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July 17, 2002
67
SDR Memory Design
5.2.2 Clock Enable Isolation During Power-down States
The processor SD_CKE[3:0] clock enable signals must be isolated from the SDR SDRAM. This is because
power states exist where the processor is powered down and the SDR SDRAM remains powered (e.g. STR).
The processor does not have a suspend power well, and like any CMOS circuit, the outputs are undefined for
short periods of time during power transitions. It is likely that all the signals glitch as power is applied or
removed from the processor. If the clock enable signal on the SDRAM remains at a stable state, preventing
activity within the SDRAM during power transitions, data integrity is maintained.
The SUSPEND_STATUS signal from the southbridge remains asserted while in STR mode, and is therefore
used to control the isolation. The output should multiplex between a pull-down resistor and the SD_CKE
signal from the processor, controlled by SUSPEND_STATUS.
5.2.3 Signal Termination
Series termination is recommended for all signals. Termination impedance should be calculated on a per-
design basis.
5.2.4 Miscellaneous Notes
If DIMMs are used, the serial presence detect (SPD) bus must be connected to the southbridge (not shown in
the block diagram).
All SDR SDRAM power supply inputs should be connected to V3_3_STR.
5.3 SDR SDRAM Layout Notes
This section describes procedures and guidelines for implementing SDR memory interface designs for
TM5500/TM5800 processors. Following the recommended SDR memory interface design rules and layout
procedures will result in reliable system designs that maximize performance while minimizing power
consumption.
5.3.1 SDR SDRAM Memory Interface Timing
The TM5500/TM5800 processor SDR SDRAM memory controller is capable of addressing up to four banks of
memory. The SDRAM configuration and size of each bank may be different.
The four clocks from the memory controller are copies of the SDR SDRAM clock. It does not matter which
clock goes to which bank, as bank selection is done with the CKE# and select lines. The clocks always have
four loads, making their behavior very predictable.
Содержание Crusoe TM5500
Страница 1: ...TM5500 TM5800 System Design Guide July 17 2002...
Страница 6: ...July 17 2002 6 List of Tables...
Страница 8: ...July 17 2002 8 List of Figures...
Страница 50: ...July 17 2002 50 Processor Power Supplies and Power Management...
Страница 110: ...July 17 2002 110 System Design Considerations...
Страница 122: ...July 17 2002 122 System Design Checklists...
Страница 128: ...July 17 2002 128 Serial Write protection PLD Data...
Страница 130: ...July 17 2002 130 Index...