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7.2.2 Data Bus
The DATA[15:0] signals are bidirectional data bus pins used to transfer data between
the chip and an external device.
7.2.3 Read/Write
The R/W output signal indicates whether the current bus access is a read or write
cycle. A high (logic one) level indicates a read cycle, and a low (logic zero) level indi-
cates a write cycle.
7.2.4 Control Signals
The OE and EB[0:1] signals are used to control the interface to the data bus.
7.2.4.1 Output Enable (OE)
This active-low output signal indicates the bus access is a read and enables slave
devices to drive the data bus with read data.
7.2.4.2 Enable Byte 0–1 (EB[0:1])
These active-low output pins indicate active data bytes for the current access. They
can be programmed in the chip-select control registers to assert for read and write
cycles or for write cycles only. EB0 corresponds to DATA[15:8], and EB1 corresponds
to DATA[7:0].
7.2.5 Boot Mode
The MOD input pin selects the initial CPU boot mode during hardware reset.
If this pin is driven to a logic-low level four LOW_REFCLK clock cycles before
RSTOUT negation, then the internal ROM will be disabled and the CPU will fetch the
first word from offset 0x0 of the external Flash memory, which is located at the abso-
lute address 0x2D00000 in the CPU address space. The internal ROM is disabled for
the first CPU cycle only and is available for subsequent accesses.
If this pin is driven to a logic-high level four LOW_REFCLK clock cycles before
RSTOUT negation, then the internal ROM is enabled and the CPU fetches the first
word from absolute address 0x0, which is the starting address of the internal ROM.
This signal is latched in the timer/reset module, and an internal version is supplied to
the EIM.
7.2.6 Chip Select Outputs
7.2.6.1 Chip Select 0 (CS0)
This active-low output signal is asserted based on a decode of bits ADDR[31:24] of
the access address, and at reset based on the value of the MOD input.
MOTOROLA
7-2
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Freescale Semiconductor, Inc.
EXTERNAL INTERFACE MODULE
For More Information On This Product,
Go to: www.freescale.com
MMC2001
REFERENCE MANUAL
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