Data Sheet
June 1999
ORCA Series 3C and 3T FPGAs
Resultant signals from the PCM must meet the FPGA
timing specifications. It is possible to specify pulses by
using duty-cycle adjustments that are too narrow to
function in the FPGA. For instance, if a 40 MHz clock is
doubled to 80 MHz and a 6.25% duty cycle is selected,
the result will be a 780 ps pulse that repeats every
12.5 ns. This pulse falls outside of the clock pulse width
specification and is not valid.
Programmable Clock Manager (PCM)
(continued)
High-Speed Internal Processing with Slow I/Os
The PCM PLL mode provides two outputs, one sent to
the global system clock routing of the FPGA and the
other to the ExpressCLK(s) that serve the FPGA I/Os.
The ExpressCLK output of the PCM has a divide capa-
bility (DIV2) that the system clock output does not. This
feature allows an input clock to be multiplied up to a
higher frequency for high-speed internal processing,
and also allows the ExpressCLK output to be divided
down to a lower frequency to accommodate off-FPGA
data transfers. For example, a 10 MHz input clock may
be multiplied (see Clock Multiplication in the Phase-
Locked Loop (PLL) Mode subsection) to 25 MHz (DIV0
= 4, DIV1 = 5, DIV2 = 2) and output to the FPGA
ExpressCLK. This allows the I/Os of the circuit to run at
25 MHz ((2 * 5)/4 * 10 MHz). The system clock will run
at DIV2 times the ExpressCLK rate, which is 2 times
25 MHz, or 50 MHz. This setup allows for internal pro-
cessing to occur at twice the rate of on/off device I/O
transfers.
Using divider DIV2, it is possible to specify a clock mul-
tiplication factor of 64 between the input clock and the
output system clock. As mentioned above, the resultant
frequency must meet all FPGA timing specifications.
The input clock must also meet the minimum specifica-
tions. An input clock rate that is below the PCM clock
minimum cannot be used even if the multiplied output is
within the allowable range.
The use of the PCM to tweak a clock signal to eliminate
a particular problem, such as a single setup time viola-
tion, is discouraged. A small shift in delay, duty cycle, or
phase to correct a single-point problem is in essence
an asynchronous patch to a synchronous system, mak-
ing the system less stable. This type of local problem,
as opposed to a global clock control issue like device-
wide clock delay, can usually be eliminated through
more robust design practices. If this type of change is
made, the designer must be aware that depending on
the extent of the change made, the design may fail to
operate correctly in a different speed grade or voltage
grade (e.g., 3C vs. 3T), or even in a different production
lot of the same device.
PCM Cautions
Cautions do apply when using the PCM. There are a
number of configurations that are possible in the PCM
that are theoretically valid, but may not produce viable
results. This section describes some of those situa-
tions, and should leave the user with an understanding
of the types of pitfalls that must be avoided when modi-
fying clock signals.
Divider DIV2 is available in DLL mode for the Express-
CLK output, but its use is not recommended with duty-
cycle adjusted clocks.
84
Lucent Technologies Inc.
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