bits of each field are determined by the states of DATA[7:1] during reset. There are
weak internal pull-up drivers for each of the data lines, so that chip-select operation
will be selected by default out of reset. However, the internal pull-up drivers can be
overcome by bus loading effects — to insure a particular configuration out of reset, use
an active device to put the data lines in a known state during reset. The base address
fields in chip-select base address registers CSBAR[0:10] and chip select option regis-
ters CSOR[0:10] have the reset values shown in Table 4-23. The BYTE fields of
CSOR[0:10] have a reset value of “disable”, so that a chip-select signal cannot be as-
serted until the base and option registers are initialized.
Table 4-23 Chip Select Base and Option Register Reset Values
Fields
Base Address
Block Size
Reset Values
$000000
2 Kbyte
Async/Sync Mode
Upper/Lower Byte
Read/Write
AS/DS
Asynchronous Mode
Disabled
Reserved
AS
DSACK
No Wait States
CPU Space
Any Level
4
Address Space
IPL
Autovector
External Interrupt Vector
Following reset, the MCU fetches initial stack pointer and program counter values from
the exception vector table, beginning at $000000 in supervisor program space. The
CSBOOT chip-select signal is used to select an external boot ROM mapped to a base
address of $000000. In order to do this, the reset values of the fields that control CS-
BOOT must be different from those of other chip select signals.
The MSB of the CSBOOT field in CSPAR0 has a reset value of one, so that chip-select
function is selected by default out of reset. The BYTE field in option register CSORBT
has a reset value of “both bytes” so that the select signal is enabled out of reset. The
LSB value of the CSBOOT field, determined by the logic level of DATA0 during reset,
selects boot ROM port size. When DATA0 is held low during reset, port size is eight
bits. When DATA0 is held high during reset, port size is 16 bits. DATA0 has a weak
internal pull-up driver, so that a 16-bit port will be selected by default out of reset. How-
ever, the internal pull-up driver can be overcome by bus loading effects — to insure a
particular configuration out of reset, use an active device to put DATA0 in a known
state during reset.
The base address field in chip-select base address register boot (CSBARBT) has a
reset value of all zeros, so that when the initial access to address $000000 is made,
an address match occurs, and the CSBOOT signal is asserted. The block size field in
CSBARBT has a reset value of 1 Mbyte. Table 4-24 shows CSBOOT reset values.
MOTOROLA
4-56
SYSTEM INTEGRATION MODULE
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
