MC68331
USER’S MANUAL
SYSTEM INTEGRATION MODULE
MOTOROLA
4-51
4
4.8.1.1 Chip-Select Pin Assignment Registers
The pin assignment registers contain twelve 2-bit fields (CS[10:0] and CSBOOT) that
determine the functions of the chip-select pins. Each pin has two or three possible
functions, as shown in
Table 4-19
.
Table 4-20
shows pin assignment field encoding. Pins that have no discrete output
function do not use the %00 encoding.
Port size determines the way in which bus transfers to an external address are allo-
cated. Port size of eight bits or sixteen bits can be selected when a pin is assigned as
a chip select. Port size and transfer size affect how the chip-select signal is asserted.
Refer to
4.8.1.3 Chip-Select Option Registers
for more information.
Out of reset, chip-select pin function is determined by the logic level on a correspond-
ing data bus pin. These pins have weak internal pull-up drivers, but can be held low
by external devices. (Refer to
4.6.3.1 Data Bus Mode Selection
for more informa-
tion.) Either 16-bit chip-select function (%11) or alternate function (%01) can be select-
ed during reset. All pins except the boot ROM select pin (CSBOOT) are disabled out
of reset. There are twelve chip-select functions and only eight associated data bus
pins. There is not a one-to-one correspondence. Refer to
4.8.4 Chip-Select Reset
Operation
for more detailed information.
The CSBOOT signal is normally enabled out of reset. The state of the DATA0 line dur-
ing reset determines 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 select-
ed. If DATA0 is held low, 8-bit port size is selected.
Table 4-19 Chip-Select Pin Functions
16-Bit
Chip Select
CSBOOT
CS0
CS1
CS2
CS3
CS4
CS5
CS6
CS7
CS8
CS9
CS10
8-Bit
Chip Select
CSBOOT
CS0
CS1
CS2
CS3
CS4
CS5
CS6
CS7
CS8
CS9
CS10
Alternate
Function
CSBOOT
BR
BG
BGACK
FC0
FC1
FC2
ADDR19
ADDR20
ADDR21
ADDR22
ADDR23
Discrete
Output
—
—
—
—
PC0
PC1
PC2
PC3
PC4
PC5
PC6
ECLK
Table 4-20 Pin Assignment Field Encoding
Bit Field
00
01
10
11
Description
Discrete Output
Alternate Function
Chip Select (8-Bit Port)
Chip Select (16-Bit Port)