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Platform RAM Array Controller (PRAM_CTL)

MAC7200 Microcontroller Family Reference Manual, Rev. 2

378

Freescale Semiconductor

19.1.6

SRAM Application Usage

19.1.6.1

SRAM Initialization

In order to avoid ECC errors on an uninitialized array, the SRAM array must be initialized using only

32-bit writes. This is because all RAM is in an indeterminate state after power up and all 8 and 16-bit writes

are actually performed as 32-bit READ + MODIFY + WRITE in order to correctly calculate the ECC

syndrome (which is based on a 32-bit value). Other than this, no other special handling is required.

19.1.6.2

SRAM Address Mirroring

Within the 512MB address space that the SRAM occupies on the crossbar switch, the actual addressable

range of the SRAM is mirrored in the memory map on a 32K boundary, as follows:

This means that accessing any mirrored SRAM address is virtually identical to accessing the SRAM at its

base address of $4000 0000. However, it should be noted that there is a posted write capability built into

the SRAM controller. As a consequence, writing/reading to/from address $0000 0000 (for example) is not

exactly equivalent (from a hardware point of view) to writing/reading to/from address $0000 8000 (for

example). The intended use of this mirroring is to allow the user to select a base address of their choosing,

but not to allow changing of this base address during the execution of an application. The default base

address for the SRAM is $4000 0000 and is not selectable during reset, but is software selectable after

reset.

19.2

Platform RAM Array Controller (PRAM_CTL)

19.2.1

Introduction

The PRAM_CTL module is an asynchronous reset, platform RAM array controller, with integrated error

detection and correction. Shown below in Figure 19-1 is a simplified block diagram of how the

PRAM_CTL would be implemented a typical platform:

Table 19-1. SRAM Address Mirroring

Address

Maps to....

$4000 0000

$4000 8000

$4000 0000

$4001 0000

$4000 0000

$5FFF 0000

$4000 0000

$5FFF 8000

$4000 0000

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