GXM datasheet(119/244 Pages) NSC

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Integrated Functions (Continued)

The SDRAM interface timings are programmable. The

SHFTSDCLK bits in the MC_MEM_CNTRL2 register can

be used to change the relationship between SDCLK and

the control/address/data signals. To meet setup and hold

time requirements for SDRAM across different board lay-

outs, the SHFTSDCLK bits are used. SHFTSDCLK bit val-

ues are selected based upon the SDRAM signals loads

and the core frequency (refer to Table 7-10 on page 190 ).

Figure 4-10 shows an example of how the SHFTSDCLK

bits setting affects SDCLK. The PCI clock is the input

clock to the GXm processor. The core clock is the internal

processor clock that is multiplied up. The memory control-

ler runs off this processor clock. The memory clock is gen-

erated by dividing down the processor clock. SDCLK is

generated from the memory clock. In the example dia-

gram, the processor clock is running 6X times the PCI

clockand thememoryclock is runningindivideby3

mode.

The SDRAM control, address, and data signals are driven

off edge "x" of the memory clock to be setup before edge

"y". With no shift applied, the control signals could end up

being latched on edge "x". A shift value of two or three

couldbeusedsothatSDCLKatthe SDRAM iscentered

around when the control signals change.

Figure 4-10. Effects of SHFTSDCLK Programming Bits Example

123

PCI Clock

Core Clock

Memory

Clock

(Internal)

CNTRL

SDCLK

(Note)

Note: The first SDCLK shows how SDCLK operates with the SHFTSDCLK bits = 000, no shift.

The second SDCLK shows how SDCLK operates with the SHFTSDCLK bits = 001, shift 0.5 core clock.

(See MC_MEMCNTRL2 bits [5:3], Table 4-16 on page 108 , for remaining decode values.)

Shift =

Valid

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