External Bus Interface (EBI)MPC5644A Microcontroller Reference Manual, Rev. 6Freescale Semiconductor 33314.6 Initialization/Application information14.6.1 Booting from external memoryThe EBI block does not support booting directly to external memory (i.e. fetching the first instruction afterreset externally). One common method for an MCU to resemble an external boot with this EBI is to usean internal Boot Assist Module on the MCU, which fetches the first instruction internally and configuresEBI registers before branching to an external address to “boot” externally. Refer to the device-specificdocumentation to see how/if external boot is supported for a particular MCU.If code in external memory needs to write EBI registers, this must be done in a way that avoids modifyingEBI registers while external accesses are being performed, such as the following method:• Copy the code that is doing the register writes (plus a return branch) to internal SRAM• Branch to internal SRAM to run this code, ending with a branch back to external flash14.6.2 Running with SDR (Single Data Rate) burst memoriesThis includes FLASH and SRAM memories with a compatible burst interface. BDIP is required only forsome SDR memories. Figure 14-38 shows a block diagram of an MCU connected to a 32-bit SDR burstmemory.Figure 14-38. MCU Connected to SDR Burst MemoryRefer to Figure 14-23 for an example of the timing of a typical Burst Read operation to an SDR burstmemory. Refer to Figure 14-14 for an example of the timing of a typical Single Write operation to SDRmemory.CLKOUT CKADDR[3:29] A[0:21]CECS0DATA[0:31] D[0:31]TSBDIPWE0/BE0ADVBAA*WE**OEOEMCU* May or may not be connected, depending on the memory used.SDR BurstableFlash or SRAM** Flash memories typically use one WE signal as shown, RAMs use 2 or 4 (16-bit or 32-bit)
