Motorola Espresso Maker MC68340 User Manual

MOTOROLA

4- 15

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.

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.

A31

A31

PORT A7

IACK7

A30

A30

PORT A6

IACK6

A29

A29

PORT A5

IACK5

A28

A28

PORT A4

IACK4

A27

A27

PORT A3

IACK3

A26

A26

PORT A2

IACK2

A25

A25

PORT A1

IACK1

A24

A24

PORT A0

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Freescale Semiconductor, Inc.

For More Information On This Product,

   Go to: www.freescale.com

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