Zynq-7000 AP SoC and 7 Series FPGAs MIS v4.1
345
UG586 November 30, 2016
Chapter 2:
QDR II+ Memory Interface Solution
• Xilinx recommends keeping the system clock pins in the data write bank.
RECOMMENDED:
Although the MIG allows system clock selection to be in different super logic regions
(SLRs), it is not recommended due to the additional clock jitter in this topology.
System Clock, PLL Location, and Constraints
The PLL is required to be in the bank that supplies the clock to the memory to meet the
specified interface performance. The system clock input is also strongly recommended to
be in this bank. The MIG tool follows these two rules whenever possible. However,
exceptions are possible where pins might not be available for the clock input in the bank as
that of the PLL. In this case, the clock input needs to come from an adjacent bank through
the frequency backbone to the PLL. The system clock input to the PLL must come from
clock-capable I/Os.
The system clock input can only be used for an interface in the same column. The system
clock input cannot be driven from another column. The additional PLL or MMCM and clock
routing required for this induces too much additional jitter.
Unused outputs from the PLL can be used as clock outputs. Only the settings for these
outputs can be changed. Settings related to the overall PLL behavior and the used outputs
must not be disturbed. A PLL cannot be shared among interfaces.
See
Clocking Architecture, page 322
for information on allowed PLL parameters.
Configuration
The XDC contains timing, pin, and I/O standard information. The
sys_clk
constraint sets
the operating frequency of the interface. It is set through the MIG GUI. This must be rerun
if this constraint needs to be altered, because other internal parameters are affected. For
example:
create_clock -period 1.875 [get_ports sys_clk_p]
The
clk_ref
constraint sets the frequency for the IDELAY reference clock, which is
typically 200 MHz. For example:
create_clock -period 5 [get_ports clk_ref_p]
The I/O standards are set appropriately for the QDR II+ SRAM interface with LVCMOS15,
HSTL15_I, HSTL15_I_DCI, DIFF_HSTL15_I, or DIFF_HSTL15_I_DCI, as appropriate. LVDS_25 is
used for the system clock (
sys_clk
) and I/O delay reference clock (
clk_ref
). These
standards can be changed, as required, for the system configuration. These signals are
brought out to the top-level for system connection: