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[/] [mips_enhanced/] [trunk/] [grlib-gpl-1.0.19-b3188/] [lib/] [techmap/] [clocks/] [clkgen.in.help] - Rev 2
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Use Virtex CLKDLL for clock synchronisation
CONFIG_CLK_INFERRED
Certain target technologies include clock generators to scale or
phase-adjust the system and SDRAM clocks. This is currently supported
for Xilinx, Altera and Proasic3 FPGAs. Depending on technology, you
can select to use the Xilinx CKLDLL macro (Virtex, VirtexE, Spartan1/2),
the Xilinx DCM (Virtex-2, Spartan3, Virtex-4), the Altera ALTDLL
(Stratix, Cyclone), or the Proasic3 PLL. Choose the 'inferred'
option to skip a clock generator.
Clock multiplier
CONFIG_CLK_MUL
When using the Xilinx DCM or Altera ALTPLL, the system clock can
be multiplied with a factor of 2 - 32, and divided by a factor of
1 - 32. This makes it possible to generate almost any desired
processor frequency. When using the Xilinx CLKDLL generator,
the resulting frequency scale factor (mul/div) must be one of
1/2, 1 or 2. On Proasic3, the factor can be 1 - 128.
WARNING: The resulting clock must be within the limits specified
by the target FPGA family.
Clock divider
CONFIG_CLK_DIV
When using the Xilinx DCM or Altera ALTPLL, the system clock can
be multiplied with a factor of 2 - 32, and divided by a factor of
1 - 32. This makes it possible to generate almost any desired
processor frequency. When using the Xilinx CLKDLL generator,
the resulting frequency scale factor (mul/div) must be one of
1/2, 1 or 2. On Proasic3, the factor can be 1 - 128.
WARNING: The resulting clock must be within the limits specified
by the target FPGA family.
Output clock divider
CONFIG_OCLK_DIV
When using the Proasic3 PLL, the system clock is generated by three
parameters: input clock multiplication, input clock division and
output clock division. Only certain values of these parameters
are allowed, but unfortunately this is not documented by Actel.
To find the correct values, run the Libero Smartgen tool and
insert you desired input and output clock frequencies in the
Static PLL configurator. The mul/div factors can then be read
out from tool.
System clock multiplier
CONFIG_CLKDLL_1_2
The Xilinx CLKDLL can scale the input clock with a factor of 0.5, 1.0,
or 2.0. Useful when the target board has an oscillator with a too high
(or low) frequency for your design. The divided clock will be used as the
main clock for the whole processor (except PCI and ethernet clocks).
System clock multiplier
CONFIG_DCM_2_3
The Xilinx DCM and Altera ALTDLL can scale the input clock with a large
range of factors. Useful when the target board has an oscillator with a
too high (or low) frequency for your design. The divided clock will
be used as the main clock for the whole processor (except PCI and
ethernet clocks). NOTE: the resulting frequency must be at least
24 MHz or the DCM and ALTDLL might not work.
Enable CLKDLL for PCI clock
CONFIG_PCI_CLKDLL
Say Y here to re-synchronize the PCI clock using a
Virtex BUFGDLL macro. Will improve PCI clock-to-output
delays on the expense of input-setup requirements.
Use PCI clock system clock
CONFIG_PCI_SYSCLK
Say Y here to the PCI clock to generate the system clock.
The PCI clock can be scaled using the DCM or CLKDLL to
generate a suitable processor clock.
External SDRAM clock feedback
CONFIG_CLK_NOFB
Say Y here to disable the external clock feedback to synchronize the
SDRAM clock. This option is necessary if your board or design does not
have an external clock feedback that is connected to the pllref input
of the clock generator.