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Peripheral Clocking
6.3.2 DDR2/mDDR Memory Controller Clocking
The DDR2/mDDR memory controller requires two input clocks to source VCLK and 2X_CLK (see
Figure
6-3):
•
VCLK is sourced from PLL0_SYSCLK2/2 that clocks the command FIFO, write FIFO, and read FIFO of
the DDR2/mDDR memory controller. From this, VCLK drives the interface to the peripheral bus.
•
2X_CLK is sourced from PLL1_SYSCLK1.
2X_CLK clock is again divided down by 2 in the DDR PHY controller to generate a clock called MCLK.
The MCLK domain consists of the DDR2/mDDR memory controller state machine and memory-mapped
registers. This clock domain is clocked at the rate of the external DDR2/mDDR memory, 2X_CLK/2.
Table 6-5
shows example PLL register settings based on the OSCIN reference clock frequency of
25 MHz. From these example configurations, the following observations are made:
•
To achieve the maximum frequency (150 MHz) supported by the DDR2/mDDR memory controller and
the typical CPU frequency of 300 MHz, the output of the PLL multiplier should be set to be 300 MHz
and the DDR_CLK source should be set to PLL1_SYSCLK1.
•
The frequency of the PLL1 direct output clock is fixed at the output frequency of the PLL1 multiplier
block.
•
The PLLDIV1 block that sets the divider ratio for SYSCLK1 can be changed to achieve various clock
frequencies.
•
For certain PLL1 multiplier and PLL1 post-divider control register (POSTDIV) settings, a higher clock
frequency can be achieved by selecting SYSCLK1 as the clock source for 2X_CLK.
If the DDR2/mDDR memory controller is not in use and the DDR_CLK and DDR_CLK are used in the
application as a free running clock that could be used by an FPGA or for some other purpose, then
2X_CLK should be used as the source for DDR_CLK and DDR_CLK and VCLK should be gated off. This
allows clock gating of the majority of the logic in the DDR2/mDDR memory controller via the LPSC while
still providing a clock on the DDR_CLK and DDR_CLK.
NOTE: DDR_CLK and DDR_CLK are output clock signals.
64
Device Clocking
Copyright © 2011, Texas Instruments Incorporated
www.ti.com
SPRUGX5A – May 2011
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