a standard). For example, in the Actel system the key programs are edn2adl, adl2edn, and als (in newer versions adl2edn is now an option to als). Hint: Actel does not use '-help' argument, but you can get instructions on the syntax for each option individually. Table 8.13 shows an example for the Xilinx xdelay program.

TABLE 8.13  Xilinx xdelay arguments.

usage: xdelay [<options>] [<lcafile> ..]

where <options> are:

-help Print this help.

-timespec Do timespec based delay analysis.

-s Write short xdelay report.

-x Write long xdelay report.

-t <template file> Read <template file>.

-r Use two letter style block names in output.

-o <file> Send output to file.

-w Write design file, after retiming net delays.

-u <speed> Use the <speed> speed grade.

-d Don't trace delay paths.

-convert <input .lca file> <new part type> <output .lca file>

Convert the input design to a new part type.

Specify no arguments to run xdelay in interactive mode.


To Select Report Specify Option


TimeSpec summary -timespec

Short path details -s

Long path details -x

Analyze summary none of -s, -x or -timespec


A template file can be specified with the -t option to further filter the selected report. Only those template commands relevant to the selected report will be used.


Using -w and -d options together will insert delay information into the design file(s), without tracing any paths.


The -convert option may not be used with any other options.

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