MOTOROLA
MC68CK338
34
MC68CK338TS/D
Port size determines the way in which bus transfers to external addresses are allocated. Port size of
eight bits or sixteen bits can be selected when a pin is assigned as a chip-select. Port size and transfer
Out of reset, chip-select pin function is determined by the logic level on a corresponding data bus pin.
These pins have weak internal pull-up drivers, but can be held low by external devices. Either 16-bit
chip-select function (%11) or alternate function (%01) can be selected during reset. All pins except the
boot ROM select pin (CSBOOT) are disabled out of reset.
The CSBOOT signal is normally enabled out of reset. The state of the DATA0 line during reset deter-
mines what port width CSBOOT uses. If DATA0 is held high (either by the weak internal pull-up driver
or by an external pull-up device), 16-bit width is selected. If DATA0 is held low, 8-bit port size is selected.
A pin programmed as a discrete output drives an external signal to the value specified in the pin data
register. No discrete output function is available on pins CSBOOT, BR, BG, or BGACK. ADDR23 pro-
vides ECLK output rather than a discrete output signal.
When a pin is programmed for discrete output or alternate function, internal chip-select logic still func-
tions and can be used to generate DSACK or AVEC internally on an address and control signal match.
3.6.3 Base Address Registers
Each chip-select has an associated base address register. A base address is the lowest address in the
block of addresses enabled by a chip-select. Block size is the extent of the address block above the
base address. Block size is determined by the value contained in a BLKSZ field. Multiple chip-selects
may be assigned to the same block of addresses so long as each chip-select uses the same number
of wait states.
The BLKSZ field determines which bits in the base address field are compared to corresponding bits on
the address bus during an access. Provided other constraints determined by option register fields are
also satisfied, when a match occurs, the associated chip-select signal is asserted.
After reset, the MCU fetches the address of the first instruction to be executed from the reset vector,
located beginning at address $000000 in program space. To support bootstrap operation from reset,
the base address field in CSBARBT has a reset value of all zeros. A memory device containing the reset
vector and an initialization routine can be automatically enabled by CSBOOT after a reset. The block
size field in CSBARBT has a reset value of one Mbyte.
Table 22 CSPAR1 Pin Assignments
CSPAR1 Field
Chip-Select Signal
Alternate Signal
Discrete Output
CS10PA[1:0]
CS10
ADDR23
ECLK
CS9PA[1:0]
CS9
ADDR22
PC6
CS8PA[1:0]
CS8
ADDR21
PC5
CS7PA[1:0]
CS7
ADDR20
PC4
CS6PA[1:0]
CS6
ADDR19
PC3
CSBARBT — Chip-Select Base Address Register Boot ROM
$YFFA48
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
ADDR
23
ADDR
22
ADDR
21
ADDR
20
ADDR
19
ADDR
18
ADDR
17
ADDR
16
ADDR
15
ADDR
14
ADDR
13
ADDR
12
ADDR
11
BLKSZ[2:0]
RESET:
0
1
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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