Signal Descriptions
MCF5272 ColdFire Integrated Microprocessor User’s Manual, Rev. 3
19-20
Freescale Semiconductor
19.6
Bus Control Signals
This section describes bus control signals.
19.6.1
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.
19.6.2
Byte Strobes (BS[3:0])
The byte strobes (BS[3:0]) define the flow of data on the data bus. During SRAM and peripheral accesses,
these outputs indicate that data is to be latched or driven onto a byte of the data when driven low. BSn
signals are asserted only to the memory bytes used during a read or write access.
BSn signals are asserted during accesses to on-chip peripherals but not to on-chip SRAM, cache, or ROM.
During SDRAM accesses, these signals indicate a byte transfer between SDRAM and the MCF5272 when
driven high.
For SRAM or FLASH devices, BS[3:0] outputs should be connected to individual byte strobe signals.
For SDRAM devices, BS[3:0] should be connected to individual SDRAM DQM signals. Note that most
SDRAMs associate DQM3 with the MSB, in which case BS3 should be connected to the SDRAM's
DQM3 input.
Table 19-3. Byte Strobe Operation for 32-Bit Data Bus
BS3
BS2
BS1
BS0
Access Type
Access Size
Data Located On
11
None
—
11
10
FLASH/SRAM
Byte
D[31:24]
11
01
FLASH/SRAM
D[23:16]
1
0
1
FLASH/SRAM
D[15:8]
01
11
FLASH/SRAM
D[7:0]
1
0
FLASH/SRAM
Word
D[31:16]
0
1
FLASH/SRAM
D[15:0]
0
FLASH/SRAM
Longword
D[31:0]
11
10
SDRAM
Byte
D[7:0]
1
0
1
SDRAM
D[15:8]
10
11
SDRAM
D[23:16]
01
11
SDRAM
D[31:24]
1
0
SDRAM
Word
D[15:0]
00
11
SDRAM
D[31:16]
0
SDRAM
Longword
D[31:0]