MOTOROLA
MC68341 USER’S MANUAL
4- 17
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.
After the V-bit is set, the CS0 pin can alternately be used as the AVEC input. The AVEC
bit in the MCR controls the function of this pin, as shown in Table 4-4.
Table 4-4. Port B Pin Assignment Register
Pin Function
Signal
AVEC Bit = 0
AVEC Bit = 1
CS0
AVEC
A system can use an 8-bit boot ROM if an external 8-bit DSACK≈ that responds in five or
less wait states is generated. The 8-bit DSACK≈ must respond in five or less wait states
so that the global chip select, which responds with six wait states, will not be used. See
Section 11 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 4-5. 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.
Table 4-5 Port A Pin Assignment Register
Pin Function
Signal
PPARA1 = 0
PPARA1 = 1
PPARA1 = 0
PPARA2 = 0
PPARA2 = X
PPARA2 = 1
A31
PORT A7
IACK7
A30
PORT A6
IACK6
A29
PORT A5
IACK5
A28
PORT A4
IACK4
A27
PORT A3
IACK3
A26
PORT A2
IACK2
A25
PORT A1
IACK1
A24
PORT A0
—
4.2.5.2 PORT B. Port B pins can be independently programmed to function as IRQ≈ or
discrete I/O pins. Selection of a pin function is accomplished by the port B pin assignment
register (PPARB). See Table 4-6 for port B selections. By changing the value of the
F
re
e
sc
a
le
S
e
m
ic
o
n
d
u
c
to
r,
I
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
n
c
..
.