Embedded Target for Motorola MPC555

Overview of Memory Organization
and the Boot Process

Memory Organization

The MPC555 has a total of 448K of on-chip flash memory. This memory is organized into 14 banks of 32K each. The first bank is always used to store the boot code and the remaining 416K is available for application code. When using the Embedded Target for Motorola MPC555, the on-chip flash memory is located at absolute address 0x0000 in the MPC555 address space.

Figure 3-3: Organization of Flash Memory

To run a stand-alone application on the MPC555, it is first necessary to program the boot code into the first bank of flash memory.

The Boot Process

The boot code is executed following power on or reset (except if a probe is connected to the BDM port). Normally, the boot code performs basic hardware initialization and then branches to the application code. Once the application code is running, there is no way to return to the boot code except by performing a reset.

One of the important functions of the boot code is to serve as agent that allows program code to be downloaded over CAN. There are two methods of initiating a program download over CAN:

• The default method for initiating a flash download is to send a special CAN message during a short window of time while the boot code is executing. In the supplied boot code, this window is set to 40ms. If this special message is received during the window while the boot code is executing, a program download sequence commences and a new application can be programmed into flash memory. See Downloading Application Code to Flash Memory via CAN for details.
• Alternatively, it is possible to commence a flash download over CAN while application code is running on the target. To initiate a download over CAN, you must include a special Simulink block in your Simulink model. See Downloading Boot or Application Code via CAN Without Manual CPU Reset for details.

Downloading Boot and Application Code

Downloading Boot Code