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If an access matches multiple chip selects, the lowest
numbered chip select will have priority. For example, if CS0
and CS2 "overlap" for a certain range, CS0 will assert when
accessing the "overlapped" address range, and CS2 will not.
Global chip select provides a 16-bit port with three wait states, which allows a boot ROM
to be located in any address space and still provide the stack pointer and program counter
values at $00000000 and $00000004, respectively. Global chip select does not provide
write protection and responds to all function codes. While CS0 is a global chip select, no
other chip select ( CS1, CS2, CS3 ) can be used. CS0 operates in this manner until the
V-bit is set in the CS0 base address register, which will then allow the use of CS3–CS1 .
Provided the desired address range is first loaded into the CS0 base address register,
CS0 can be programmed to continue decode for a range of addresses after the V-bit is
set, After the V-bit is set for CS0 , global chip select can only be restarted with a system
reset.
A system can use an 8-bit boot ROM if an external 8-bit DSACK that responds in two or
less wait states is generated. The 8-bit DSACK must respond in two or less wait states
so that the global chip select, which responds with three wait states, will not be used. See
Section 10 Applications for a detailed discussion.
4.2.5 External Bus Interface Operation
This section describes port A and port B functions. Refer to Section 3 Bus Operation for
more information about the EBI.
4.2.5.1 PORT A. Port A pins can be independently programmed to function as either
addresses A31–A24, discrete I/O pins, or IACKx pins. The port A pin assignment
registers (PPARA1 and PPARA2) control the function of the port A pins as listed in Table
4-4. Upon reset, port A is configured as input pins. If the system uses these signals as
addresses, pulldowns should be put on these signals to avoid indeterminate values until
the port A registers can be programmed.
MOTOROLA
Freescale Semiconductor, Inc.
NOTE
Table 4-4. Port A Pin Assignment Register
Signal
PPARA1 = 0
PPARA2 = 0
A31
A31
A30
A30
A29
A29
A28
A28
A27
A27
A26
A26
A25
A25
A24
A24
MC68340 USER'S MANUAL
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Pin Function
PPARA1 = 1
PPARA1 = 0
PPARA2 = X
PPARA2 = 1
IACK7
PORT A7
IACK6
PORT A6
PORT A5
IACK5
PORT A4
IACK4
IACK3
PORT A3
IACK2
PORT A2
PORT A1
IACK1
PORT A0
—
4- 15
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