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July 17, 2002
66
SDR Memory Design
The frequency setting for the SDR SDRAM interface is initialized during the boot sequence from data stored
in the configuration ROM. SDR interface frequency settings vary at each LongRun power management step.
SDR interface timing specifications and operating frequencies at various LongRun power management steps
are provided in the
Data Book
. The
Data Book
also provides SDR memory interface configuration constraints,
as well as recommended and example system memory configurations. See the
OEM Configuration Table
chapter of the
Development and Manufacturing Guide
for further LongRun configuration and memory
frequency information.
5.2 SDR Memory Interface Design Guidelines
The SDR SDRAM interface operates at frequencies up to 133 MHz and with edge rates as fast as 100 pS.
Use careful high-speed design, layout, and routing practices to minimize inductance and crosstalk,
maintaining the integrity of the transmission line structure throughout. Some specific guidelines for SDR
memory layout are provided below.
•
Place the memory as close as possible to the processor and oriented to reduce routing lengths.
•
A nominal SDR layout is based on the total trace length of any SDR address or command signal
(motherboard plus DIMM, if used). For a given number of loads, the rules are as follows:
-
For up to 16 loads the maximum total trace length must be <= 5”.
-
For up to 12 loads the maximum total trace length must be <= 8”.
•
Source termination of 33
Ω
(at the processor) is recommended for the data and mask signals.
•
Source termination of 10
Ω
(at the processor) is recommended for the address and command signals.
•
The clock signals should be of matched lengths. Source termination of 22
Ω
(at the processor) is
recommended for the clock signals.
•
The SDR_CLKOUT signal should have a 33
Ω
source termination resistor. The trace routing from the
output of this resistor to the SDR_CLKIN processor input is a timing delay. This trace length should be
equal to the sum of the total length of a clock trace and the length of the shortest data signal route. This
is explained in detail in
•
Characteristic impedance should be 55
Ω
± 10% for SDR-only systems. If DDR is present, the
characteristic impedance should be 60
Ω
± 10%.
•
For specific information about memory timing, see the
Data Book
. Refer to the
Development and
Manufacturing Guide
for additional information on memory configuration.
5.2.1 Bank Selection
The memory banks are selected with the SD_CS[3:0] signals. Use them in-order from highest to lowest. For
example, if only one bank of memory is used, connect it to SD_CS[3]. For one DIMM, which may have two
banks, connect SD_CS[3:2]. If both soldered-down memory and DIMM are used, connect the highest-order
bits to the soldered-down memory.
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...