Remap HCS12 RAM memory

Introduction

This Technical Note applies to IAR Embedded Workbench for HCS12 version 3.x and newer.

Discussion

The SFR area hide some RAM, and that RAM is needed by the application.

Background

IAR Embedded Workbench for HCS12 have chip setup files for each derivative. These setup files do not move any of the memory types. This means that the reset locations are used for SFRs, EEPROM and RAM. Thus, at address 0x00 and up you typically find these three types of memory covering each other.

A Freescale MC9S12DG128 (for example) have this memory map (lowest 16 kb) from reset...

... with 1 kB of the RAM memory and all (2kb) of the EEPROM memory covered by memory types with precedence. (Precedence is SFRs, RAM, EEPROM.)

Solution

During startup of the device, set the SFR INITRM to a value that moves the RAM memory to the wanted location.

These are the changes (in the IAR files):

• In the .ddf file add the line...

  Sfr3 = INITRM         2     YES     YES     0x20

  ...and note that the numbering "Sfr#" must be corrected when adding the line for writing the INITRM SFR. The value "0x20" should be read:
  "Place RAM at address 0x2000 and place the RAM upwards from this address."
• In the .ddf file edit the line...

  Memory1 = RAM         Memory       0x00000    0x01FFF  RW

  ...so that it instead reads...
  

  Memory1 = RAM         Memory       0x02000    0x03FFF  RW

• In the .xcl file change from...

  -D_RAM_BEGIN=00000
  -D_RAM_END=01FFF

  ...to...

  -D_RAM_BEGIN=02000
  -D_RAM_END=03FFF

• Finally make changes in the settings of the project so that the project "points to" the edited .ddf file and the edited .xcl file instead of the files supplied as default.
  These settings are found at... Project > Options > Linker > Config
  ...and at... Project > Options > Debugger > Setup

Example (for Freescale MC9S12DG128)

The attached example (Example for MC9S12DG128.zip) is configured with a moved RAM memory from 0x2000 to 0x3FFF.

Notes

When a .ddf file has been edited, it is a must to close the project and reopen it. (The .ddf file is read once, when the project is opened.)

The suggested solution works when downloading through the C-SPY BDM debugger, since the "BDM debugger driver" writes the SFRs with the values supplied in the .ddf file. If you want to use the device in stand-alone mode, the writes to the SFRs must be moved into __low_level_init.

The example is built, adapted and tested for "Special Single Chip Mode", so the write to INITRM need to be added in the .ddf file when using any other mode.

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