Chapter 19. Signal Descriptions
19-17
Chip Selects (CS7/SDCS, CS6/AEN, CS[5:1], CS0)
When a 16-bit data bus is used, mode parallel port C pins can be multiplexed onto D[15:0].
Data read from or written to on-chip peripherals is visible on the external data bus when the
device’s external bus width is 32 bits. When the device is configured for external 16-bit
wide data bus and the data access is 32 bits wide, the lower 16 bits of on-chip data are not
visible externally. On-chip cache, ROM, and SRAM accesses are not visible externally.
19.3.1 Dynamic Data Bus Sizing
When the device is in normal mode, dynamic bus sizing lets the programmer change data
bus width between 8, 32, and 16 bits for each chip select. The initial width for the bootstrap
program chip select, CS0, is determined by the state of BUSW[1:0]. The program should
select bus widths for the other chip selects before accessing the associated memory space.
19.4 Chip Selects (CS7/SDCS, CS6/AEN, CS[5:1],
CS0)
The eight chip selects, CS[7:0], allow the MCF5272 to interface directly to SRAM,
EPROM, EEPROM, and external memory-mapped peripherals. These signals can be
programmed for an address location, with masking capabilities, port size, burst capability
indication, and wait-state generation.
CS0 provides a special function as a global chip select that allows access to boot ROM at
at reset. CS0 can have its address redefined after reset. CS0 is the only chip select initialized
and enabled during reset. All other chip selects are disabled at reset and must be initialized
by device initialization software.
CS7/SDCS can be configured to access RAM or ROM or one physical bank of SDRAM.
Only CS7 can be used for SDRAM chip select.
19.5 Bus Control Signals
This section describes bus control signals.
19.5.1 Output Enable/Read (OE/RD)
The output enable/read signal (OE/RD) defines the data transfer direction for the data bus
D[31:0] for accesses to SRAM, ROM or external peripherals. A low (logic zero) level
indicates a read cycle while a high (logic one) indicates a write cycle.
This signal is normally connected to the OE pins of external SRAM, ROM, or FLASH.
19.5.2 Byte Strobes (BS[3:0])
The byte strobes (BS[3:0]) define the flow of data on the data bus. During SRAM and
Содержание DigitalDNA ColdFire MCF5272
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