MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
12-56
Freescale Semiconductor
The EBI does not provide memory controller services to an external master that accesses shared external
memories. Each master must properly configure its own memory controller and drive its own chip selects
when sharing a memory between two masters.
The EBI does not support burst accesses from an external master; only single accesses of 8, 16, or 32 bits
can be performed.
1
12.4.2.10.1 Address Decoding for External Master Accesses
The EBI allows external masters to access internal address space when the EBI is configured for external
master mode. The external address is compared for any external master access, in order to determine if
EBI operation is required. Because only 24 address bits are available on the external bus, special decoding
logic is required to allow an external master to access on-chip locations whose upper 8 address bits are
non-zero. This is done by using the upper 4 external address bits (ADDR[8:11]) as a code to determine
whether the access is on-chip and if so, which internal slave it is targeted for.
Below is the address compare sequence:
•
External master access to another device — If ADDR[8] = 0, then the access is assumed to be to
another device and is ignored by the EBI.
•
External master access to valid internal slave — If ADDR[8] = 1, then ADDR[9:11] are checked
versus a list of 3-bit codes to determine which internal slave to forward the access to. The upper 8
internal address bits are set appropriately by the EBI according to this 3-bit code, and internal
address bits [8:11] are set appropriately to match the internal slave selected.
•
External master access to invalid internal slave — If the 3-bit code does not match a valid internal
slave, then the EBI responds with a bus error.
shows the possible 3-bit codes that are associated with various slaves in the MCU, as well as
the resulting upper 12 address bits required to appropriately match up with the memory map of each
internal slave.
1. Except for the special case of a 32-bit non-chip select access in 16-bit data bus mode. See
“Non-Chip-Select Burst in 16-bit Data Bus Mode
”.
Table 12-24. EBI Internal Slave Address Decoding
Internal Slave
External ADDR[8:11]
1
1
Value on upper 4 bits of 24-bit external address bus ADDR[8:31]. ADDR[8] determines whether
the access is on or off chip.
Internal Addr [0:7]
2
2
Value on upper 8 bits of 32-bit internal address bus.
Internal Addr [8:11]
3
3
Value on bits 8:11 of 32-bit internal address bus.
(off-chip)
0b0xxx
—
—
Internal flash
0b10xx
0b0000_0000
0b00, ADDR[10:11]
Internal SRAM
0b1100
0b0100_0000
0b0000
Reserved
0b1101
0b0110_0000
0b0000
Bridge A
Peripherals
0b1110
0b1100_0011
0b1111
Bridge B
Peripherals
0b1111
0b1111_1111
0b1111
Summary of Contents for MPC5553
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