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Chapter 6. MPC8240 Memory Interface
6-19
SDRAM Interface Operation
from memory in the order 2-3-0-1. In 32-bit data bus mode, if the processor core requests
the third double word of a cache block, the MPC8240 reads words from memory in the
order 4-5-6-7-0-1-2-3.
For single-beat read transactions, the MPC8240 masks the extraneous data in the burst by
driving the DQM[0:7] signals high on the irrelevant cycles. For single-beat write
transactions, the MPC8240 protects non-targeted addresses by driving the DQM[0:7]
signals high on the irrelevant cycles. For single-beat transactions, the bursts cannot be
terminated early. That is, if the relevant data is in the first data phase, the subsequent data
phases of the burst must run to completion even though the data is irrelevant.
6.2.7 SDRAM Page Mode
Under certain conditions, the MPC8240 retains four active SDRAM pages for burst or
single-beat accesses. These conditions are as follows:
•
A pending transaction (read or write) hits one of the currently active internal pages.
•
There are no pending refreshes.
•
The burst-to-precharge interval (controlled by BSTOPRE[0:9]) has not been
exceeded.
•
The maximum activate-to-precharge interval (controlled by PGMAX) has not been
exceeded.
•
MCCR2[RSV_PG] = 0b0. In this case only three active pages are allowed.
Note that the BSTOPRE[0:9] parameter is composed of BSTOPRE[0:1] (bits 19–18 of
MCCR4), BSTOPRE[2:5] (bits 31–28 of MCCR3), and BSTOPRE[6:9] (bits 3–0 of
MCCR4).
Page mode can dramatically reduce access latencies for page hits. Depending on the
memory system design and timing parameters, using page mode can save clock cycles from
subsequent burst accesses that hit in an active page. SDRAM page mode is controlled by
the BSTOPRE[0:9], and PGMAX parameters. Page mode is disabled by clearing the
PGMAX or BSTOPRE[0:9] parameters.
The page open duration counter is loaded with BSTOPRE[0:9] every time the page is
accessed (including page hits). When the counter expires (or when PGMAX expires) the
open page is closed with a precharge bank command. Page hits can occur at any time in the
interval specified by BSTOPRE.
The 1-byte memory page mode register (MPMR) contains the PGMAX parameter that
controls how long the MPC8240 retains the currently accessed page (row) in memory. The
PGMAX parameter specifies the activate-to-precharge interval (sometimes called row
active time or t
RAS
). The PGMAX value is multiplied by 64 to generate the actual number
of clock cycles for the interval. When PGMAX is programmed to 0x00, page mode is
disabled.
Summary of Contents for MPC8240
Page 1: ...MPC8240UM D Rev 1 1 2001 MPC8240 Integrated Processor User s Manual ...
Page 38: ...xviii MPC8240 Integrated Processor User s Manual TABLES Table Number Title Page Number ...
Page 48: ...xlviii MPC8240 Integrated Processor User s Manual Acronyms and Abbreviations ...
Page 312: ...6 94 MPC8240 Integrated Processor User s Manual ROM Flash Interface Operation ...
Page 348: ...7 36 MPC8240 Integrated Processor User s Manual PCI Host and Agent Modes ...
Page 372: ...8 24 MPC8240 Integrated Processor User s Manual DMA Register Descriptions ...
Page 394: ...9 22 MPC8240 Integrated Processor User s Manual I2O Interface ...
Page 412: ...10 18 MPC8240 Integrated Processor User s Manual Programming Guidelines ...
Page 454: ...12 14 MPC8240 Integrated Processor User s Manual Internal Arbitration ...
Page 466: ...13 12 MPC8240 Integrated Processor User s Manual Exception Latencies ...
Page 516: ...16 14 Watchpoint Trigger Applications ...
Page 538: ...B 16 MPC8240 Integrated Processor User s Manual Setting the Endian Mode of Operation ...
Page 546: ...C 8 MPC8240 Integrated Processor User s Manual ...
Page 640: ...INDEX Index 16 MPC8240 Integrated Processor User s Manual ...