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 4-19 Chip-Select Pin Functions
16-Bit
Chip Select
8-Bit
Chip Select
Alternate
Function
Discrete
Output
CSBOOT
CS0
CSBOOT
CS0
CSBOOT
BR
—
—
CS1
CS1
BG
—
CS2
CS2
BGACK
FC0
—
CS3
CS3
PC0
PC1
PC2
PC3
PC4
PC5
PC6
ECLK
CS4
CS4
FC1
CS5
CS5
FC2
CS6
CS6
ADDR19
ADDR20
ADDR21
ADDR22
ADDR23
CS7
CS7
CS8
CS8
CS9
CS9
4
CS10
CS10
Table 4-20 shows pin assignment field encoding. Pins that have no discrete output
function do not use the %00 encoding.
Table 4-20 Pin Assignment Field Encoding
Bit Field
Description
Discrete Output
00
01
10
11
Alternate Function
Chip Select (8-Bit Port)
Chip Select (16-Bit Port)
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.
MC68331
SYSTEM INTEGRATION MODULE
MOTOROLA
4-51
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
