Digital Signal Processing Reference
In-Depth Information
adjust the layout and run the timing analysis again, continuing
until timing analysis passes.
For the timing analysis stage the designer must provide scripts
to tell the tool what timing behavior is required. These scripts are
written by the hardware designer and shipped with the library
component (if you write your own hardware with multiple clock
domains then you will need to provide these scripts). If these
scripts are incorrect, or if the clock speeds set in the scripts are
lower than the actual speed of the clock, then the design can fail
even if it meets timing.
A timing failure in one part of the circuit can cause problems
elsewhere in the design, because if one part of the design fails to
meet timing then the tool will stop rearranging the design
throughout the chip. It will report the errors that have caused it to
stop processing, but may suppress errors for other areas of the
design which have not been completely processed. Thus a timing
failure in one part of the chip is said to hide failures elsewhere in
the design.
The propagation delay varies with several factors:
The temperature of the silicon within the chip: recent chips
run fastest at room temperature and slowest at the top and
bottom ends of their temperature range.
Manufacturing variations can change the propagation delay
between one batch of chips and the next. Manufacturers partly
deal with this by measuring the speed of chips after produc-
tion and assigning a higher speed grade (and price) to those
with lower propagation delays but there is still a small varia-
tion within each speed grade.
Small changes in the supply voltage: tolerances within the
power supply components allow for difference between the
supply voltages from one board to another.
The timing analysis tool will usually check the timing multiple
times with different timing modes
for example it will check
both the maximum and minimum propagation delays for the
temperature and manufacturing variation.
All timing models for a design must pass before it can be used
in a production system is used when timing passes only at lower
temperatures: a liberal application of freezer spray to the chip can
make a design work for a minute or two
e
often long enough to
indicate that timing is the cause of failures.
e
21.1.2 Fix Your Design if it Does Not Meet Timing
Here we will be referring back to the two basic timing equa-
tions above. In most chips the propagation delay is significantly
